code review comments

pkg/snapshot/layered_map.go

@@ -27,10 +27,11 @@ import (
 )
 
 type LayeredMap struct {
-	layers      []map[string]string
-	whiteouts   []map[string]string
-	added       []map[string]string
-	hasher      func(string) (string, error)
+	layers    []map[string]string
+	whiteouts []map[string]string
+	added     []map[string]string
+	hasher    func(string) (string, error)
+	// cacheHasher doesn't include mtime in it's hash so that filesystem cache keys are stable
 	cacheHasher func(string) (string, error)
 }
 

pkg/snapshot/layered_map_test.go

@@ -32,11 +32,15 @@ func Test_CacheKey(t *testing.T) {
 				"a": "apple",
 				"b": "bat",
 				"c": "cat",
+				"d": "dog",
+				"e": "egg",
 			},
 			map2: map[string]string{
 				"c": "cat",
+				"d": "dog",
 				"b": "bat",
 				"a": "apple",
+				"e": "egg",
 			},
 			equal: true,
 		},
@@ -67,11 +71,8 @@ func Test_CacheKey(t *testing.T) {
 			if err != nil {
 				t.Fatalf("error getting key for map 2: %v", err)
 			}
-			if test.equal && k1 != k2 {
-				t.Fatalf("keys differ.\nExpected\n%+v\nActual\n%+v", k1, k2)
-			}
-			if !test.equal && k1 == k2 {
-				t.Fatal("keys are the same, expected different keys")
+			if test.equal != (k1 == k2) {
+				t.Fatalf("unexpected result: \nExpected\n%s\nActual\n%s\n", k1, k2)
 			}
 		})
 	}

pkg/util/util.go

@@ -122,5 +122,8 @@ func MtimeHasher() func(string) (string, error) {
 func SHA256(r io.Reader) (string, error) {
 	hasher := sha256.New()
 	_, err := io.Copy(hasher, r)
-	return hex.EncodeToString(hasher.Sum(make([]byte, 0, hasher.Size()))), err
+	if err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(hasher.Sum(make([]byte, 0, hasher.Size()))), nil
 }