Fix data race in expandConnections: remove orphaned waitForCompletion goroutine

The go f.waitForCompletion() goroutine in expandConnections was redundant
since the downloadLoop's slow start loop already handles stateSteady by
calling waitForCompletion directly. The orphaned goroutine could race with
doStart() when writing f.ctx during pause/resume.

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

internal/protocol/http/fetcher.go

@@ -872,9 +872,9 @@ func (f *Fetcher) expandConnections() {
 	f.connMu.Unlock()
 
 	if len(newConns) == 0 {
-		// No new connections could be created, stop expansion
+		// No new connections could be created, stop expansion.
+		// The downloadLoop will check stateSteady and call waitForCompletion.
 		f.setState(stateSteady)
-		go f.waitForCompletion()
 		return
 	}
 
@@ -1195,9 +1195,9 @@ func (f *Fetcher) runConnectionWithResolveResp(conn *connection) {
 		if n > 0 {
 			// For non-range downloads with known size, enforce size limit
 			if !f.meta.Res.Range && f.meta.Res.Size > 0 {
+				f.connMu.Lock()
 				remaining := f.meta.Res.Size - conn.Downloaded
 				if remaining <= 0 {
-					f.connMu.Lock()
 					conn.Completed = true
 					conn.State = connCompleted
 					f.connMu.Unlock()
@@ -1206,6 +1206,7 @@ func (f *Fetcher) runConnectionWithResolveResp(conn *connection) {
 				if remaining < int64(n) {
 					n = int(remaining)
 				}
+				f.connMu.Unlock()
 			}
 
 			f.fileMu.Lock()
@@ -1225,17 +1226,18 @@ func (f *Fetcher) runConnectionWithResolveResp(conn *connection) {
 			}
 			f.fileMu.Unlock()
 
+			f.connMu.Lock()
 			conn.Chunk.Downloaded += int64(n)
 			conn.Downloaded += int64(n)
 
 			// Check if size limit reached after updating counters
 			if !f.meta.Res.Range && f.meta.Res.Size > 0 && conn.Downloaded >= f.meta.Res.Size {
-				f.connMu.Lock()
 				conn.Completed = true
 				conn.State = connCompleted
 				f.connMu.Unlock()
 				return
 			}
+			f.connMu.Unlock()
 		}
 		if err != nil {
 			if err == io.EOF {