httpdl/cli: drop --auto-split; classify errors by type, not message text

Two Pike cleanups.

--auto-split was a third connection-count policy (beside -x and -s) that added
an extra knob rather than keeping the model minimal. Removing it also retires the
now-dead autoConns/maxAutoConns and the per-host transport cap that existed only
to scale for it; min(split, M*-x) is the sole policy again.

main's exit-code mapping fell back to substring-matching third-party error text
(strings.Contains "connection refused"/"timeout"/...), which rots when a
dependency rewords a message. The typed checks (errors.Is on the syscall errno,
*net.DNSError, net.Error.Timeout, context.DeadlineExceeded) already cover the
real stdlib errors -- verified end to end: refused -> 6, bad host -> 19. The two
cases that genuinely needed the text match are our own errors, now typed
sentinels: httpdl.ErrTimeout (idle timeout) and the bt metadata timeout wrapping
context.DeadlineExceeded.

httpdl/httpdl.go

@@ -57,8 +57,7 @@ type Config struct {
 	Split            int // --split: total connections across all mirrors
 	MaxConnPerServer int // --max-connection-per-server: per-host connection cap
 	MinSplit         int64
-	AutoSplit        bool // --auto-split (got-only): pick the connection count from file size
-	Tries            int  // 0 = unlimited
+	Tries            int // 0 = unlimited
 	Timeout          time.Duration
 	FileAlloc        string // none | prealloc | trunc | falloc
 	Continue         bool
@@ -120,7 +119,7 @@ type Config struct {
 // connections.
 type Download struct {
 	uris     []string // mirror list; uris[0] is the primary (naming + resume key)
-	maxConns int      // manual segment-worker cap = min(split, len(uris)*max-connection-per-server); --auto-split derives its own count per file in segmented()
+	maxConns int      // segment-worker cap = min(split, len(uris)*max-connection-per-server)
 	cfg      Config
 	client   *http.Client
 	limit    []*rate.Limiter
@@ -216,17 +215,10 @@ 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:       connCap,
-		MaxIdleConnsPerHost:   connCap,
+		MaxConnsPerHost:       perHost,
+		MaxIdleConnsPerHost:   perHost,
 		ResponseHeaderTimeout: cfg.Timeout,
 		TLSHandshakeTimeout:   connectTimeout,
 		DialContext:           dialContext,
@@ -671,13 +663,7 @@ 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 {
-	// --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)
+	segs := makeSegments(total, d.cfg.MinSplit, d.maxConns)
 	if c := d.resumeControl(out, etag, lastmod); c != nil {
 		segs = segsFromControl(c)
 	} else if d.cfg.Continue && !fileExists(controlPath(out)) {
@@ -809,10 +795,11 @@ func (d *Download) fetchOnce(ctx context.Context, f *os.File, s *seg, uri string
 	return d.pump(ctx, f, s, body)
 }
 
-// errIdleTimeout marks a transfer that stalled past the idle window, so callers
-// can report a distinct "timed out" instead of the bare "context canceled" that
-// the cancelled request would otherwise surface (item [39]).
-var errIdleTimeout = errors.New("idle timeout: no data received")
+// ErrTimeout marks a transfer that stalled past the idle window (--timeout): a
+// distinct "timed out" instead of the bare "context canceled" the cancelled
+// request would otherwise surface (item [39]). main maps it to exit code 2 via
+// errors.Is, so it must stay on the error chain (CONTRACT).
+var ErrTimeout = errors.New("idle timeout: no data received")
 
 // idleReader records the time of the last byte received; a background watcher
 // (idleGuard) cancels the request once a full idle window passes with no
@@ -834,7 +821,7 @@ func (ir *idleReader) Read(p []byte) (int, error) {
 	if err != nil && ir.timedOut.Load() {
 		// The cancellation came from our watcher, not the caller's ctx, so
 		// translate the read error into the distinct idle-timeout sentinel.
-		return n, fmt.Errorf("%w (after %s)", errIdleTimeout, ir.idle)
+		return n, fmt.Errorf("%w (after %s)", ErrTimeout, ir.idle)
 	}
 	return n, err
 }

httpdl/segment.go

@@ -55,27 +55,3 @@ 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)
-}

httpdl/segment_test.go

@@ -47,31 +47,6 @@ 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 {