gitignore: ignore downloaded media so an in-tree run can't commit content

running got in the repo root drops downloaded files next to the source;
torrents/control files were already ignored, add common media (mkv/mp4/
avi/mov/webm/iso/mp3/flac) so anime/ISO downloads don't show up as
untracked and can't be committed by accident.

.gitignore

@@ -7,3 +7,13 @@ ref/
 bin/
 .torrent.db*
 *.part
+
+# media downloaded by running got in the repo tree (never commit content)
+*.mkv
+*.mp4
+*.avi
+*.mov
+*.webm
+*.iso
+*.mp3
+*.flac

README.md

@@ -1,13 +1,8 @@
 # got
 
-A small, Rob-Pike-style command-line download tool in Go: one program that
-downloads HTTP(S) URLs over several connections and BitTorrent
-magnets/torrents, with a focused set of options.
-
-The design leans on Go's runtime instead of a hand-rolled reactor. There is no
-single-threaded event-poll loop: one goroutine per download blocks on real I/O,
-a buffered channel of slots bounds concurrency, and `context` carries shutdown.
-What remains is the data — and, per Pike, the data is the design.
+A command-line download tool in Go. It fetches HTTP(S) files over multiple
+connections and downloads BitTorrent magnets and `.torrent` files — one binary,
+familiar flags.
 
 ## Build
 
@@ -15,148 +10,67 @@ What remains is the data — and, per Pike, the data is the design.
 go build -o got .
 ```
 
-Requires Go 1.25+. The BitTorrent engine is
-[`anacrolix/torrent`](https://github.com/anacrolix/torrent); everything else is
-the standard library plus `golang.org/x/{term,time}`.
+Needs Go 1.25+.
 
-## Use
+## Usage
 
 ```sh
-# segmented HTTP download, 16 connections, into ./downloads
-got -x16 -s16 -d downloads https://example.com/big.iso
+# HTTP download with 16 connections
+got -x16 -s16 https://example.com/big.iso
+
+# let got pick the connection count from the file size
+got --auto-split https://example.com/big.iso
 
 # resume an interrupted download
-got -c -x16 https://example.com/big.iso
+got -c https://example.com/big.iso
 
-# the same file from several mirrors at once (one file, connections spread
-# across servers, a dead mirror falls over); -Z downloads them separately instead
+# one file from several mirrors (connections spread across servers,
+# a dead mirror falls over); -Z downloads them as separate files instead
 got -x4 https://a.example/big.iso https://b.example/big.iso
 
 # a torrent or magnet; seed for 30 minutes after finishing
 got --seed-time=30 ubuntu.torrent
 got 'magnet:?xt=urn:btih:...'
 
-# list the files in a torrent without downloading
-got -S -T some.torrent
-
-# several downloads at once, two running in parallel
+# many downloads, two at a time
 got -j2 -i urls.txt
 ```
 
-Run `got --help` (or `--help=http`, `--help=bittorrent`, ...) for the full
-option list.
+Run `got --help` (or `--help=all`) for every option.
 
-### Options
-
-A focused subset of options, with familiar short flags and sensible defaults.
+## Common options
 
 | flag | meaning | default |
 |------|---------|---------|
 | `-d, --dir` | output directory | `.` |
-| `-o, --out` | output filename (single download) | from URL |
-| `-i, --input-file` | read downloads line by line (TAB-separated URLs = mirrors; `-` = stdin) | |
+| `-o, --out` | output filename | from URL |
 | `-c, --continue` | resume a partial download | false |
 | `-x, --max-connection-per-server` | connections to one server (1–16) | 1 |
 | `-s, --split` | split a download into N connections | 5 |
-| `-Z, --force-sequential` | download each command-line URL separately, not as mirrors | false |
-| `-k, --min-split-size` | do not split below this size | 20M |
-| `-j, --max-concurrent-downloads` | parallel downloads | 5 |
+| `--auto-split` | choose connections from file size (overrides `-x`/`-s`) | false |
+| `-j, --max-concurrent-downloads` | downloads at once | 5 |
 | `--max-overall-download-limit` | global speed cap | 0 (off) |
-| `--max-download-limit` | per-download speed cap | 0 (off) |
-| `--retry-wait` | seconds to wait between retries | 0 |
-| `--load-cookies` / `--save-cookies` | read/write a Netscape `cookies.txt` | |
-| `--conditional-get` | skip the download if the local file is up to date | false |
-| `--remote-time` | set the file's mtime from the server | false |
-| `--checksum` | verify the finished file: `TYPE=DIGEST` (sha-256, sha-1, md5, …) | |
-| `--dry-run` | check the file is available but do not download it | false |
-| `--ca-certificate` | verify HTTPS against the CA certificates in FILE (PEM) | |
-| `--stop` | stop the program after N seconds | 0 (off) |
-| `--disable-ipv6` | force IPv4-only connections | false |
-| `-T, --torrent-file` | a `.torrent` file | |
-| `--seed-time` | minutes to seed after finishing | seed by ratio |
+| `--checksum` | verify the finished file: `TYPE=DIGEST` (sha-256, sha-1, …) | |
+| `--seed-time` | minutes to seed after a torrent finishes | by ratio |
 | `--seed-ratio` | stop seeding at this ratio | 1.0 |
-| `--listen-port` | port (range) for incoming peers | 6881-6999 |
-| `--enable-dht` | use the BitTorrent DHT | true |
-| `--select-file` | fetch only these file indexes (`1,3-5`) | all |
 | `-S, --show-files` | list torrent files and exit | |
-| `--bt-stop-timeout` | give up if no download progress for N s | 0 (off) |
-| `--bt-metadata-timeout` | give up if a magnet can't fetch metadata in N s | 60 |
-| `--save-session` | on exit, write unfinished downloads to FILE | |
-| `-q, --quiet` | no progress readout | false |
+| `-q, --quiet` | no progress output | false |
 
-### Resuming across runs
+## Resume
 
-There is no daemon — resume is a plain file. Point
-`--save-session` and `-i` at the same file and add `-c`: unfinished downloads
-are loaded at start and the still-unfinished ones are written back on exit.
+HTTP downloads resume from a small `<file>.got` sidecar — just re-run with `-c`.
+Torrents resume from the data already on disk. To resume across runs, point
+`--save-session` and `-i` at the same file:
 
 ```sh
-got -c --save-session=got.session -i got.session  <new urls/magnets...>
+got -c --save-session=got.session -i got.session  <urls/magnets...>
 ```
 
-On the first run the session file need not exist. HTTP downloads resume from
-their `.got` control file; torrents resume from the data already on disk
-(re-checked by the engine). A magnet with no reachable peers no longer hangs:
-`--bt-metadata-timeout` bounds the metadata fetch and defaults to 60s (set it to
-0 to wait forever).
+## Notes
 
-## Design
-
-```
-main            classify URIs, wire options, run, report exit code
- └─ download    the Download interface + the scheduler (-j semaphore, ctx)
-     ├─ httpdl  []Segment + worker pool + WriteAt + JSON resume sidecar
-     └─ bt      anacrolix/torrent behind the Download interface
- ├─ cli         one flat option table -> hand parser -> layered config
- └─ progress    one ticker goroutine, pull-snapshot, \r line redraw
-```
-
-The contract is one small interface:
-
-```go
-type Download interface {
-        Name() string
-        Run(ctx context.Context) error   // blocks in its own goroutine
-        Stat() Stat                       // a value snapshot, lock-free to read
-}
-```
-
-- **HTTP** splits the file into byte-range `Segment`s handed to a pool of
-  workers. Each worker streams its range straight to one shared file with
-  `os.File.WriteAt` — safe for concurrent non-overlapping writes, so there is no
-  shared seek offset and no mutex. Resume is a small JSON sidecar
-  (`<file>.got`) holding per-segment progress plus validators (length +
-  ETag/Last-Modified) so a stale file is never trusted.
-- **BitTorrent** is `anacrolix/torrent` configured from the CLI options and
-  driven to completion, then seeded for `--seed-time` minutes or up to
-  `--seed-ratio`.
-- **Options** are one flat `[]Opt` table — the single source of truth for
-  parsing, validation, defaults and `--help`. Layers apply in order:
-  built-in defaults, config file, proxy environment, command line.
-- **Progress** is a single goroutine that ticks once a second, pulls a snapshot
-  of the running downloads, derives speeds from the change since the last tick,
-  and redraws one line with `\r` + erase-to-end-of-line.
-
-## Status
-
-Implemented: segmented HTTP(S) with resume — including a foreign/browser-started
-partial file (`-c`) — multi-mirror downloads (several URLs, or TAB-grouped `-i`
-lines, fetch one file with connections spread across servers and fallover; `-Z`
-to download them separately), retries, rate limits, auto-renaming,
-`Content-Disposition` naming, conditional GET, cookies
-(`--load-cookies`/`--save-cookies`), HTTP basic auth, `--remote-time`, whole-file
-checksum verification (`--checksum`), and a `--dry-run` availability check;
-BitTorrent download + seeding (magnet and `.torrent`,
-DHT, trackers, file selection, show-files, metadata-fetch timeout), seeding that
-stops on whichever of `--seed-time`/`--seed-ratio` comes first, and a listen port
-chosen from the whole range so a busy port no longer disables BitTorrent;
-a command-line interface with documented exit codes, config file, live progress,
-and session save/reload for resume across runs.
