From 64b1cbc28945f193a604ca1f52825f43b8f52041 Mon Sep 17 00:00:00 2001
From: Vincent Batts <vbatts@redhat.com>
Date: Wed, 19 Nov 2014 15:46:03 -0500
Subject: [PATCH 1/2] pkg/tarsum: actually init the TarSum struct

closes #9241

Signed-off-by: Vincent Batts <vbatts@redhat.com>
---
 tarsum/tarsum.go | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/tarsum/tarsum.go b/tarsum/tarsum.go
index 34386ff..ba09d4a 100644
--- a/tarsum/tarsum.go
+++ b/tarsum/tarsum.go
@@ -27,11 +27,7 @@ const (
 // including the byte payload of the image's json metadata as well, and for
 // calculating the checksums for buildcache.
 func NewTarSum(r io.Reader, dc bool, v Version) (TarSum, error) {
-	headerSelector, err := getTarHeaderSelector(v)
-	if err != nil {
-		return nil, err
-	}
-	return &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v, headerSelector: headerSelector}, nil
+	return NewTarSumHash(r, dc, v, DefaultTHash)
 }
 
 // Create a new TarSum, providing a THash to use rather than the DefaultTHash
@@ -40,7 +36,9 @@ func NewTarSumHash(r io.Reader, dc bool, v Version, tHash THash) (TarSum, error)
 	if err != nil {
 		return nil, err
 	}
-	return &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v, headerSelector: headerSelector, tHash: tHash}, nil
+	ts := &tarSum{Reader: r, DisableCompression: dc, tarSumVersion: v, headerSelector: headerSelector, tHash: tHash}
+	err = ts.initTarSum()
+	return ts, err
 }
 
 // TarSum is the generic interface for calculating fixed time
@@ -134,12 +132,6 @@ func (ts *tarSum) initTarSum() error {
 }
 
 func (ts *tarSum) Read(buf []byte) (int, error) {
-	if ts.writer == nil {
-		if err := ts.initTarSum(); err != nil {
-			return 0, err
-		}
-	}
-
 	if ts.finished {
 		return ts.bufWriter.Read(buf)
 	}

From c91f8d09a5d5f7bfc47a402e66c926bb546c57b6 Mon Sep 17 00:00:00 2001
From: Josh Hawn <josh.hawn@docker.com>
Date: Wed, 19 Nov 2014 12:15:20 -0800
Subject: [PATCH 2/2] Add unit test for tarSum.Sum() with no data

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
---
 tarsum/tarsum_test.go | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tarsum/tarsum_test.go b/tarsum/tarsum_test.go
index 1e06cda..c5dca6a 100644
--- a/tarsum/tarsum_test.go
+++ b/tarsum/tarsum_test.go
@@ -230,6 +230,17 @@ func TestEmptyTar(t *testing.T) {
 	if resultSum != expectedSum {
 		t.Fatalf("expected [%s] but got [%s]", expectedSum, resultSum)
 	}
+
+	// Test without ever actually writing anything.
+	if ts, err = NewTarSum(bytes.NewReader([]byte{}), true, Version0); err != nil {
+		t.Fatal(err)
+	}
+
+	resultSum = ts.Sum(nil)
+
+	if resultSum != expectedSum {
+		t.Fatalf("expected [%s] but got [%s]", expectedSum, resultSum)
+	}
 }
 
 var (