fix(http): wait for the prefetch goroutine to exit before reusing its file (#1606)

* fix(http): wait for the prefetch goroutine to exit before reusing its file

stopPrefetchAndCopyData waited at most 10 s for the Resolve prefetch, but a
stalled body read only times out after readTimeout (15 s). The caller then
closed and removed the temporary file while the prefetch goroutine could
still write to it.

Stopping the prefetch now closes the Resolve body, which interrupts a
pending read, and waits on a channel closed when the goroutine exits. Pause
uses the same path. The stop channel is closed under resolveRespLock so a
concurrent Pause and first-connection takeover cannot close it twice.

The proxy test helper now calls Setup before Resolve, as the downloader
does, so the Resolve request also goes through the proxy.

Fixes #1563

* refactor(http): wait on prefetchExitCh in completeFromResolveFallback

Replaces the 10 ms polling loop with the exit channel added in the
previous commit, so every place that waits for the prefetch uses it.

---------

Co-authored-by: Levi <[email protected]>

internal/protocol/http/fetcher.go

@@ -314,6 +314,7 @@ type Fetcher struct {
 	prefetchDone     atomic.Bool   // Prefetch completed or stopped
 	prefetchErr      error         // Error during prefetch (if any)
 	prefetchStopCh   chan struct{} // Signal to stop prefetch
+	prefetchExitCh   chan struct{} // Closed when the prefetch goroutine exits
 	resolveFallback  atomic.Bool   // Resolve prefetch is still available until the first Range request takes over
 
 	// Target file
@@ -562,6 +563,7 @@ func (f *Fetcher) Resolve(req *base.Request) error {
 	// For non-range resources, the response will be used directly in Start
 	if res.Range && reuseResolveBody {
 		f.prefetchStopCh = make(chan struct{})
+		f.prefetchExitCh = make(chan struct{})
 		f.resolveFallback.Store(true)
 		go f.asyncPrefetch()
 	}
@@ -579,6 +581,9 @@ func (f *Fetcher) Resolve(req *base.Request) error {
 func (f *Fetcher) asyncPrefetch() {
 	defer func() {
 		f.prefetchDone.Store(true)
+		if f.prefetchExitCh != nil {
+			close(f.prefetchExitCh)
+		}
 	}()
 
 	// Get the resolve response
@@ -654,23 +659,38 @@ func (f *Fetcher) asyncPrefetch() {
 	}
 }
 
+// stopPrefetch signals the async prefetch to stop and waits until it exits.
+// Closing the resolve body unblocks a pending Read, so a stalled server cannot
+// keep the prefetch goroutine writing to the temporary file after we reuse it.
+func (f *Fetcher) stopPrefetch() {
+	if f.prefetchStopCh == nil {
+		return
+	}
+
+	// Pause and the first connection's takeover can stop concurrently, so the
+	// check-and-close must be under the lock to avoid closing the channel twice.
+	f.resolveRespLock.Lock()
+	select {
+	case <-f.prefetchStopCh:
+		// Already closed
+	default:
+		close(f.prefetchStopCh)
+	}
+	if f.resolveResp != nil {
+		f.resolveResp.Body.Close()
+		f.resolveResp = nil
+	}
+	f.resolveRespLock.Unlock()
+
+	if f.prefetchExitCh != nil {
+		<-f.prefetchExitCh
+	}
+}
+
 // stopPrefetchAndGetData stops the async prefetch and returns prefetched bytes
 // It also copies prefetched data to the target file
 func (f *Fetcher) stopPrefetchAndCopyData() int64 {
-	// Signal prefetch to stop (safely)
-	if f.prefetchStopCh != nil {
-		select {
-		case <-f.prefetchStopCh:
-			// Already closed
-		default:
-			close(f.prefetchStopCh)
-		}
-	}
-
-	// Wait for prefetch to finish (with timeout)
-	for i := 0; f.prefetchStopCh != nil && i < 1000 && !f.prefetchDone.Load(); i++ {
-		time.Sleep(10 * time.Millisecond)
-	}
+	f.stopPrefetch()
 
 	prefetched := f.prefetchSize.Load()
 	if prefetched == 0 {
@@ -1631,11 +1651,10 @@ func (f *Fetcher) completeFromResolveFallback(conn *connection) bool {
 		return false
 	}
 
