net/http: fix server/transport data race when sharing the request body

Introduced in https://go-review.googlesource.com/12865 (git rev c2db5f4c).

This fix doesn't add any new lock acquistions: it just moves the
existing one taken by the unreadDataSize method and moves it out
wider.

It became flaky at rev c2db5f4c, but now reliably passes again:
$ go test -v -race -run=TestTransportAndServerSharedBodyRace -count=100 net/http

Fixes #11985

Change-Id: I6956d62839fd7c37e2f7441b1d425793f4a0db30
Reviewed-on: https://go-review.googlesource.com/12909
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/src/net/http/transfer.go b/src/net/http/transfer.go
index d1762ebbd2..c128a1d3cd 100644
--- a/src/net/http/transfer.go
+++ b/src/net/http/transfer.go
@@ -737,11 +737,10 @@ func mergeSetHeader(dst *Header, src Header) {
 	}
 }
 
-// unreadDataSize returns the number of bytes of unread input.
+// unreadDataSizeLocked returns the number of bytes of unread input.
 // It returns -1 if unknown.
-func (b *body) unreadDataSize() int64 {
-	b.mu.Lock()
-	defer b.mu.Unlock()
+// b.mu must be held.
+func (b *body) unreadDataSizeLocked() int64 {
 	if lr, ok := b.src.(*io.LimitedReader); ok {
 		return lr.N
 	}