-
-Deferred (not implemented yet): FTP/SFTP, Metalink, and the
-JSON-RPC server. The architecture leaves room for
-each — a new protocol is just another `Download`. One known limitation: the
-BitTorrent engine (`anacrolix/torrent`) applies download/upload rate limits
-client-wide, so `--max-upload-limit`/`--max-download-limit` act per-run rather
-than per-torrent when several torrents run at once (`--max-overall-*` are exact);
-DHT/PEX likewise cannot yet be disabled per private torrent.
+- BitTorrent uses [`anacrolix/torrent`](https://github.com/anacrolix/torrent);
+  HTTP uses the standard library.
+- Not implemented: FTP/SFTP, Metalink, and the JSON-RPC server.
+- When several torrents run at once, `--max-download-limit` /
+  `--max-upload-limit` apply per run rather than per torrent (`--max-overall-*`
+  are exact).

bt/bt.go

@@ -9,12 +9,16 @@ package bt
 import (
 	"context"
 	"fmt"
+	"io"
+	"log/slog"
+	"os"
 	"path/filepath"
 	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
 
+	alog "github.com/anacrolix/log"
 	missinggo "github.com/anacrolix/missinggo/v2"
 	"github.com/anacrolix/torrent"
 	"github.com/anacrolix/torrent/metainfo"
@@ -73,6 +77,14 @@ func ParsePorts(spec string) []int {
 	return ports
 }
 
+// quietSlogger filters anacrolix's structured logging down to Error and above,
+// so the routine webseed/peer/tracker warning chatter (the 403/429 noise) no
+// longer interleaves with — and corrupts — the live progress display. Genuine
+// errors still reach w.
+func quietSlogger(w io.Writer) *slog.Logger {
+	return slog.New(slog.NewTextHandler(w, &slog.HandlerOptions{Level: slog.LevelError}))
+}
+
 // NewClient builds the shared anacrolix client from the run-wide config. When a
 // listen-port spec is given it tries each candidate port in turn, advancing past
 // any that is already in use, and falls back to an OS-chosen port if none bind,
@@ -91,6 +103,11 @@ func ParsePorts(spec string) []int {
 //     prevention must come from a run-wide setting (e.g. --enable-dht=false).
 func NewClient(cfg ClientConfig) (*torrent.Client, error) {
 	c := torrent.NewDefaultClientConfig()
+	// Keep the library's own logging out of the readout. Webseed/peer warnings go
+	// through slog, the rest through the legacy analog logger, so filter both to
+	// Error and above (the analog default is Warning, which is what leaks today).
+	c.Slogger = quietSlogger(os.Stderr)
+	c.Logger = alog.Default.FilterLevel(alog.Error)
 	c.DataDir = cfg.Dir
 	if c.DataDir == "" {
 		c.DataDir = "."

bt/bt_test.go

@@ -1,10 +1,38 @@
 package bt
 
 import (
+	"bytes"
+	"context"
+	"log/slog"
 	"reflect"
+	"strings"
 	"testing"
 )
 
+// TestQuietSloggerDropsWarnings: the library's warning chatter (the webseed
+// 403/429 noise) is filtered out so it can't corrupt the live display, while a
+// genuine error still gets through.
+func TestQuietSloggerDropsWarnings(t *testing.T) {
+	var buf bytes.Buffer
+	lg := quietSlogger(&buf)
+
+	lg.Warn("webseed request error", "err", "429 Too Many Requests")
+	if buf.Len() != 0 {
+		t.Errorf("warning leaked into output: %q", buf.String())
+	}
+	// anacrolix's webseed code logs via Log(ctx, level, ...); confirm that path is
+	// filtered too, not just the Warn helper.
+	lg.Log(context.Background(), slog.LevelWarn, "another warning")
+	if buf.Len() != 0 {
+		t.Errorf("Log-at-Warn leaked: %q", buf.String())
+	}
+
+	lg.Error("real failure")
+	if !strings.Contains(buf.String(), "real failure") {
+		t.Errorf("error was suppressed, want it kept: %q", buf.String())
+	}
+}
+
 func TestParsePorts(t *testing.T) {
 	tests := []struct {
 		spec string

cli/option.go

@@ -78,9 +78,10 @@ var options = []Opt{
 	{Long: "human-readable", Kind: Bool, Default: "true", Help: "show sizes as Ki/Mi/Gi", Tag: Basic},
 
 	// --- http ---
-	{Long: "max-connection-per-server", Short: 'x', Kind: Int, Default: "1", Min: 1, Max: 16, Help: "max connections to one server (1-16)", Tag: HTTP},
-	{Long: "split", Short: 's', Kind: Int, Default: "5", Min: 1, Help: "split a download into N connections; actual connections are min(max-connection-per-server, split), and -x defaults to 1", Tag: HTTP},
-	{Long: "min-split-size", Short: 'k', Kind: Size, Default: "20M", Min: 1 << 20, Max: 1 << 30, Help: "do not split a piece smaller than SIZE (1M-1024M)", Tag: HTTP},
+	{Long: "max-connection-per-server", Short: 'x', Kind: Int, Default: "1", Min: 1, Max: 16, Help: "max connections to one server (1-16)", Tag: Basic},
+	{Long: "split", Short: 's', Kind: Int, Default: "5", Min: 1, Help: "split a download into N connections; actual connections are min(max-connection-per-server, split), and -x defaults to 1", Tag: Basic},
+	{Long: "min-split-size", Short: 'k', Kind: Size, Default: "20M", Min: 1 << 20, Max: 1 << 30, Help: "do not split a piece smaller than SIZE (1M-1024M)", Tag: Basic},
+	{Long: "auto-split", Kind: Bool, Default: "false", Help: "got-only: pick the connection count from file size (one per min-split-size, up to 16); overrides -x/-s", Tag: HTTP},
 	{Long: "force-sequential", Short: 'Z', Kind: Bool, Default: "false", Help: "download each command-line URI as its own file instead of mirroring them", Tag: HTTP},
 	{Long: "max-tries", Short: 'm', Kind: Int, Default: "5", Min: 0, Help: "max retries per segment (0 = unlimited)", Tag: HTTP},
 	{Long: "timeout", Short: 't', Kind: Int, Default: "60", Min: 1, Help: "connection timeout in seconds", Tag: HTTP},

download/engine.go

@@ -91,6 +91,10 @@ func (e *Engine) track(d Download) {
 	e.mu.Unlock()
 }
 
+// untrack removes d from the active set. Every Download implementation is a
+// pointer type, so the == compares identities — never the fields of a value,
+// which could panic on a non-comparable one. (Keying on Stat().ID is not an
+// option: a torrent's ID changes from source to infohash once metadata loads.)
 func (e *Engine) untrack(d Download) {
 	e.mu.Lock()
 	defer e.mu.Unlock()

go.mod

@@ -3,6 +3,7 @@ module github.com/hanbok/got
 go 1.25
 
 require (
+	github.com/anacrolix/log v0.17.1-0.20251118025802-918f1157b7bb
 	github.com/anacrolix/missinggo/v2 v2.10.0
 	github.com/anacrolix/torrent v1.61.0
 	golang.org/x/sys v0.38.0
@@ -19,7 +20,6 @@ require (
 	github.com/anacrolix/envpprof v1.4.0 // indirect
 	github.com/anacrolix/generics v0.1.1-0.20251125230353-15d98d46693b // indirect
 	github.com/anacrolix/go-libutp v1.3.2 // indirect
-	github.com/anacrolix/log v0.17.1-0.20251118025802-918f1157b7bb // indirect
 	github.com/anacrolix/missinggo v1.3.0 // indirect
 	github.com/anacrolix/missinggo/perf v1.0.0 // indirect
 	github.com/anacrolix/mmsg v1.0.1 // indirect

httpdl/control.go

@@ -27,11 +27,13 @@ type segState struct {
 
 func controlPath(out string) string { return out + ".got" }
 
-// snapshot builds a control record from the live segments.
-func snapshot(url string, total int64, etag, lastmod string, segs []seg) control {
+// snapshot builds a control record from the live segments. Callers hold the
+// pool lock (see pool.snapshot) so the slice and each segment's frontier are
+// stable for the read.
