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, 49 insertions, 0 deletions

diff --git a/utils/sync/bytes_test.go b/utils/sync/bytes_test.go
new file mode 100644
index 0000000..b233429
--- /dev/null
+++ b/utils/sync/bytes_test.go
@@ -0,0 +1,49 @@
+package sync
+
+import (
+	"testing"
+)
+
+func TestGetAndPutBytesBuffer(t *testing.T) {
+	buf := GetBytesBuffer()
+	if buf == nil {
+		t.Error("nil was not expected")
+	}
+
+	initialLen := buf.Len()
+	buf.Grow(initialLen * 2)
+	grownLen := buf.Len()
+
+	PutBytesBuffer(buf)
+
+	buf = GetBytesBuffer()
+	if buf.Len() != grownLen {
+		t.Error("bytes buffer was not reused")
+	}
+
+	buf2 := GetBytesBuffer()
+	if buf2.Len() != initialLen {
+		t.Errorf("new bytes buffer length: wanted %d got %d", initialLen, buf2.Len())
+	}
+}
+
+func TestGetAndPutByteSlice(t *testing.T) {
+	slice := GetByteSlice()
+	if slice == nil {
+		t.Error("nil was not expected")
+	}
+
+	wanted := 16 * 1024
+	got := len(*slice)
+	if wanted != got {
+		t.Errorf("byte slice length: wanted %d got %d", wanted, got)
+	}
+
+	newByteSlice := make([]byte, wanted*2)
+	PutByteSlice(&newByteSlice)
+
+	newSlice := GetByteSlice()
+	if len(*newSlice) != len(newByteSlice) {
+		t.Error("byte slice was not reused")
+	}
+}