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, 51 insertions, 0 deletions

diff --git a/utils/sync/bytes.go b/utils/sync/bytes.go
new file mode 100644
index 0000000..dd06fc0
--- /dev/null
+++ b/utils/sync/bytes.go
@@ -0,0 +1,51 @@
+package sync
+
+import (
+	"bytes"
+	"sync"
+)
+
+var (
+	byteSlice = sync.Pool{
+		New: func() interface{} {
+			b := make([]byte, 16*1024)
+			return &b
+		},
+	}
+	bytesBuffer = sync.Pool{
+		New: func() interface{} {
+			return bytes.NewBuffer(nil)
+		},
+	}
+)
+
+// GetByteSlice returns a *[]byte that is managed by a sync.Pool.
+// The initial slice length will be 16384 (16kb).
+//
+// After use, the *[]byte should be put back into the sync.Pool
+// by calling PutByteSlice.
+func GetByteSlice() *[]byte {
+	buf := byteSlice.Get().(*[]byte)
+	return buf
+}
+
+// PutByteSlice puts buf back into its sync.Pool.
+func PutByteSlice(buf *[]byte) {
+	byteSlice.Put(buf)
+}
+
+// GetBytesBuffer returns a *bytes.Buffer that is managed by a sync.Pool.
+// Returns a buffer that is resetted and ready for use.
+//
+// After use, the *bytes.Buffer should be put back into the sync.Pool
+// by calling PutBytesBuffer.
+func GetBytesBuffer() *bytes.Buffer {
+	buf := bytesBuffer.Get().(*bytes.Buffer)
+	buf.Reset()
+	return buf
+}
+
+// PutBytesBuffer puts buf back into its sync.Pool.
+func PutBytesBuffer(buf *bytes.Buffer) {
+	bytesBuffer.Put(buf)
+}