+func snapshot(url string, total int64, etag, lastmod string, segs []*seg) control {
 	c := control{URL: url, Total: total, ETag: etag, LastModified: lastmod, Segs: make([]segState, len(segs))}
 	for i := range segs {
-		c.Segs[i] = segState{segs[i].start, segs[i].end, atomic.LoadInt64(&segs[i].written)}
+		c.Segs[i] = segState{segs[i].start, segs[i].endOff(), atomic.LoadInt64(&segs[i].written)}
 	}
 	return c
 }

httpdl/httpdl.go

@@ -57,7 +57,8 @@ type Config struct {
 	Split            int // --split: total connections across all mirrors
 	MaxConnPerServer int // --max-connection-per-server: per-host connection cap
 	MinSplit         int64
-	Tries            int // 0 = unlimited
+	AutoSplit        bool // --auto-split (got-only): pick the connection count from file size
+	Tries            int  // 0 = unlimited
 	Timeout          time.Duration
 	FileAlloc        string // none | prealloc | trunc | falloc
 	Continue         bool
@@ -119,7 +120,7 @@ type Config struct {
 // connections.
 type Download struct {
 	uris     []string // mirror list; uris[0] is the primary (naming + resume key)
-	maxConns int      // segment workers = min(split, len(uris)*max-connection-per-server)
+	maxConns int      // manual segment-worker cap = min(split, len(uris)*max-connection-per-server); --auto-split derives its own count per file in segmented()
 	cfg      Config
 	client   *http.Client
 	limit    []*rate.Limiter
@@ -214,10 +215,17 @@ func New(uris []string, cfg Config) *Download {
 	if perHost < 1 {
 		perHost = 1
 	}
+	// The transport must allow as many concurrent connections per host as we may
+	// actually open. --auto-split scales up to maxAutoConns per host once the file
+	// size is known (in segmented), so raise the cap to that ceiling when it is on.
+	connCap := perHost
+	if cfg.AutoSplit {
+		connCap = maxAutoConns
+	}
 	tr := &http.Transport{
 		Proxy:                 http.ProxyFromEnvironment,
-		MaxConnsPerHost:       perHost,
-		MaxIdleConnsPerHost:   perHost,
+		MaxConnsPerHost:       connCap,
+		MaxIdleConnsPerHost:   connCap,
 		ResponseHeaderTimeout: cfg.Timeout,
 		TLSHandshakeTimeout:   connectTimeout,
 		DialContext:           dialContext,
@@ -528,29 +536,43 @@ func (d *Download) withRetries(ctx context.Context, what string, attempt func()
 	return retriesExhausted(what, tries, lastErr)
 }
 
+// overMirrors calls fn against each mirror URL in turn, starting at offset start
+// so callers can spread work across mirrors (segment index) and rotate on retry.
+// It returns nil on the first success, ctx.Err() if cancelled between mirrors, or
+// the last error once every mirror is spent.
+func (d *Download) overMirrors(ctx context.Context, start int, fn func(uri string) error) error {
+	var lastErr error
+	for i := range d.uris {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+		if err := fn(d.mirror(start + i)); err != nil {
+			lastErr = err
+			continue
+		}
+		return nil
+	}
+	return lastErr
+}
+
 // probeRetry runs probe within the shared retry policy (item [18]). The probe
 // result is captured through pr because withRetries only carries an error.
 func (d *Download) probeRetry(ctx context.Context) (probeResult, error) {
 	// Probe mirrors in order (primary first); the first that answers anchors the
 	// size and validators for the whole download. A dead/404 primary falls over
 	// to the next mirror.
-	var lastErr error
-	for _, u := range d.uris {
-		if ctx.Err() != nil {
-			return probeResult{}, ctx.Err()
-		}
-		var pr probeResult
-		err := d.withRetries(ctx, u, func() error {
+	var pr probeResult
+	err := d.overMirrors(ctx, 0, func(uri string) error {
+		return d.withRetries(ctx, uri, func() error {
 			var perr error
-			pr, perr = d.probe(ctx, u)
+			pr, perr = d.probe(ctx, uri)
 			return perr
 		})
-		if err == nil {
-			return pr, nil
-		}
-		lastErr = err
+	})
+	if err != nil {
+		return probeResult{}, err
 	}
-	return probeResult{}, lastErr
+	return pr, nil
 }
 
 // probe issues a one-byte ranged GET to learn the total size, whether the
@@ -610,7 +632,13 @@ func (d *Download) probe(ctx context.Context, uri string) (probeResult, error) {
 // segmented downloads total bytes over d.maxConns workers, with per-segment
 // resume from the control file.
 func (d *Download) segmented(ctx context.Context, out string, total int64, etag, lastmod string) error {
-	segs := makeSegments(total, d.cfg.MinSplit, d.maxConns)
+	// --auto-split picks the connection count from the file size now that total is
+	// known; otherwise use the manual cap fixed at construction.
+	conns := d.maxConns
+	if d.cfg.AutoSplit {
+		conns = autoConns(total, d.cfg.MinSplit)
+	}
+	segs := makeSegments(total, d.cfg.MinSplit, conns)
 	if c := d.resumeControl(out, etag, lastmod); c != nil {
 		segs = segsFromControl(c)
 	} else if d.cfg.Continue && !fileExists(controlPath(out)) {
@@ -648,36 +676,31 @@ func (d *Download) segmented(ctx context.Context, out string, total int64, etag,
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
+	p := newPool(segs, d.cfg.MinSplit)
+
 	// Periodically persist progress so a crash can resume.
 	saveDone := make(chan struct{})
-	go d.saveLoop(ctx, out, total, etag, lastmod, segs, saveDone)
-
-	jobs := make(chan *seg)
-	go func() {
-		defer close(jobs)
-		for i := range segs {
-			if segs[i].done() {
-				continue
-			}
-			select {
-			case jobs <- &segs[i]:
-			case <-ctx.Done(): // stop feeding once we're tearing down
-				return
-			}
-		}
-	}()
+	go d.saveLoop(ctx, out, total, etag, lastmod, p, saveDone)
 
+	// One worker per connection. A worker fetches segments until acquire runs
+	// dry, which happens only once every remaining byte is owned; a worker that
+	// finishes early steals the tail of a slower segment rather than sitting idle
+	// while the last segment drains over a single connection.
 	var (
 		wg      sync.WaitGroup
 		errOnce sync.Once
 		runErr  error
 	)
-	for w := 0; w < d.maxConns; w++ {
+	for w := 0; w < conns; w++ {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			for s := range jobs {
-				if err := d.fetchSeg(ctx, f, s, total); err != nil {
+			for {
+				s := p.acquire()
+				if s == nil {
+					return
+				}
+				if err := d.fetchSeg(ctx, f, p, s, total); err != nil {
 					errOnce.Do(func() { runErr = err; cancel() })
 					return
 				}
@@ -691,7 +714,7 @@ func (d *Download) segmented(ctx context.Context, out string, total int64, etag,
 	if runErr != nil {
 		return runErr // keep the control file for a later -c
 	}
-	snapshot(d.primary(), total, etag, lastmod, segs).save(out)
+	p.snapshot(d.primary(), total, etag, lastmod).save(out)
 	removeControl(out)
 	return nil
 }
@@ -699,37 +722,27 @@ func (d *Download) segmented(ctx context.Context, out string, total int64, etag,
 // fetchSeg downloads one segment, retrying from its resume point on error.
 // Each attempt re-reads s.offset(), so a retry continues from the bytes already
 // written rather than restarting the range.
-func (d *Download) fetchSeg(ctx context.Context, f *os.File, s *seg, total int64) error {
+func (d *Download) fetchSeg(ctx context.Context, f *os.File, p *pool, s *seg, total int64) error {
 	// Try each mirror in turn for this segment: a transient error retries the
 	// same mirror under withRetries; an exhausted budget or a per-mirror permanent
 	// error (a 404, a non-206, a length mismatch) falls over to the next mirror.
 	// The segment fails only once every mirror is spent. Starting at s.index
 	// spreads the segments across mirrors round-robin, and each attempt resumes
 	// from s.offset().
-	var lastErr error
-	for i := range d.uris {
-		if ctx.Err() != nil {
-			return ctx.Err()
-		}
-		uri := d.mirror(s.index + i)
-		err := d.withRetries(ctx, fmt.Sprintf("segment %d", s.index), func() error {
+	return d.overMirrors(ctx, s.index, func(uri string) error {
+		return d.withRetries(ctx, fmt.Sprintf("segment %d", s.index), func() error {
 			if s.done() {
 				return nil
 			}
-			return d.fetchOnce(ctx, f, s, uri, total)
+			return d.fetchOnce(ctx, f, p, s, uri, total)
 		})
-		if err == nil {
-			return nil
-		}
-		lastErr = err
-	}
-	return lastErr
+	})
 }
 
-func (d *Download) fetchOnce(ctx context.Context, f *os.File, s *seg, uri string, total int64) error {
+func (d *Download) fetchOnce(ctx context.Context, f *os.File, p *pool, s *seg, uri string, total int64) error {
 	reqCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
-	req, err := d.request(reqCtx, uri, fmt.Sprintf("bytes=%d-%d", s.offset(), s.end))
+	req, err := d.request(reqCtx, uri, fmt.Sprintf("bytes=%d-%d", s.offset(), s.endOff()))
 	if err != nil {
 		return err
 	}
@@ -755,7 +768,7 @@ func (d *Download) fetchOnce(ctx context.Context, f *os.File, s *seg, uri string
 	}
 	body, stop := d.idleGuard(resp.Body, cancel)
 	defer stop()
-	return d.pump(ctx, f, s, body)
+	return d.pump(ctx, f, p, s, body)
 }
 
 // errIdleTimeout marks a read that stalled past the idle window, so callers can
@@ -807,10 +820,9 @@ const speedStartupGrace = 10 * time.Second
 
 // belowSpeed reports whether the bytes transferred between two cumulative
 // d.completed samples (prev -> now over one window) put the download at or
-// below limit. A negative delta is not a slow second: d.completed is not
-// strictly monotonic — singleOnce rewrites it with an absolute StoreInt64 and a
-// resumed Range answered by a 200 resets the offset to 0, so the counter can
-// jump backward on a retry. Treating that backward jump as "below limit" would
+// below limit. A negative delta is not a slow second: when a resumed Range is
+// answered by a 200 the transfer restarts from offset 0, so d.completed jumps
+// backward on that retry. Treating that backward jump as "below limit" would
 // false-abort a healthy download, so a reset (delta < 0) reports false; the
 // caller has already advanced its baseline, so the next full window measures the
 // real speed. Only a genuine non-negative delta at or under the limit aborts.
