http2: pool transport gzip readers

This is a copy of the CL 510255 with the difference
that it preserves use of fs.ErrClosed when reading after close.

goos: darwin
goarch: arm64
pkg: golang.org/x/net/http2
cpu: Apple M4
              │   HEAD~1    │             HEAD              │
              │   sec/op    │   sec/op     vs base          │
ClientGzip-10   752.8µ ± 1%   750.7µ ± 2%  ~ (p=0.393 n=10)

              │    HEAD~1    │                 HEAD                 │
              │     B/op     │     B/op      vs base                │
ClientGzip-10   75.49Ki ± 0%   33.98Ki ± 3%  -54.99% (p=0.000 n=10)

              │   HEAD~1   │               HEAD                │
              │ allocs/op  │ allocs/op   vs base               │
ClientGzip-10   705.0 ± 0%   698.5 ± 0%  -0.92% (p=0.000 n=10)

Updates golang/go#61353

http2/transport.go

@@ -9,6 +9,7 @@ package http2
 import (
 	"bufio"
 	"bytes"
+	"compress/flate"
 	"compress/gzip"
 	"context"
 	"crypto/rand"
@@ -3076,35 +3077,102 @@ type erringRoundTripper struct{ err error }
 func (rt erringRoundTripper) RoundTripErr() error                             { return rt.err }
 func (rt erringRoundTripper) RoundTrip(*http.Request) (*http.Response, error) { return nil, rt.err }
 
+var errConcurrentReadOnResBody = errors.New("http2: concurrent read on response body")
+
 // 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 io.ReadCloser // underlying Response.Body
-	zr   *gzip.Reader  // lazily-initialized gzip reader
-	zerr error         // sticky error
+	mu   sync.Mutex    // guards zr and zerr
+	zr   *gzip.Reader  // stores gzip reader from the pool between reads
+	zerr error         // sticky gzip reader init error or sentinel value to detect concurrent read and read after close
+}
+
+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 { // fs.ErrClosed
+		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 = fs.ErrClosed
 }
 
 func (gz *gzipReader) Read(p []byte) (n int, err error) {
-	if gz.zerr != nil {
-		return 0, gz.zerr
+	zr, err := gz.acquire()
+	if err != nil {
+		return 0, err
 	}
-	if gz.zr == nil {
-		gz.zr, err = gzip.NewReader(gz.body)
-		if err != nil {
-			gz.zerr = err
-			return 0, err
-		}
-	}
-	return gz.zr.Read(p)
+	defer gz.release(zr)
+
+	return zr.Read(p)
 }
 
 func (gz *gzipReader) Close() error {
-	if err := gz.body.Close(); err != nil {
-		return err
-	}
-	gz.zerr = fs.ErrClosed
-	return nil
+	gz.close()
+
+	return gz.body.Close()
 }
 
 type errorReader struct{ err error }