Optimize zlib reader and consolidate sync.pools

Expands on the optimisations from https://github.com/fluxcd/go-git/pull/5
and ensures that zlib reader does not need to recreate a deflate
dictionary at every use.

The use of sync pools was consolidated into a new sync utils package.

name       old time/op    new time/op    delta
Parser-16    7.51ms ± 3%    7.71ms ± 6%     ~     (p=0.222 n=5+5)

name       old alloc/op   new alloc/op   delta
Parser-16    4.65MB ± 3%    1.90MB ± 3%  -59.06%  (p=0.008 n=5+5)

name       old allocs/op  new allocs/op  delta
Parser-16     3.48k ± 0%     3.32k ± 0%   -4.57%  (p=0.016 n=5+4)

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

1 files changed, 74 insertions, 0 deletions

diff --git a/utils/sync/zlib.go b/utils/sync/zlib.go
new file mode 100644
index 0000000..c613885
--- /dev/null
+++ b/utils/sync/zlib.go
@@ -0,0 +1,74 @@
+package sync
+
+import (
+	"bytes"
+	"compress/zlib"
+	"io"
+	"sync"
+)
+
+var (
+	zlibInitBytes = []byte{0x78, 0x9c, 0x01, 0x00, 0x00, 0xff, 0xff, 0x00, 0x00, 0x00, 0x01}
+	zlibReader    = sync.Pool{
+		New: func() interface{} {
+			r, _ := zlib.NewReader(bytes.NewReader(zlibInitBytes))
+			return ZLibReader{
+				Reader: r.(zlibReadCloser),
+			}
+		},
+	}
+	zlibWriter = sync.Pool{
+		New: func() interface{} {
+			return zlib.NewWriter(nil)
+		},
+	}
+)
+
+type zlibReadCloser interface {
+	io.ReadCloser
+	zlib.Resetter
+}
+
+type ZLibReader struct {
+	dict   *[]byte
+	Reader zlibReadCloser
+}
+
+// GetZlibReader returns a ZLibReader that is managed by a sync.Pool.
+// Returns a ZLibReader that is resetted using a dictionary that is
+// also managed by a sync.Pool.
+//
+// After use, the ZLibReader should be put back into the sync.Pool
+// by calling PutZlibReader.
+func GetZlibReader(r io.Reader) (ZLibReader, error) {
+	z := zlibReader.Get().(ZLibReader)
+	z.dict = GetByteSlice()
+
+	err := z.Reader.Reset(r, *z.dict)
+
+	return z, err
+}
+
+// PutZlibReader puts z back into its sync.Pool, first closing the reader.
+// The Byte slice dictionary is also put back into its sync.Pool.
+func PutZlibReader(z ZLibReader) {
+	z.Reader.Close()
+	PutByteSlice(z.dict)
+	zlibReader.Put(z)
+}
+
+// GetZlibWriter returns a *zlib.Writer that is managed by a sync.Pool.
+// Returns a writer that is resetted with w and ready for use.
+//
+// After use, the *zlib.Writer should be put back into the sync.Pool
+// by calling PutZlibWriter.
+func GetZlibWriter(w io.Writer) *zlib.Writer {
+	z := zlibWriter.Get().(*zlib.Writer)
+	z.Reset(w)
+	return z
+}
+
+// PutZlibWriter puts w back into its sync.Pool.
+func PutZlibWriter(w *zlib.Writer) {
+	zlibWriter.Put(w)
+}