httpdl: drop work-stealing segment pool for one worker per segment

The pool let an idle worker steal the back half of a slower in-flight
segment, behind a mutex that also serialized every advance. At the default
-x 1 it never fired, and it carried the trickiest invariant in the tree
(minSplit >= readBuf keeps an owner's in-flight write out of the stolen tail).

makeSegments already caps the segment count at the connection count, so the
segments map one-to-one onto workers: give each its own goroutine, advance
written with a plain atomic add, and snapshot the fixed slice with no lock.
pool/newPool/acquire/steal/advance and the seg.owned field all go away.

Net -156 lines. Behaviour at the user's flags is unchanged, except a slow
mirror's tail segment is no longer rebalanced onto idle connections.

httpdl/segment.go

@@ -1,27 +1,22 @@
 package httpdl
 
-import (
-	"sync"
-	"sync/atomic"
-)
+import "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 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).
+// BitTorrent. start and end are fixed when the file is divided; written is
+// advanced (atomically) by the segment's one worker, so Stat() and the resume
+// snapshot can read its frontier while it downloads.
 type seg struct {
 	index   int
 	start   int64 // first byte offset, inclusive
-	end     int64 // last byte offset, inclusive (a steal may shrink this)
+	end     int64 // last byte offset, inclusive
 	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) 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) length() int64      { return s.end - s.start + 1 }
+func (s *seg) endOff() int64      { return s.end }
 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) }
@@ -30,7 +25,9 @@ func (s *seg) remaining() int64   { return s.length() - atomic.LoadInt64(&s.writ
 
 // 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
-// the last segment. With conns==1 (the default) this yields one segment.
+// the last segment. With conns==1 (the default) this yields one segment. Each
+// segment gets its own worker, so the division here fixes the parallelism: there
+// is no later rebalancing.
 func makeSegments(total, minSplit int64, conns int) []seg {
 	if conns < 1 {
 		conns = 1
@@ -82,103 +79,3 @@ func autoConns(total, minSplit int64) int {
 	}
 	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)
-}