@@ -901,8 +913,10 @@ func statusError(ctxMsg string, code int) error {
 }
 
 // pump copies the response body into the file at the segment's running offset,
-// stopping at the segment end and respecting rate limits.
-func (d *Download) pump(ctx context.Context, f *os.File, s *seg, body io.Reader) error {
+// stopping at the segment end and respecting rate limits. The loop reads
+// remaining() afresh each turn, so a steal that shrinks s mid-transfer simply
+// ends the loop early at the new end, leaving the stolen tail to its new worker.
+func (d *Download) pump(ctx context.Context, f *os.File, p *pool, s *seg, body io.Reader) error {
 	buf := make([]byte, readBuf)
 	for s.remaining() > 0 {
 		n := int64(len(buf))
@@ -914,7 +928,7 @@ func (d *Download) pump(ctx context.Context, f *os.File, s *seg, body io.Reader)
 			if _, werr := f.WriteAt(buf[:rd], s.offset()); werr != nil {
 				return werr
 			}
-			s.advance(int64(rd))
+			p.advance(s, int64(rd))
 			atomic.AddInt64(&d.completed, int64(rd))
 			d.throttle(ctx, rd)
 		}
@@ -953,23 +967,13 @@ func (d *Download) single(ctx context.Context, out string) error {
 	first := true
 	// One connection at a time, but try each mirror: a mirror that fails its
 	// retry budget falls over to the next (resuming from what's already on disk).
-	var lastErr error
-	for i := range d.uris {
-		if ctx.Err() != nil {
-			return ctx.Err()
-		}
-		uri := d.mirror(i)
-		err := d.withRetries(ctx, uri, func() error {
+	return d.overMirrors(ctx, 0, func(uri string) error {
+		return d.withRetries(ctx, uri, func() error {
 			resumeFromDisk := !first || foreignResume
 			first = false
 			return d.singleOnce(ctx, out, uri, resumeFromDisk)
 		})
-		if err == nil {
-			return nil
-		}
-		lastErr = err
-	}
-	return lastErr
+	})
 }
 
 // singleOnce performs one single-stream transfer attempt. When resumeFromDisk is
@@ -1047,7 +1051,7 @@ func (d *Download) singleOnce(ctx context.Context, out, uri string, resumeFromDi
 			}
 			off += int64(rd)
 			got += int64(rd)
-			atomic.StoreInt64(&d.completed, off)
+			atomic.AddInt64(&d.completed, int64(rd)) // add deltas, as pump does
 			d.throttle(ctx, rd)
 		}
 		if err == io.EOF {
@@ -1066,7 +1070,7 @@ func (d *Download) singleOnce(ctx context.Context, out, uri string, resumeFromDi
 	}
 }
 
-func (d *Download) saveLoop(ctx context.Context, out string, total int64, etag, lastmod string, segs []seg, done chan<- struct{}) {
+func (d *Download) saveLoop(ctx context.Context, out string, total int64, etag, lastmod string, p *pool, done chan<- struct{}) {
 	defer close(done)
 	interval := d.cfg.AutoSaveInterval
 	if interval <= 0 {
@@ -1077,10 +1081,10 @@ func (d *Download) saveLoop(ctx context.Context, out string, total int64, etag,
 	for {
 		select {
 		case <-ctx.Done():
-			snapshot(d.primary(), total, etag, lastmod, segs).save(out)
+			p.snapshot(d.primary(), total, etag, lastmod).save(out)
 			return
 		case <-t.C:
-			snapshot(d.primary(), total, etag, lastmod, segs).save(out)
+			p.snapshot(d.primary(), total, etag, lastmod).save(out)
 		}
 	}
 }

httpdl/segment.go

@@ -1,25 +1,32 @@
 package httpdl
 
-import "sync/atomic"
+import (
+	"sync"
+	"sync/atomic"
+)
 
 // seg is one contiguous byte range of the output file, downloaded by a single
 // ranged GET. A segment IS a byte range — a plain HTTP downloader does not need
 // the Piece/Segment/block layering that exists only to share code with
-// BitTorrent. written is updated with atomic ops so Stat() can read it while a
-// worker advances it.
+// BitTorrent. written and end are updated with atomic ops so Stat() and the
+// resume snapshot can read them while a worker advances written or a steal
+// shrinks end (see pool).
 type seg struct {
 	index   int
 	start   int64 // first byte offset, inclusive
-	end     int64 // last byte offset, inclusive
+	end     int64 // last byte offset, inclusive (a steal may shrink this)
 	written int64 // bytes already written into this segment (the resume point)
+	owned   bool  // a worker is fetching this segment; guarded by pool.mu
 }
 
-func (s *seg) length() int64    { return s.end - s.start + 1 }
-func (s *seg) done() bool       { return atomic.LoadInt64(&s.written) >= s.length() }
-func (s *seg) advance(n int64)  { atomic.AddInt64(&s.written, n) }
-func (s *seg) progress() int64  { return atomic.LoadInt64(&s.written) }
-func (s *seg) offset() int64    { return s.start + atomic.LoadInt64(&s.written) }
-func (s *seg) remaining() int64 { return s.length() - atomic.LoadInt64(&s.written) }
+func (s *seg) endOff() int64      { return atomic.LoadInt64(&s.end) }
+func (s *seg) setEnd(v int64)     { atomic.StoreInt64(&s.end, v) }
+func (s *seg) length() int64      { return s.endOff() - s.start + 1 }
+func (s *seg) done() bool         { return atomic.LoadInt64(&s.written) >= s.length() }
+func (s *seg) addWritten(n int64) { atomic.AddInt64(&s.written, n) }
+func (s *seg) progress() int64    { return atomic.LoadInt64(&s.written) }
+func (s *seg) offset() int64      { return s.start + atomic.LoadInt64(&s.written) }
+func (s *seg) remaining() int64   { return s.length() - atomic.LoadInt64(&s.written) }
 
 // makeSegments divides a file of total bytes into contiguous segments, using at
 // most conns of them and never splitting below minSplit. The remainder lands in
@@ -51,3 +58,127 @@ func makeSegments(total, minSplit int64, conns int) []seg {
 	}
 	return segs
 }
+
+// maxAutoConns is the ceiling --auto-split scales to. aria2 caps
+// max-connection-per-server (-x) at 16, so a got-chosen count honours the same
+// per-server limit rather than inventing a looser one.
+const maxAutoConns = 16
+
+// autoConns picks a connection count from the file size, used only when
+// --auto-split is set: one connection per min-split-size of content, at least 1
+// and at most maxAutoConns. It is the got-only stand-in for hand-tuning -x/-s,
+// and because the count never exceeds total/minSplit, makeSegments yields exactly
+// that many balanced segments.
+func autoConns(total, minSplit int64) int {
+	if minSplit < 1 {
+		minSplit = 1
+	}
+	n := total / minSplit
+	if n < 1 {
+		n = 1
+	}
+	if n > maxAutoConns {
+		n = maxAutoConns
+	}
+	return int(n)
+}
+
+// pool is the live set of segments for one segmented download. A worker takes a
+// segment with acquire and fetches it to completion; once no fresh segment is
+// left, an idle worker steals the back half of whichever in-flight segment has
+// the most still to download, so the last slow segment is shared across the idle
+// connections instead of draining alone. aria2 does the same on-demand split via
+// its SegmentMan; without it a fixed pre-division leaves connections idle while
+// one slow mirror finishes.
+//
+// The mutex serialises a steal against the byte accounting it splits: advance (a
+// worker committing a write) takes it too, so a steal reading a victim's frontier
+// can never race the owner advancing past the chosen split point. A steal fires
+// only when the victim still has at least 2*minSplit to go and cuts at the
+// midpoint, so both halves stay >= minSplit (--min-split-size) and the half the
+// owner keeps is far larger than one read buffer — the owner's in-flight write
+// therefore can never reach into the stolen tail.
