net/http: optimize request body writes

net.TCPConn has the ability to send data out using system calls such as
sendfile when the source data comes from an *os.File. However, the way
that I/O has been laid out in the transport means that the File is
actually wrapped behind two outer io.Readers, and as such the TCP stack
cannot properly type-assert the reader, ensuring that it falls back to
genericReadFrom. Or it would, if persistConnWriter implemented
io.ReaderFrom, which is missing as well.

As such, this commit does the following:

* Removes transferBodyReader and moves its functionality to a new
doBodyCopy helper. This is not an io.Reader implementation, but no
functionality is lost this way, and it allows us to unwrap one layer
from the body.

* The second layer of the body is unwrapped if the original writer
was wrapped with ioutil.NopCloser, which is what NewRequest wraps the
body in if it's not a ReadCloser on its own. The unwrap operation
passes through the existing body if there's no nopCloser.

* Finally, io.ReaderFrom is implemented for persistConnWriter in the
higher-level transport, which was preventing ReadFrom in net.TCPConn
from being used in the first place. This has the additional benefit of
facilitating the fallback to genericReadFrom if there's still no
*os.File, which by itself was giving significant performance gains over
the io.Writer implementation.

Fixes #30377.

Author: vancluever

src/net/http/transfer.go

@@ -53,19 +53,6 @@ func (br *byteReader) Read(p []byte) (n int, err error) {
 	return 1, io.EOF
 }
 
-// transferBodyReader is an io.Reader that reads from tw.Body
-// and records any non-EOF error in tw.bodyReadError.
-// It is exactly 1 pointer wide to avoid allocations into interfaces.
-type transferBodyReader struct{ tw *transferWriter }
-
-func (br transferBodyReader) Read(p []byte) (n int, err error) {
-	n, err = br.tw.Body.Read(p)
-	if err != nil && err != io.EOF {
-		br.tw.bodyReadError = err
-	}
-	return
-}
-
 // transferWriter inspects the fields of a user-supplied Request or Response,
 // sanitizes them without changing the user object and provides methods for
 // writing the respective header, body and trailer in wire format.
@@ -347,15 +334,18 @@ func (t *transferWriter) writeBody(w io.Writer) error {
 	var err error
 	var ncopy int64
 
-	// Write body
+	// Write body. We "unwrap" the body first if it was wrapped in a
+	// nopCloser. This is to ensure that we can take advantage of
+	// OS-level optimizations in the event that the body is an
+	// *os.File.
 	if t.Body != nil {
-		var body = transferBodyReader{t}
+		var body = t.unwrapBody()
 		if chunked(t.TransferEncoding) {
 			if bw, ok := w.(*bufio.Writer); ok && !t.IsResponse {
 				w = &internal.FlushAfterChunkWriter{Writer: bw}
 			}
 			cw := internal.NewChunkedWriter(w)
-			_, err = io.Copy(cw, body)
+			_, err = t.doBodyCopy(cw, body)
 			if err == nil {
 				err = cw.Close()
 			}
@@ -364,14 +354,14 @@ func (t *transferWriter) writeBody(w io.Writer) error {
 			if t.Method == "CONNECT" {
 				dst = bufioFlushWriter{dst}
 			}
-			ncopy, err = io.Copy(dst, body)
+			ncopy, err = t.doBodyCopy(dst, body)
 		} else {
-			ncopy, err = io.Copy(w, io.LimitReader(body, t.ContentLength))
+			ncopy, err = t.doBodyCopy(w, io.LimitReader(body, t.ContentLength))
 			if err != nil {
 				return err
 			}
 			var nextra int64
-			nextra, err = io.Copy(ioutil.Discard, body)
+			nextra, err = t.doBodyCopy(ioutil.Discard, body)
 			ncopy += nextra
 		}
 		if err != nil {
@@ -402,6 +392,31 @@ func (t *transferWriter) writeBody(w io.Writer) error {
 	return err
 }
 
+// doBodyCopy wraps a copy operation, with any resulting error also
+// being saved in bodyReadError.
+//
+// This function is only intended for use in writeBody.
+func (t *transferWriter) doBodyCopy(dst io.Writer, src io.Reader) (n int64, err error) {
+	n, err = io.Copy(dst, src)
+	if err != nil && err != io.EOF {
+		t.bodyReadError = err
+	}
+	return
+}
+
+// unwrapBodyReader unwraps the body's inner reader if it's a
+// nopCloser. This is to ensure that body writes sourced from local
+// files (*os.File types) are properly optimized.
+//
+// This function is only intended for use in writeBody.
+func (t *transferWriter) unwrapBody() io.Reader {
+	if reflect.TypeOf(t.Body) == nopCloserType {
+		return reflect.ValueOf(t.Body).Field(0).Interface().(io.Reader)
+	}
+
+	return t.Body
+}
+
 type transferReader struct {
 	// Input
 	Header        Header

src/net/http/transport.go

@@ -1375,6 +1375,12 @@ func (w persistConnWriter) Write(p []byte) (n int, err error) {
 	return
 }
 
+func (w persistConnWriter) ReadFrom(r io.Reader) (n int64, err error) {
+	n, err = io.Copy(w.pc.conn, r)
+	w.pc.nwrite += n
+	return
+}
+
 // connectMethod is the map key (in its String form) for keeping persistent
 // TCP connections alive for subsequent HTTP requests.
 //