net/http: document concurrency expectations for Request.Body

This is primarily aimed at client requests where the user can supply
their own io.ReadCloser, but also clarifies server request behavior.
A server request body can be one of:

- *body
- *http2RequestBody
- *expectContinueReader
- *maxBytesReader

Of those, *expectContinueReader did not meet these expectations, so this
change also removes the data race.

Change-Id: Id4f1ae573d938347b1123a7b612b271aabb045a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/251087
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/net/http/request.go

@@ -175,6 +175,10 @@ type Request struct {
 	// but will return EOF immediately when no body is present.
 	// The Server will close the request body. The ServeHTTP
 	// Handler does not need to.
+	//
+	// Body must allow Read to be called concurrently with Close.
+	// In particular, calling Close should unblock a Read waiting
+	// for input.
 	Body io.ReadCloser
 
 	// GetBody defines an optional func to return a new copy of

src/net/http/server.go

@@ -890,12 +890,12 @@ func (srv *Server) initialReadLimitSize() int64 {
 type expectContinueReader struct {
 	resp       *response
 	readCloser io.ReadCloser
-	closed     bool
+	closed     atomicBool
 	sawEOF     atomicBool
 }
 
 func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
-	if ecr.closed {
+	if ecr.closed.isSet() {
 		return 0, ErrBodyReadAfterClose
 	}
 	w := ecr.resp
@@ -917,7 +917,7 @@ func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
 }
 
 func (ecr *expectContinueReader) Close() error {
-	ecr.closed = true
+	ecr.closed.setTrue()
 	return ecr.readCloser.Close()
 }