+type pool struct {
+	mu       sync.Mutex
+	segs     []*seg
+	minSplit int64
+}
+
+// newPool wraps the pre-divided segments in a pool. Each is copied into its own
+// allocation so a later steal can append a tail without invalidating the pointers
+// workers already hold.
+//
+// minSplit is floored at readBuf: a steal leaves the owner the front half of the
+// split, which is at least minSplit, while the owner's in-flight write is at most
+// readBuf, so minSplit >= readBuf is exactly what keeps that write out of the
+// stolen tail. The CLI already holds --min-split-size well above readBuf (>= 1
+// MiB), so the floor only guards direct callers and tests — but it keeps the
+// no-overlap invariant inside this file rather than resting on a distant flag.
+func newPool(segs []seg, minSplit int64) *pool {
+	if minSplit < readBuf {
+		minSplit = readBuf
+	}
+	p := &pool{minSplit: minSplit, segs: make([]*seg, len(segs))}
+	for i := range segs {
+		s := segs[i]
+		p.segs[i] = &s
+	}
+	return p
+}
+
+// acquire returns the next segment for a worker to fetch: a fresh one if any
+// remain, otherwise the tail split off the busiest in-flight segment. It returns
+// nil when every remaining byte is already owned, i.e. the download is finishing
+// and there is nothing left to steal.
+func (p *pool) acquire() *seg {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	for _, s := range p.segs {
+		if !s.owned && !s.done() {
+			s.owned = true
+			return s
+		}
+	}
+	return p.steal()
+}
+
+// steal splits the back half off the in-flight segment with the most remaining
+// and returns it, or nil if none has enough left to be worth splitting. The
+// caller holds p.mu, which keeps every owner's advance out so each victim's
+// frontier is stable while we choose and commit the split.
+func (p *pool) steal() *seg {
+	var victim *seg
+	for _, s := range p.segs {
+		if s.owned && !s.done() && s.remaining() >= 2*p.minSplit {
+			if victim == nil || s.remaining() > victim.remaining() {
+				victim = s
+			}
+		}
+	}
+	if victim == nil {
+		return nil
+	}
+	mid := victim.offset() + victim.remaining()/2
+	// index only feeds the mirror round-robin (d.mirror) and the "segment N" log
+	// label; a tail's index need not be contiguous or unique, so len is fine.
+	tail := &seg{index: len(p.segs), start: mid, end: victim.endOff(), owned: true}
+	victim.setEnd(mid - 1)
+	p.segs = append(p.segs, tail)
+	return tail
+}
+
+// advance commits n freshly written bytes to s under the pool lock, so a
+// concurrent steal sees a stable frontier for the segment it may split.
+func (p *pool) advance(s *seg, n int64) {
+	p.mu.Lock()
+	s.addWritten(n)
+	p.mu.Unlock()
+}
+
+// snapshot builds the resume record from the live set, including any stolen
+// tails, under the lock so it never races a steal appending a segment.
+func (p *pool) snapshot(url string, total int64, etag, lastmod string) control {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return snapshot(url, total, etag, lastmod, p.segs)
+}

httpdl/segment_test.go

@@ -3,6 +3,9 @@ package httpdl
 import (
 	"os"
 	"path/filepath"
+	"sort"
+	"sync"
+	"sync/atomic"
 	"testing"
 )
 
@@ -44,6 +47,31 @@ func TestMakeSegments(t *testing.T) {
 	}
 }
 
+func TestAutoConns(t *testing.T) {
+	const m = int64(20 << 20) // 20 MiB min-split, aria2's default
+	tests := []struct {
+		name     string
+		total    int64
+		minSplit int64
+		want     int
+	}{
+		{"under one min-split is a single connection", 5 << 20, m, 1},
+		{"exactly one min-split", m, m, 1},
+		{"five min-splits", 100 << 20, m, 5},
+		{"caps at maxAutoConns", 10 << 30, m, maxAutoConns},
+		{"rounds down to whole pieces", m*3 + 1, m, 3},
+		{"tiny min-split still capped", 1 << 30, 1 << 20, maxAutoConns},
+		{"zero total is one connection", 0, m, 1},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := autoConns(tc.total, tc.minSplit); got != tc.want {
+				t.Errorf("autoConns(%d, %d) = %d, want %d", tc.total, tc.minSplit, got, tc.want)
+			}
+		})
+	}
+}
+
 func TestSegProgress(t *testing.T) {
 	s := seg{start: 100, end: 199} // length 100
 	if s.length() != 100 {
@@ -52,14 +80,14 @@ func TestSegProgress(t *testing.T) {
 	if s.done() {
 		t.Fatalf("new segment should not be done")
 	}
-	s.advance(60)
+	s.addWritten(60)
 	if s.offset() != 160 {
 		t.Errorf("offset = %d, want 160", s.offset())
 	}
 	if s.remaining() != 40 {
 		t.Errorf("remaining = %d, want 40", s.remaining())
 	}
-	s.advance(40)
+	s.addWritten(40)
 	if !s.done() {
 		t.Errorf("segment should be done after writing full length")
 	}
@@ -186,3 +214,151 @@ func TestParseContentRange(t *testing.T) {
 		t.Errorf("parseContentRange(garbage) ok = true, want false")
 	}
 }
+
+// assertTiles checks that the pool's segments still cover [0,total) exactly:
+// sorted by start they must be contiguous with no gap and no overlap.
+func assertTiles(t *testing.T, p *pool, total int64) {
+	t.Helper()
+	p.mu.Lock()
+	type rng struct{ start, end int64 }
+	rs := make([]rng, len(p.segs))
+	for i, s := range p.segs {
+		rs[i] = rng{s.start, s.endOff()}
+	}
+	p.mu.Unlock()
+	sort.Slice(rs, func(i, j int) bool { return rs[i].start < rs[j].start })
+	var next int64
+	for _, r := range rs {
+		if r.start != next {
+			t.Fatalf("segment gap/overlap: next byte %d, got start %d (ranges %v)", next, r.start, rs)
+		}
+		next = r.end + 1
+	}
+	if next != total {
+		t.Fatalf("segments cover %d bytes, want %d", next, total)
+	}
+}
+
+func TestPoolAcquireThenSteal(t *testing.T) {
+	// Sizes are in units of readBuf because that is what newPool floors minSplit
+	// to and what the no-overlap invariant is stated against.
+	const total = 8 * readBuf
+	p := newPool(makeSegments(total, readBuf, 1), readBuf) // one segment [0,total)
+
+	first := p.acquire()
+	if first == nil || first.start != 0 || first.endOff() != total-1 {
+		t.Fatalf("first acquire = %+v, want the whole [0,%d] segment", first, total-1)
+	}
+	if p.acquire() == nil {
+		// nothing fresh left, so this must steal first's back half
+		t.Fatal("second acquire returned nil, want a stolen tail")
+	}
+	// first kept the front half, a tail took the back half; together they still tile.
+	if first.endOff() != total/2-1 {
+		t.Errorf("victim end = %d, want %d after midpoint split", first.endOff(), total/2-1)
+	}
+	assertTiles(t, p, total)
+}
+
+func TestPoolNoStealBelowThreshold(t *testing.T) {
+	// remaining is below 2*minSplit, so the lone segment is not worth splitting
+	// and an idle worker is told there is nothing to do.
+	p := newPool(makeSegments(readBuf+readBuf/2, readBuf, 1), readBuf)
+	if s := p.acquire(); s == nil {
+		t.Fatal("first acquire returned nil, want the only segment")
+	}
+	if s := p.acquire(); s != nil {
+		t.Fatalf("second acquire = %+v, want nil (tail would be below min-split)", s)
+	}
+}
+
+// TestPoolConcurrentCoverage drives the pool the way real workers do — acquire a
+// segment, copy it in small chunks, commit each with p.advance, repeat — across
+// more workers than initial segments so stealing is forced. The coverage array
+// proves the core invariant: every byte is written exactly once, so a steal
+// never overlaps the owner's writes and never leaves a gap.
+func TestPoolConcurrentCoverage(t *testing.T) {
+	const (
+		total    = int64(64 * readBuf) // 2 MiB
+		minSplit = int64(readBuf)      // steal threshold is 2*this
+		chunk    = int64(readBuf / 4)  // one "read", well below minSplit
+		workers  = 8
+	)
+	p := newPool(makeSegments(total, minSplit, 1), minSplit) // start with a single segment
+	cover := make([]uint32, total)
+
+	var wg sync.WaitGroup
+	for w := 0; w < workers; w++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for {
+				s := p.acquire()
+				if s == nil {
+					return
+				}
+				for s.remaining() > 0 {
+					n := chunk
+					if s.remaining() < n {
+						n = s.remaining()
+					}
+					off := s.offset()
+					for j := off; j < off+n; j++ {
+						atomic.AddUint32(&cover[j], 1)
+					}
+					p.advance(s, n)
+				}
+			}
+		}()
+	}
+	wg.Wait()
+
+	for i, c := range cover {
+		if c != 1 {
+			t.Fatalf("byte %d written %d times, want exactly 1", i, c)
+		}
+	}
+	assertTiles(t, p, total)
+	if len(p.segs) == 1 {
+		t.Error("no stealing happened: still one segment after 8 workers drained it")
+	}
+}
+
+// TestPoolSnapshotAfterStealRoundTrips checks that a steal which happens before a
+// crash survives the control file: the snapshot records the stolen tail, and a
+// resume rebuilds a segment set that still tiles the file and keeps the bytes
+// already written. This is the persistence path an interrupt-and-resume relies on
+// but cannot deterministically trigger from the outside.