-	for !f.prefetchDone.Load() {
-		if conn.ctx.Err() != nil {
-			return false
-		}
-		time.Sleep(10 * time.Millisecond)
+	select {
+	case <-f.prefetchExitCh:
+	case <-conn.ctx.Done():
+		return false
 	}
 	knownSize := f.hasKnownSize()
 	if f.prefetchErr != nil || (knownSize && f.prefetchSize.Load() != f.meta.Res.Size) {
@@ -2442,14 +2461,7 @@ func (f *Fetcher) Pause() error {
 	}
 
 	// Stop prefetch if running
-	if f.prefetchStopCh != nil {
-		select {
-		case <-f.prefetchStopCh:
-			// Already closed
-		default:
-			close(f.prefetchStopCh)
-		}
-	}
+	f.stopPrefetch()
 
 	// Wait for downloadLoop to exit first (it will call wg.Wait internally)
 	if f.downloadLoopDone != nil {
@@ -2459,11 +2471,6 @@ func (f *Fetcher) Pause() error {
 	// Wait for all connection goroutines to stop
 	f.wg.Wait()
 
-	// Wait for prefetch to finish
-	for f.prefetchStopCh != nil && !f.prefetchDone.Load() {
-		time.Sleep(10 * time.Millisecond)
-	}
-
 	// If Pause wins before the first Range response is validated, materialize
 	// the Resolve prefix before discarding its temporary file. The first
 	// connection was allocated after the earlier prefetch snapshot, so advance

internal/protocol/http/fetcher_test.go

@@ -3261,16 +3261,17 @@ func downloadResume(listener net.Listener, connections int, t *testing.T) {
 }
 
 func downloadWithProxy(httpListener net.Listener, proxyListener net.Listener, t *testing.T) {
-	fetcher := downloadReady(httpListener, 4, t)
-	ctl := controller.NewController()
-	ctl.GetProxy = func(requestProxy *base.RequestProxy) func(*gohttp.Request) (*url.URL, error) {
+	// Setup must run before Resolve, as in the downloader, so the Resolve
+	// request also goes through the proxy.
+	f := buildFetcher()
+	f.ctl.GetProxy = func(requestProxy *base.RequestProxy) func(*gohttp.Request) (*url.URL, error) {
 		return (&base.DownloaderProxyConfig{
 			Enable: true,
 			Scheme: "socks5",
 			Host:   proxyListener.Addr().String(),
 		}).ToHandler()
 	}
-	fetcher.Setup(ctl)
+	fetcher := doDownloadReady(f, httpListener, 4, t)
 	err := fetcher.Start()
 	if err != nil {
 		t.Fatal(err)

internal/protocol/http/stop_prefetch_test.go

@@ -0,0 +1,56 @@
+package http
+
+import (
+	"bytes"
+	"fmt"
+	gohttp "net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/GopeedLab/gopeed/pkg/base"
+	"github.com/GopeedLab/gopeed/pkg/protocol/http"
+)
+
+// A stalled Resolve body must not keep the prefetch goroutine alive after
+// stopPrefetchAndCopyData returns, or it writes to a file we already removed.
+func TestFetcher_StopPrefetchUnblocksStalledRead(t *testing.T) {
+	prefix := bytes.Repeat([]byte("p"), 64*1024)
+	stall := make(chan struct{})
+	defer close(stall)
+
+	server := httptest.NewServer(gohttp.HandlerFunc(func(w gohttp.ResponseWriter, r *gohttp.Request) {
+		w.Header().Set(base.HttpHeaderAcceptRanges, base.HttpHeaderBytes)
+		w.Header().Set(base.HttpHeaderContentLength, fmt.Sprintf("%d", len(prefix)*4))
+		w.WriteHeader(gohttp.StatusOK)
+		_, _ = w.Write(prefix)
+		w.(gohttp.Flusher).Flush()
+		select {
+		case <-stall:
+		case <-r.Context().Done():
+		}
+	}))
+	defer server.Close()
+
+	f := buildFetcher()
+	f.Meta().Opts = &base.Options{
+		Path:  t.TempDir(),
+		Name:  "stalled.data",
+		Extra: &http.OptsExtra{Connections: 1},
+	}
+	if err := f.Resolve(&base.Request{URL: server.URL + "/stalled.data"}); err != nil {
+		t.Fatal(err)
+	}
+	for f.prefetchSize.Load() < int64(len(prefix)) {
+		time.Sleep(time.Millisecond)
+	}
+
+	start := time.Now()
+	f.stopPrefetch()
+	if elapsed := time.Since(start); elapsed > 2*time.Second {
+		t.Fatalf("stopPrefetch took %v, want it to interrupt the stalled read", elapsed)
+	}
+	if !f.prefetchDone.Load() {
+		t.Fatal("prefetch goroutine still running after stopPrefetch")
+	}
+}