[release-branch.go1.15] 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.

For #42935.
Updates #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>
(cherry picked from commit 212d385a2f723a8dd5e7d2e83efb478ddd139349)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297909
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

2 files changed, 60 insertions, 4 deletions

diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index d37b52b13d0..6fb2ea5663f 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -2560,6 +2560,7 @@ 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 {
@@ -2579,11 +2580,15 @@ 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.")
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index 0a47687d9af..3c7b9eb4de8 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -6289,3 +6289,54 @@ func TestTransportRejectsSignInContentLength(t *testing.T) {
 		t.Fatalf("Error mismatch\nGot: %q\nWanted substring: %q", got, want)
 	}
 }
+
+// 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()
+}