+func TestPoolSnapshotAfterStealRoundTrips(t *testing.T) {
+	const total = 8 * readBuf
+	p := newPool(makeSegments(total, readBuf, 1), readBuf)
+	a := p.acquire()      // the whole file
+	p.advance(a, readBuf) // owner makes some progress, then idles out
+	b := p.acquire()      // steals a's back half
+	if b == nil {
+		t.Fatal("expected a stolen tail")
+	}
+	p.advance(b, readBuf) // the tail's worker makes progress too
+
+	c := p.snapshot("http://x", total, "", "")
+	if len(c.Segs) != 2 {
+		t.Fatalf("snapshot has %d segments, want 2 (original + stolen tail)", len(c.Segs))
+	}
+
+	rebuilt := segsFromControl(&c)
+	sort.Slice(rebuilt, func(i, j int) bool { return rebuilt[i].start < rebuilt[j].start })
+	var next, written int64
+	for _, s := range rebuilt {
+		if s.start != next {
+			t.Fatalf("rebuilt gap/overlap: next byte %d, got start %d", next, s.start)
+		}
+		next = s.endOff() + 1
+		written += s.progress()
+	}
+	if next != total {
+		t.Fatalf("rebuilt covers %d bytes, want %d", next, total)
+	}
+	if written != 2*readBuf {
+		t.Errorf("rebuilt written = %d, want %d (bytes must survive the round trip)", written, 2*readBuf)
+	}
+}

main.go

@@ -190,9 +190,9 @@ type failedDownload struct {
 	err  error
 }
 
-func (f failedDownload) Name() string              { return f.name }
-func (f failedDownload) Run(context.Context) error { return f.err }
-func (f failedDownload) Stat() download.Stat {
+func (f *failedDownload) Name() string              { return f.name }
+func (f *failedDownload) Run(context.Context) error { return f.err }
+func (f *failedDownload) Stat() download.Stat {
 	return download.Stat{Name: f.name, Status: download.Errored, Total: -1}
 }
 
@@ -224,6 +224,24 @@ func dedupeTorrents(uris []string, follow bool) map[string]string {
 	return dup
 }
 
+// markInfoHash records the v1 infohash of the .torrent at path in seen and
+// reports whether it repeats one already seen, returning the path first recorded
+// under that infohash. Callers fail the duplicate rather than let two jobs share
+// one torrent on the client (see bt.SourceInfoHash for why that aliasing is
+// unsafe). A v2-only or unreadable infohash is never a duplicate — the dedup
+// can't see it, so it runs.
+func markInfoHash(seen map[metainfo.Hash]string, path string) (first string, dup bool) {
+	h, ok := bt.SourceInfoHash(path, true)
+	if !ok {
+		return "", false
+	}
+	if first, dup = seen[h]; dup {
+		return first, true
+	}
+	seen[h] = path
+	return "", false
+}
+
 func build(opts *cli.Options, targets []target) ([]job, *torrent.Client, error) {
 	flat := allURIs(targets)
 	// Two sources naming the same torrent (a magnet and its own .torrent, say)
@@ -263,7 +281,7 @@ func build(opts *cli.Options, targets []target) ([]job, *torrent.Client, error)
 		u := t.uris[0] // primary: classifies the target and names the session source
 		src := strings.Join(t.uris, "\t")
 		if first, isDup := dups[u]; isDup {
-			out = append(out, job{source: src, dl: failedDownload{
+			out = append(out, job{source: src, dl: &failedDownload{
 				name: u,
 				err:  fmt.Errorf("%w: same torrent as %s", errDuplicate, first),
 			}})
@@ -358,6 +376,7 @@ func runJobs(ctx context.Context, eng *download.Engine, jobs []job) []download.R
 // so a regular download that merely ends in ".torrent" is left alone.
 func followUps(jobs []job, client *torrent.Client, bo bt.Options) []job {
 	var out []job
+	seen := map[metainfo.Hash]string{}
 	for _, j := range jobs {
 		h, ok := j.dl.(*httpdl.Download)
 		if !ok || j.dl.Stat().Status != download.Complete {
@@ -373,6 +392,17 @@ func followUps(jobs []job, client *torrent.Client, bo bt.Options) []job {
 		if _, err := bt.Files(path); err != nil {
 			continue // downloaded file is not a valid torrent
 		}
+		// Two fetched .torrent files that name the same torrent would share one
+		// torrent on the refcount-less client: the first to finish Drops it out
+		// from under the other, stranding it at 0%. build() dedupes pass-1 sources
+		// but cannot reach a not-yet-fetched HTTP .torrent, so dedupe here too.
+		if first, dup := markInfoHash(seen, path); dup {
+			out = append(out, job{source: path, dl: &failedDownload{
+				name: path,
+				err:  fmt.Errorf("%w: same torrent as %s", errDuplicate, first),
+			}})
+			continue
+		}
 		out = append(out, job{source: path, dl: bt.New(client, path, true, bo)})
 	}
 	return out
@@ -408,22 +438,23 @@ func allURIs(ts []target) []string {
 // gatherURIs collects download targets from the command line, --torrent-file,
 // and --input-file. Plain command-line http/https URIs are grouped into ONE
 // mirrored download by default (every command-line URI names the same file);
-// -Z/--force-sequential makes each its own download. Torrents and magnets
-// are always one target each; --input-file groups TAB-separated URIs per line.
+// -Z/--force-sequential makes each its own download. Torrents, magnets, and
+// .torrent URLs (fetched over HTTP then followed) are always one target each,
+// since distinct torrents are not mirrors of one another; --input-file groups
+// TAB-separated URIs per line.
 func gatherURIs(opts *cli.Options, cmdURIs []string) []target {
 	var ts []target
 	follow := opts.Bool("follow-torrent")
 	seq := opts.Bool("force-sequential")
 	var httpGroup []string
 	for _, u := range cmdURIs {
-		switch uriKind(u, follow) {
-		case kindHTTP, kindFollow:
-			if seq {
-				ts = append(ts, target{uris: []string{u}})
-			} else {
-				httpGroup = append(httpGroup, u)
-			}
-		default:
+		// Only plain HTTP URLs collapse into one mirror download (alternate sources
+		// for the same file); -Z opts out. A .torrent URL (kindFollow), like a
+		// torrent or magnet, is its own download — distinct torrents are never
+		// mirrors of one another.
+		if uriKind(u, follow) == kindHTTP && !seq {
+			httpGroup = append(httpGroup, u)
+		} else {
 			ts = append(ts, target{uris: []string{u}})
 		}
 	}
@@ -493,6 +524,7 @@ func httpConfig(opts *cli.Options, single bool, overallDL *rate.Limiter) httpdl.
 		Split:            opts.Int("split"),
 		MaxConnPerServer: opts.Int("max-connection-per-server"),
 		MinSplit:         opts.Size("min-split-size"),
+		AutoSplit:        opts.Bool("auto-split"),
 		Tries:            opts.Int("max-tries"),
 		Timeout:          time.Duration(opts.Int("timeout")) * time.Second,
 		FileAlloc:        opts.Str("file-allocation"),

main_test.go

@@ -11,6 +11,9 @@ import (
 	"syscall"
 	"testing"
 
+	"github.com/anacrolix/torrent/bencode"
+	"github.com/anacrolix/torrent/metainfo"
+	"github.com/hanbok/got/bt"
 	"github.com/hanbok/got/cli"
 	"github.com/hanbok/got/download"
 	"github.com/hanbok/got/httpdl"
@@ -112,8 +115,8 @@ func TestDedupeTorrents(t *testing.T) {
 }
 
 // TestGatherURIsGrouping checks the command-line grouping: plain http URIs
-// become one mirrored download by default, -Z splits them, and torrents/magnets
-// stay separate.
+// become one mirrored download by default, -Z splits them, and torrents,
+// magnets, and .torrent URLs stay separate.
 func TestGatherURIsGrouping(t *testing.T) {
 	parse := func(args ...string) []target {
 		res, err := cli.Parse(append([]string{"--no-conf"}, args...))
@@ -131,6 +134,16 @@ func TestGatherURIsGrouping(t *testing.T) {
 	if ts := parse("http://a/x", "magnet:?xt=urn:btih:abc"); len(ts) != 2 {
 		t.Errorf("http + magnet: got %d targets, want 2 (magnet never mirrors)", len(ts))
 	}
+	// Distinct .torrent URLs are separate downloads, not mirrors of one file
+	// (follow-torrent defaults on, so each is fetched then followed).
