net/http: improve performance for parsePostForm

Remove the use of io.ReadAll in http.parsePostForm to avoid converting
the whole input from []byte to string and not performing well
space-allocated-wise.

Instead a new function called parsePostFormURLEncoded is used and is
fed directly an io.Reader that is parsed using a bufio.Reader.

Benchmark:

name         old time/op    new time/op    delta
PostQuery-4    2.90µs ± 6%    2.82µs ± 4%     ~       (p=0.094 n=9+9)

name         old alloc/op   new alloc/op   delta
PostQuery-4    1.05kB ± 0%    0.90kB ± 0%  -14.49%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
PostQuery-4      6.00 ± 0%      7.00 ± 0%  +16.67%  (p=0.000 n=10+10)

Fixes #14655

Change-Id: I112c263d4221d959ed6153cfe88bc57a2aa8ea73
Reviewed-on: https://go-review.googlesource.com/20301
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/request.go

@@ -1015,18 +1015,8 @@ func parsePostForm(r *Request) (vs url.Values, err error) {
 			maxFormSize = int64(10 << 20) // 10 MB is a lot of text.
 			reader = io.LimitReader(r.Body, maxFormSize+1)
 		}
-		b, e := ioutil.ReadAll(reader)
-		if e != nil {
-			if err == nil {
-				err = e
-			}
-			break
-		}
-		if int64(len(b)) > maxFormSize {
-			err = errors.New("http: POST too large")
-			return
-		}
-		vs, e = url.ParseQuery(string(b))
+		vs = make(url.Values)
+		e := parsePostFormURLEncoded(vs, reader, maxFormSize)
 		if err == nil {
 			err = e
 		}
@@ -1041,6 +1031,52 @@ func parsePostForm(r *Request) (vs url.Values, err error) {
 	return
 }
 
+// parsePostFormURLEncoded reads from r, the reader of a POST form to populate vs which is a url-type values.
+// maxFormSize indicates the maximum number of bytes that will be read from r.
+func parsePostFormURLEncoded(vs url.Values, r io.Reader, maxFormSize int64) error {
+	br := newBufioReader(r)
+	defer putBufioReader(br)
+
+	var readSize int64
+	for {
+		// Read next "key=value&" or "justkey&".
+		// If this is the last pair, b will contain just "key=value" or "justkey".
+		b, err := br.ReadBytes('&')
+		if err != nil && err != io.EOF && err != bufio.ErrBufferFull {
+			return err
+		}
+		isEOF := err == io.EOF
+		readSize += int64(len(b))
+		if readSize >= maxFormSize {
+			return errors.New("http: POST too large")
+		}
+
+		// Remove last delimiter
+		if len(b) > 0 && b[len(b)-1] == '&' {
+			b = b[:len(b)-1]
+		}
+
+		// Parse key and value
+		k := string(b)
+		var v string
+		if i := strings.Index(k, "="); i > -1 {
+			k, v = k[:i], k[i+1:]
+		}
+		if k, err = url.QueryUnescape(k); err != nil {
+			return err
+		}
+		if v, err = url.QueryUnescape(v); err != nil {
+			return err
+		}
+
+		// Populate vs
+		vs[k] = append(vs[k], v)
+		if isEOF {
+			return nil
+		}
+	}
+}
+
 // ParseForm parses the raw query from the URL and updates r.Form.
 //
 // For POST or PUT requests, it also parses the request body as a form and