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_test.go b/utils/sync/zlib_test.go
new file mode 100644
index 0000000..b736fb2
--- /dev/null
+++ b/utils/sync/zlib_test.go
@@ -0,0 +1,74 @@
+package sync
+
+import (
+	"bytes"
+	"compress/zlib"
+	"io"
+	"testing"
+)
+
+func TestGetAndPutZlibReader(t *testing.T) {
+	_, err := GetZlibReader(bytes.NewReader(zlibInitBytes))
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	dict := &[]byte{}
+	reader := FakeZLibReader{}
+	PutZlibReader(ZLibReader{dict: dict, Reader: &reader})
+
+	if !reader.wasClosed {
+		t.Errorf("reader was not closed")
+	}
+
+	z2, err := GetZlibReader(bytes.NewReader(zlibInitBytes))
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	if dict != z2.dict {
+		t.Errorf("zlib dictionary was not reused")
+	}
+
+	if &reader != z2.Reader {
+		t.Errorf("zlib reader was not reused")
+	}
+
+	if !reader.wasReset {
+		t.Errorf("reader was not reset")
+	}
+}
+
+func TestGetAndPutZlibWriter(t *testing.T) {
+	w := GetZlibWriter(nil)
+	if w == nil {
+		t.Errorf("nil was not expected")
+	}
+
+	newW := zlib.NewWriter(nil)
+	PutZlibWriter(newW)
+
+	w2 := GetZlibWriter(nil)
+	if w2 != newW {
+		t.Errorf("zlib writer was not reused")
+	}
+}
+
+type FakeZLibReader struct {
+	wasClosed bool
+	wasReset  bool
+}
+
+func (f *FakeZLibReader) Reset(r io.Reader, dict []byte) error {
+	f.wasReset = true
+	return nil
+}
+
+func (f *FakeZLibReader) Read(p []byte) (n int, err error) {
+	return 0, nil
+}
+
+func (f *FakeZLibReader) Close() error {
+	f.wasClosed = true
+	return nil
+}