Revert "io: allocate copy buffers from a pool"

This reverts CL 456555.

Reason for revert: This seems too likely to exercise race conditions
in code where a Write call continues to access its buffer after
returning. The HTTP/2 ResponseWriter is one such example.

Reverting this change while we think about this some more.

For #57202

Change-Id: Ic86823f81d7da410ea6b3f17fb5b3f9a979e3340
Reviewed-on: https://go-review.googlesource.com/c/go/+/467095
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 18 insertions, 12 deletions

diff --git a/src/io/io.go b/src/io/io.go
index 374e20bf8c..630ab73b56 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -400,13 +400,6 @@ func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	return copyBuffer(dst, src, buf)
 }
 
-var bufPool = sync.Pool{
-	New: func() any {
-		b := make([]byte, 32*1024)
-		return &b
-	},
-}
-
 // copyBuffer is the actual implementation of Copy and CopyBuffer.
 // if buf is nil, one is allocated.
 func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
@@ -420,9 +413,15 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		return rt.ReadFrom(src)
 	}
 	if buf == nil {
-		bufp := bufPool.Get().(*[]byte)
-		defer bufPool.Put(bufp)
-		buf = *bufp
+		size := 32 * 1024
+		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
+			if l.N < 1 {
+				size = 1
+			} else {
+				size = int(l.N)
+			}
+		}
+		buf = make([]byte, size)
 	}
 	for {
 		nr, er := src.Read(buf)
@@ -638,14 +637,21 @@ func (discard) WriteString(s string) (int, error) {
 	return len(s), nil
 }
 
+var blackHolePool = sync.Pool{
+	New: func() any {
+		b := make([]byte, 8192)
+		return &b
+	},
+}
+
 func (discard) ReadFrom(r Reader) (n int64, err error) {
-	bufp := bufPool.Get().(*[]byte)
+	bufp := blackHolePool.Get().(*[]byte)
 	readSize := 0
 	for {
 		readSize, err = r.Read(*bufp)
 		n += int64(readSize)
 		if err != nil {
-			bufPool.Put(bufp)
+			blackHolePool.Put(bufp)
 			if err == EOF {
 				return n, nil
 			}