net/http: fix race

Fixes #4724

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7225071

diff --git a/src/pkg/net/http/transport.go b/src/pkg/net/http/transport.go
index 98e198e78af71bf4983c770a88c8ec10bd5cc20f..5cd3997a26f5cde8cb24c083db67414254968c1d 100644 (file)
--- a/src/pkg/net/http/transport.go
+++ b/src/pkg/net/http/transport.go
@@ -529,14 +529,13 @@ type persistConn struct {
        closech  chan struct{}       // broadcast close when readLoop (TCP connection) closes
        isProxy  bool
 
+       lk                   sync.Mutex // guards following 3 fields
+       numExpectedResponses int
+       broken               bool // an error has happened on this connection; marked broken so it's not reused.
        // mutateHeaderFunc is an optional func to modify extra
        // headers on each outbound request before it's written. (the
        // original Request given to RoundTrip is not modified)
        mutateHeaderFunc func(Header)
-
-       lk                   sync.Mutex // guards numExpectedResponses and broken
-       numExpectedResponses int
-       broken               bool // an error has happened on this connection; marked broken so it's not reused.
 }
 
 func (pc *persistConn) isBroken() bool {
@@ -711,8 +710,13 @@ type writeRequest struct {
 }
 
 func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
-       if pc.mutateHeaderFunc != nil {
-               pc.mutateHeaderFunc(req.extraHeaders())
+       pc.lk.Lock()
+       pc.numExpectedResponses++
+       headerFn := pc.mutateHeaderFunc
+       pc.lk.Unlock()
+
+       if headerFn != nil {
+               headerFn(req.extraHeaders())
        }
 
        // Ask for a compressed version if the caller didn't set their
@@ -728,10 +732,6 @@ func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err err
                req.extraHeaders().Set("Accept-Encoding", "gzip")
        }
 
-       pc.lk.Lock()
-       pc.numExpectedResponses++
-       pc.lk.Unlock()
-
        // Write the request concurrently with waiting for a response,
        // in case the server decides to reply before reading our full
        // request body.