net/http: pool transport gzip readers

goos: linux
goarch: amd64
pkg: net/http
             │   HEAD~1    │              HEAD              │
             │   sec/op    │    sec/op     vs base          │
ClientGzip-8   621.0µ ± 2%   616.3µ ± 10%  ~ (p=0.971 n=10)

             │    HEAD~1     │                 HEAD                 │
             │     B/op      │     B/op      vs base                │
ClientGzip-8   49.765Ki ± 0%   9.514Ki ± 2%  -80.88% (p=0.000 n=10)

             │   HEAD~1   │               HEAD                │
             │ allocs/op  │ allocs/op   vs base               │
ClientGzip-8   57.00 ± 0%   52.00 ± 0%  -8.77% (p=0.000 n=10)

Allocation saving comes from absent compress/flate.(*dictDecoder).init

Updates #61353

src/net/http/transport.go

@@ -11,6 +11,7 @@ package http
 
 import (
 	"bufio"
+	"compress/flate"
 	"compress/gzip"
 	"container/list"
 	"context"
@@ -2976,6 +2977,7 @@ type bodyEOFSignal struct {
 }
 
 var errReadOnClosedResBody = errors.New("http: read on closed response body")
+var errConcurrentReadOnResBody = errors.New("http: concurrent read on response body")
 
 func (es *bodyEOFSignal) Read(p []byte) (n int, err error) {
 	es.mu.Lock()
@@ -3025,37 +3027,98 @@ func (es *bodyEOFSignal) condfn(err error) error {
 }
 
 // gzipReader wraps a response body so it can lazily
-// call gzip.NewReader on the first call to Read
+// get gzip.Reader from the pool on the first call to Read.
+// After Close is called it puts gzip.Reader to the pool immediately
+// if there is no Read in progress or later when Read completes.
 type gzipReader struct {
 	_    incomparable
 	body *bodyEOFSignal // underlying HTTP/1 response body framing
-	zr   *gzip.Reader   // lazily-initialized gzip reader
-	zerr error          // any error from gzip.NewReader; sticky
+	mu   sync.Mutex     // guards zr and zerr
+	zr   *gzip.Reader
+	zerr error
+}
+
+type eofReader struct{}
+
+func (eofReader) Read([]byte) (int, error) { return 0, io.EOF }
+func (eofReader) ReadByte() (byte, error)  { return 0, io.EOF }
+
+var gzipPool = sync.Pool{New: func() any { return new(gzip.Reader) }}
+
+// gzipPoolGet gets a gzip.Reader from the pool and resets it to read from r.
+func gzipPoolGet(r io.Reader) (*gzip.Reader, error) {
+	zr := gzipPool.Get().(*gzip.Reader)
+	if err := zr.Reset(r); err != nil {
+		gzipPoolPut(zr)
+		return nil, err
+	}
+	return zr, nil
+}
+
+// gzipPoolPut puts a gzip.Reader back into the pool.
+func gzipPoolPut(zr *gzip.Reader) {
+	// Reset will allocate bufio.Reader if we pass it anything
+	// other than a flate.Reader, so ensure that it's getting one.
+	var r flate.Reader = eofReader{}
+	zr.Reset(r)
+	gzipPool.Put(zr)
+}
+
+// acquire returns a gzip.Reader for reading response body.
+// The reader must be released after use.
+func (gz *gzipReader) acquire() (*gzip.Reader, error) {
+	gz.mu.Lock()
+	defer gz.mu.Unlock()
+	if gz.zerr != nil {
+		return nil, gz.zerr
+	}
+	if gz.zr == nil {
+		gz.zr, gz.zerr = gzipPoolGet(gz.body)
+		if gz.zerr != nil {
+			return nil, gz.zerr
+		}
+	}
+	ret := gz.zr
+	gz.zr, gz.zerr = nil, errConcurrentReadOnResBody
+	return ret, nil
+}
+
+// release returns the gzip.Reader to the pool if Close was called during Read.
+func (gz *gzipReader) release(zr *gzip.Reader) {
+	gz.mu.Lock()
+	defer gz.mu.Unlock()
+	if gz.zerr == errConcurrentReadOnResBody {
+		gz.zr, gz.zerr = zr, nil
+	} else { // errReadOnClosedResBody
+		gzipPoolPut(zr)
+	}
+}
+
+// close returns the gzip.Reader to the pool immediately or
+// signals release to do so after Read completes.
+func (gz *gzipReader) close() {
+	gz.mu.Lock()
+	defer gz.mu.Unlock()
+	if gz.zerr == nil && gz.zr != nil {
+		gzipPoolPut(gz.zr)
+		gz.zr = nil
+	}
+	gz.zerr = errReadOnClosedResBody
 }
 
 func (gz *gzipReader) Read(p []byte) (n int, err error) {
-	if gz.zr == nil {
-		if gz.zerr == nil {
-			gz.zr, gz.zerr = gzip.NewReader(gz.body)
-		}
-		if gz.zerr != nil {
-			return 0, gz.zerr
-		}
-	}
-
-	gz.body.mu.Lock()
-	if gz.body.closed {
-		err = errReadOnClosedResBody
-	}
-	gz.body.mu.Unlock()
-
+	zr, err := gz.acquire()
 	if err != nil {
 		return 0, err
 	}
-	return gz.zr.Read(p)
+	defer gz.release(zr)
+
+	return zr.Read(p)
 }
 
 func (gz *gzipReader) Close() error {
+	gz.close()
+
 	return gz.body.Close()
 }