+	if ts := parse("http://a/x.torrent", "http://b/y.torrent", "http://c/z.torrent"); len(ts) != 3 {
+		t.Errorf(".torrent URLs: got %d targets, want 3 separate downloads", len(ts))
+	}
+	// A plain http URL still mirror-groups even alongside a .torrent URL: one
+	// http group of 1 + one torrent target = 2.
+	if ts := parse("http://a/x", "http://b/y.torrent"); len(ts) != 2 {
+		t.Errorf("http + .torrent: got %d targets, want 2", len(ts))
+	}
 }
 
 // TestReadInputFileTAB checks the input-file grouping: TAB-separated URIs on
@@ -152,6 +165,65 @@ func TestReadInputFileTAB(t *testing.T) {
 	}
 }
 
+// writeTorrent creates a .torrent file at dir/name whose info dict is built from
+// a file of the given content and internal name. Two calls with the same
+// internalName and content produce the same infohash regardless of the output
+// filename, so the test can make two .torrent files that name one torrent.
+func writeTorrent(t *testing.T, dir, name, internalName string, content []byte) string {
+	t.Helper()
+	data := filepath.Join(dir, internalName)
+	if err := os.WriteFile(data, content, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	info := metainfo.Info{PieceLength: 32 * 1024}
+	if err := info.BuildFromFilePath(data); err != nil {
+		t.Fatal(err)
+	}
+	b, err := bencode.Marshal(info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tp := filepath.Join(dir, name)
+	f, err := os.Create(tp)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	if err := (&metainfo.MetaInfo{InfoBytes: b}).Write(f); err != nil {
+		t.Fatal(err)
+	}
+	return tp
+}
+
+// TestFollowUpInfoHashDedup covers the fix for two fetched .torrent files that
+// name the same torrent: the second must be flagged a duplicate so followUps
+// fails it rather than letting both share — and Drop — one torrent on the client.
+func TestFollowUpInfoHashDedup(t *testing.T) {
+	dir := t.TempDir()
+	a := writeTorrent(t, dir, "a.torrent", "same", []byte("AAAAAAAA"))
+	b := writeTorrent(t, dir, "b.torrent", "same", []byte("AAAAAAAA")) // same infohash as a
+	c := writeTorrent(t, dir, "c.torrent", "diff", []byte("BBBBBBBB")) // a distinct torrent
+
+	// Sanity-check the fixture: a and b really share an infohash, c differs.
+	ha, _ := bt.SourceInfoHash(a, true)
+	hb, _ := bt.SourceInfoHash(b, true)
+	hc, _ := bt.SourceInfoHash(c, true)
+	if ha != hb || ha == hc {
+		t.Fatalf("fixture: want a==b!=c infohashes, got a=%v b=%v c=%v", ha, hb, hc)
+	}
+
+	seen := map[metainfo.Hash]string{}
+	if first, dup := markInfoHash(seen, a); dup {
+		t.Fatalf("a is the first occurrence, wrongly flagged a duplicate of %s", first)
+	}
+	if first, dup := markInfoHash(seen, b); !dup || first != a {
+		t.Errorf("b should duplicate a: got first=%q dup=%v, want %q true", first, dup, a)
+	}
+	if _, dup := markInfoHash(seen, c); dup {
+		t.Errorf("c is a distinct torrent, wrongly flagged a duplicate")
+	}
+}
+
 func TestReportExit(t *testing.T) {
 	canceled := download.Result{Name: "a", Err: context.Canceled}
 	done := download.Result{Name: "b", Err: nil}

progress/progress.go

@@ -25,9 +25,11 @@ type Reporter struct {
 	w        io.Writer
 	tty      bool
 	width    int
+	height   int
 
-	win     map[string]*speedWindow // keyed by Stat.ID
-	lastLog time.Time               // last non-TTY line emitted
+	win       map[string]*speedWindow // keyed by Stat.ID
+	prevLines int                     // lines drawn in the previous TTY frame, redrawn in place
+	lastLog   time.Time               // last non-TTY line emitted
 }
 
 // logEvery bounds how often progress is printed when output is not a TTY, so a
@@ -62,6 +64,7 @@ func New(snap func() []download.Stat, human bool) *Reporter {
 		w:        os.Stdout,
 		tty:      term.IsTerminal(int(os.Stdout.Fd())),
 		width:    80,
+		height:   24,
 		win:      map[string]*speedWindow{},
 	}
 }
@@ -103,29 +106,21 @@ func (r *Reporter) render(final bool) {
 		}
 	}
 
-	var line string
-	switch len(stats) {
-	case 0:
-		if !final && r.tty {
-			return // nothing active; leave the line as-is
-		}
-	case 1:
-		line = r.lineOne(stats[0])
-	default:
-		line = r.lineMany(stats)
-	}
-
 	if r.tty {
 		r.width = termWidth(r.width)
-		line = clampRunes(line, r.width)
-		fmt.Fprint(r.w, "\r"+line+"\x1b[K")
-		if final {
-			fmt.Fprintln(r.w)
-		}
+		r.height = termHeight(r.height)
+		r.redraw(r.block(stats), final)
 		return
 	}
-	// Not a TTY: can't redraw one line, so print sparingly (and always the
-	// final line) to keep redirected output and logs readable.
+	// Not a TTY: no cursor control, so emit a single line sparingly (and always
+	// the final one) to keep redirected output and logs readable.
+	var line string
+	switch {
+	case len(stats) == 1:
+		line = r.lineOne(stats[0])
+	case len(stats) > 1:
+		line = r.summary(stats)
+	}
 	if line != "" && (final || now.Sub(r.lastLog) >= logEvery) {
 		fmt.Fprintln(r.w, line)
 		r.lastLog = now
@@ -165,27 +160,161 @@ func (r *Reporter) lineOne(s download.Stat) string {
 	return b.String()
 }
 
-func (r *Reporter) lineMany(stats []download.Stat) string {
+// nameW is the fixed rune width of the name column in the table, matching
+// shortName's cap so a padded name lines the columns up.
+const nameW = 20
+
+// block builds the lines of the live display: the single-download dashboard for
+// one download, a name/bar/speed/ETA table for several, or a one-line summary
+// when the table would not fit the window.
+func (r *Reporter) block(stats []download.Stat) []string {
+	switch {
+	case len(stats) == 0:
+		return nil
+	case len(stats) == 1:
+		return []string{r.lineOne(stats[0])}
+	case len(stats)+1 > r.height: // header + one row each would overflow the screen
+		return []string{r.summary(stats)}
+	default:
+		out := make([]string, 0, len(stats)+1)
+		out = append(out, r.summary(stats))
+		for _, s := range stats {
+			out = append(out, r.tableRow(s))
+		}
+		return out
+	}
+}
+
+// redraw repaints the block in place: move to the top of the previous frame,
+// clear it, and print the new lines clamped to the terminal width. A stray line
+// from stderr that lands in the block is painted over on the next tick.
+func (r *Reporter) redraw(lines []string, final bool) {
+	if r.prevLines > 0 {
+		if r.prevLines > 1 {
+			fmt.Fprintf(r.w, "\x1b[%dA", r.prevLines-1) // up to the first line
+		}
+		fmt.Fprint(r.w, "\r\x1b[J") // column 0, clear to end of screen
+	}
+	for i, ln := range lines {
+		if i > 0 {
+			fmt.Fprint(r.w, "\n")
+		}
+		fmt.Fprint(r.w, clampRunes(ln, r.width))
+	}
+	r.prevLines = len(lines)
+	if final && len(lines) > 0 {
+		fmt.Fprintln(r.w) // leave the cursor below the block for the OK/FAIL lines
+		r.prevLines = 0
+	}
+}
+
+// summary is the header/aggregate line: the active count and total down/up
+// speed, plus a stalled count when any download has gone quiet.
+func (r *Reporter) summary(stats []download.Stat) string {
+	if len(stats) == 0 {
+		return ""
+	}
 	var totDL, totUL int64
+	stalled := 0
 	for _, s := range stats {
 		dl, ul := r.rates(s)
 		totDL += dl
 		totUL += ul
-	}
-	var b strings.Builder
-	fmt.Fprintf(&b, "[%d active DL:%s UL:%s]", len(stats), speed(totDL, r.human), speed(totUL, r.human))
-	for i, s := range stats {
-		if i >= 4 {
-			fmt.Fprintf(&b, "[+%d]", len(stats)-i)
-			break
-		}
-		if s.Total > 0 {
-			fmt.Fprintf(&b, "[%s %d%%]", shortName(s.Name), percent(s.Completed, s.Total))
-		} else {
-			fmt.Fprintf(&b, "[%s %s]", shortName(s.Name), humanSize(s.Completed, r.human))
+		if r.isStalled(s, dl) {
+			stalled++
 		}
 	}
-	return b.String()
+	line := fmt.Sprintf("%d active   DL:%s UL:%s", len(stats), speed(totDL, r.human), speed(totUL, r.human))
+	if stalled > 0 {
+		line += fmt.Sprintf("  (%d stalled)", stalled)
+	}
+	return line
+}
+
+// tableRow renders one download as "name |bar| pct metric tail": the bar and
+// percent for progress, then the field that answers "stalled or fast" (DL speed
+// or seeding) and a tail (ETA, upload rate, or a stalled flag).
