net/http: ignore connection closes once done with the connection

Once the connection is put back into the idle pool, the request should
not take any action if the connection is closed.

Fixes #41600

Change-Id: I5e4ddcdc03cd44f5197ecfbe324638604961de84
Reviewed-on: https://go-review.googlesource.com/c/go/+/257818
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Damien Neil <dneil@google.com>

src/net/http/transport.go

@@ -2599,6 +2599,7 @@ func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err err
 	var respHeaderTimer <-chan time.Time
 	cancelChan := req.Request.Cancel
 	ctxDoneChan := req.Context().Done()
+	pcClosed := pc.closech
 	for {
 		testHookWaitResLoop()
 		select {
@@ -2618,11 +2619,15 @@ func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err err
 				defer timer.Stop() // prevent leaks
 				respHeaderTimer = timer.C
 			}
-		case <-pc.closech:
-			if debugRoundTrip {
-				req.logf("closech recv: %T %#v", pc.closed, pc.closed)
+		case <-pcClosed:
+			pcClosed = nil
+			// check if we are still using the connection
+			if pc.t.replaceReqCanceler(req.cancelKey, nil) {
+				if debugRoundTrip {
+					req.logf("closech recv: %T %#v", pc.closed, pc.closed)
+				}
+				return nil, pc.mapRoundTripError(req, startBytesWritten, pc.closed)
 			}
-			return nil, pc.mapRoundTripError(req, startBytesWritten, pc.closed)
 		case <-respHeaderTimer:
 			if debugRoundTrip {
 				req.logf("timeout waiting for response headers.")

src/net/http/transport_test.go

@@ -6433,3 +6433,54 @@ func TestErrorWriteLoopRace(t *testing.T) {
 		testTransportRace(req)
 	}
 }
+
+// Issue 41600
+// Test that a new request which uses the connection of an active request
+// cannot cause it to be canceled as well.
+func TestCancelRequestWhenSharingConnection(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, req *Request) {
+		w.Header().Add("Content-Length", "0")
+	}))
+	defer ts.Close()
+
+	client := ts.Client()
+	transport := client.Transport.(*Transport)
+	transport.MaxIdleConns = 1
+	transport.MaxConnsPerHost = 1
+
+	var wg sync.WaitGroup
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for ctx.Err() == nil {
+				reqctx, reqcancel := context.WithCancel(ctx)
+				go reqcancel()
+				req, _ := NewRequestWithContext(reqctx, "GET", ts.URL, nil)
+				res, err := client.Do(req)
+				if err == nil {
+					res.Body.Close()
+				}
+			}
+		}()
+	}
+
+	for ctx.Err() == nil {
+		req, _ := NewRequest("GET", ts.URL, nil)
+		if res, err := client.Do(req); err != nil {
+			t.Errorf("unexpected: %p %v", req, err)
+			break
+		} else {
+			res.Body.Close()
+		}
+	}
+
+	cancel()
+	wg.Wait()
+}