fix(http): copy a fully prefetched body when Start runs before prefetchDone (#1608)

The prefetch goroutine sets prefetchDone only after one more read and
closing the body. A Start in that window took the whole file as the
fallback snapshot, expandConnections marked the task done, and the temp
file was never copied, leaving a zero-filled target.

Read prefetchSize once and treat the prefetch as complete when it reached
the known size.

internal/protocol/http/fetcher.go

@@ -872,12 +872,16 @@ func (f *Fetcher) doStart() error {
 	// For non-range resources, the response will be used directly
 	var prefetchedBytes int64
 	if f.meta.Res.Range {
-		resolveComplete := f.knownSize() > 0 && f.prefetchDone.Load() && f.prefetchErr == nil && f.prefetchSize.Load() >= f.knownSize()
+		// Read the size once and judge completeness by it alone: Resolve may have
+		// written every byte before it sets prefetchDone, and a snapshot that
+		// covers the whole file would otherwise finish without copying it.
+		prefetched := f.prefetchSize.Load()
+		resolveComplete := f.knownSize() > 0 && prefetched >= f.knownSize()
 		if f.resolveFallback.Load() && !resolveComplete {
 			// Keep Resolve downloading until the first Range response has been
 			// validated. The Range request starts at a stable snapshot; a valid
 			// takeover discards any overlap downloaded by Resolve in the meantime.
-			prefetchedBytes = f.prefetchSize.Load()
+			prefetchedBytes = prefetched
 		} else {
 			// Resolve already completed, or no live Resolve fallback remains.
 			prefetchedBytes = f.stopPrefetchAndCopyData()

internal/protocol/http/prefetch_start_test.go

@@ -0,0 +1,52 @@
+package http
+
+import (
+	"bytes"
+	gohttp "net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/GopeedLab/gopeed/pkg/base"
+	"github.com/GopeedLab/gopeed/pkg/protocol/http"
+)
+
+// Start can run after the Resolve prefetch wrote every byte but before it set
+// prefetchDone. The prefetched body must still be copied to the target.
+func TestFetcher_StartBeforePrefetchDoneCopiesFullBody(t *testing.T) {
+	payload := []byte("prefetched before Start")
+	server := httptest.NewServer(gohttp.HandlerFunc(func(w gohttp.ResponseWriter, r *gohttp.Request) {
+		gohttp.ServeContent(w, r, "f.txt", time.Time{}, bytes.NewReader(payload))
+	}))
+	defer server.Close()
+
+	f := buildFetcher()
+	f.Meta().Opts = &base.Options{Path: t.TempDir(), Name: "f.txt", Extra: &http.OptsExtra{Connections: 1}}
+	if err := f.Resolve(&base.Request{URL: server.URL + "/f.txt"}); err != nil {
+		t.Fatal(err)
+	}
+	for !f.prefetchDone.Load() {
+		time.Sleep(time.Millisecond)
+	}
+	// Reopen the window between the last write and prefetchDone.
+	f.prefetchDone.Store(false)
+	go func() {
+		time.Sleep(50 * time.Millisecond)
+		f.prefetchDone.Store(true)
+	}()
+
+	if err := f.Start(); err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Wait(); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(f.meta.SingleFilepath())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(got, payload) {
+		t.Fatalf("content = %q, want %q", got, payload)
+	}
+}