+func (r *Reporter) tableRow(s download.Stat) string {
+	dl, ul := r.rates(s)
+	metric, tail := r.rowMetric(s, dl, ul)
+	return fmt.Sprintf("  %s  %s %s  %-9s %s", padRune(shortName(s.Name), nameW), barOf(s), pctField(s), metric, tail)
+}
+
+// rowMetric returns a table row's two trailing fields for the download's state:
+// the primary metric (DL speed, "seeding", "done", "failed") and a tail (ETA,
+// upload rate, or a "stalled" flag).
+func (r *Reporter) rowMetric(s download.Stat, dl, ul int64) (metric, tail string) {
+	switch s.Status {
+	case download.Seeding:
+		return "seeding", "UL:" + speed(ul, r.human)
+	case download.Complete:
+		return "done", ""
+	case download.Errored:
+		return "failed", ""
+	default:
+		metric = "DL:" + speed(dl, r.human)
+		switch {
+		case dl > 0 && s.Total > 0:
+			eta := time.Duration(float64(s.Total-s.Completed)/float64(dl)) * time.Second
+			tail = secfmt(eta)
+		case r.isStalled(s, dl):
+			tail = "stalled"
+		}
+		return metric, tail
+	}
+}
+
+// isStalled reports an active download that has gone a while with no new bytes,
+// so the table can flag it rather than leave the eye to read DL:0B. A download
+// too young to have a rate yet is not stalled.
+func (r *Reporter) isStalled(s download.Stat, dl int64) bool {
+	if s.Status != download.Active || dl > 0 {
+		return false
+	}
+	w := r.win[s.ID]
+	if w == nil || len(w.samples) < 2 {
+		return false
+	}
+	return w.samples[len(w.samples)-1].t.Sub(w.samples[0].t) >= 4*time.Second
+}
+
+// barOf renders a fixed-width progress bar. A seeding or complete download is
+// full; an unknown total (no Content-Length, or pre-metadata) shows empty.
+func barOf(s download.Stat) string {
+	const cells = 12
+	k := 0
+	switch {
+	case s.Status == download.Seeding || s.Status == download.Complete:
+		k = cells
+	case s.Total > 0:
+		k = int(int64(cells) * s.Completed / s.Total)
+		if k > cells {
+			k = cells
+		}
+	}
+	return "|" + strings.Repeat("#", k) + strings.Repeat("-", cells-k) + "|"
+}
+
+// pctField is the percent column: a number, full for seeding/complete, or "--"
+// when the total is unknown.
+func pctField(s download.Stat) string {
+	switch {
+	case s.Status == download.Seeding || s.Status == download.Complete:
+		return "100%"
+	case s.Total > 0:
+		return fmt.Sprintf("%3d%%", percent(s.Completed, s.Total))
+	default:
+		return "  --"
+	}
+}
+
+// padRune right-pads s with spaces to w runes (it is never longer, having been
+// through shortName) so table columns line up on rune count.
+func padRune(s string, w int) string {
+	if n := utf8.RuneCountInString(s); n < w {
+		return s + strings.Repeat(" ", w-n)
+	}
+	return s
 }
 
 // add records a sample and drops any that have fallen out of the window. A
@@ -272,3 +401,13 @@ func termWidth(prev int) int {
 	}
 	return 80
 }
+
+func termHeight(prev int) int {
+	if _, h, err := term.GetSize(int(os.Stdout.Fd())); err == nil && h > 0 {
+		return h
+	}
+	if prev > 0 {
+		return prev
+	}
+	return 24
+}

progress/progress_test.go

@@ -1,6 +1,7 @@
 package progress
 
 import (
+	"bytes"
 	"strings"
 	"testing"
 	"time"
@@ -112,6 +113,100 @@ func TestLineOneCNSD(t *testing.T) {
 	}
 }
 
+func TestBarOf(t *testing.T) {
+	tests := []struct {
+		name string
+		s    download.Stat
+		want string
+	}{
+		{"empty", download.Stat{Status: download.Active, Total: 100, Completed: 0}, "|------------|"},
+		{"half", download.Stat{Status: download.Active, Total: 100, Completed: 50}, "|######------|"},
+		{"full by complete", download.Stat{Status: download.Complete}, "|############|"},
+		{"full by seeding", download.Stat{Status: download.Seeding}, "|############|"},
+		{"unknown total is empty", download.Stat{Status: download.Active, Total: -1, Completed: 9}, "|------------|"},
+	}
+	for _, tc := range tests {
+		if got := barOf(tc.s); got != tc.want {
+			t.Errorf("%s: barOf = %q, want %q", tc.name, got, tc.want)
+		}
+	}
+}
+
+func TestPctField(t *testing.T) {
+	if got := pctField(download.Stat{Status: download.Active, Total: 100, Completed: 50}); got != " 50%" {
+		t.Errorf("active 50%% = %q, want ' 50%%'", got)
+	}
+	if got := pctField(download.Stat{Status: download.Seeding}); got != "100%" {
+		t.Errorf("seeding = %q, want 100%%", got)
+	}
+	if got := pctField(download.Stat{Status: download.Active, Total: -1}); got != "  --" {
+		t.Errorf("unknown total = %q, want '  --'", got)
+	}
+}
+
+// TestRowMetric: the two trailing fields per download state.
+func TestRowMetric(t *testing.T) {
+	r := newReporter(true)
+	if m, tail := r.rowMetric(download.Stat{Status: download.Active, Total: 100, Completed: 50}, 10, 0); !strings.HasPrefix(m, "DL:") || tail == "" {
+		t.Errorf("active downloading: metric=%q tail=%q, want DL:… + an ETA", m, tail)
+	}
+	if m, tail := r.rowMetric(download.Stat{Status: download.Seeding}, 0, 128<<10); m != "seeding" || !strings.HasPrefix(tail, "UL:") {
+		t.Errorf("seeding: metric=%q tail=%q, want seeding + UL:…", m, tail)
+	}
+	if m, _ := r.rowMetric(download.Stat{Status: download.Complete}, 0, 0); m != "done" {
+		t.Errorf("complete metric=%q, want done", m)
+	}
+}
+
+// TestBlockTableAndFallback: several downloads render as header+rows when they
+// fit, and collapse to a single summary line when the window is too short.
+func TestBlockTableAndFallback(t *testing.T) {
+	r := newReporter(true)
+	stats := []download.Stat{
+		{ID: "a", Name: "a", Status: download.Active, Total: 100, Completed: 50},
+		{ID: "b", Name: "b", Status: download.Active, Total: 100, Completed: 10},
+	}
+	r.height = 24
+	if got := r.block(stats); len(got) != 3 { // header + 2 rows
+		t.Errorf("table block = %d lines, want 3:\n%v", len(got), got)
+	}
+	r.height = 2 // header + 2 rows = 3 > 2
+	if got := r.block(stats); len(got) != 1 {
+		t.Errorf("overflow block = %d lines, want 1 summary:\n%v", len(got), got)
+	}
+}
+
+// TestRenderRedraw: the TTY path draws the block, then on the next frame moves
+// the cursor up over the previous frame and clears it before repainting.
+func TestRenderRedraw(t *testing.T) {
+	snap := []download.Stat{
+		{ID: "a", Name: "alpha", IsBT: true, Status: download.Active, Total: 100, Completed: 50},
+		{ID: "b", Name: "beta", IsBT: true, Status: download.Seeding, Total: 100, Completed: 100, Uploaded: 200},
+	}
+	var buf bytes.Buffer
+	r := &Reporter{
+		snap:  func() []download.Stat { return snap },
+		human: true, w: &buf, tty: true, width: 200, height: 24,
+		win: map[string]*speedWindow{},
+	}
+	r.render(false)
+	first := buf.String()
+	for _, want := range []string{"2 active", "alpha", "seeding", "|"} {
+		if !strings.Contains(first, want) {
+			t.Fatalf("first frame missing %q:\n%q", want, first)
+		}
+	}
+	buf.Reset()
+	r.render(false)
+	second := buf.String()
+	if !strings.Contains(second, "\x1b[2A") { // up prevLines-1 = 3-1
+		t.Errorf("second frame should move cursor up 2 lines:\n%q", second)
+	}
+	if !strings.Contains(second, "\x1b[J") {
+		t.Errorf("second frame should clear to end of screen:\n%q", second)
+	}
+}
+
 // TestShortNameRune ensures CJK names are truncated on runes, not bytes.
 func TestShortNameRune(t *testing.T) {
 	name := strings.Repeat("あ", 30) // 30 runes, 90 bytes