interface: Add algorithm to Put(...)

To allow per-put algorithm selection.  Touching an engine-level config
for that sort of thing would probably lead to all sorts of racy bugs
allow them to set it per-put.

Signed-off-by: W. Trevor King <wking@tremily.us>

dir/dir.go

@@ -176,8 +176,11 @@ func (engine *Engine) Digests(ctx context.Context, algorithm digest.Algorithm, p
 }
 
 // Put implements Writer.Put.
-func (engine *Engine) Put(ctx context.Context, reader io.Reader) (dig digest.Digest, err error) {
-	digester := engine.Algorithm.Digester()
+func (engine *Engine) Put(ctx context.Context, algorithm digest.Algorithm, reader io.Reader) (dig digest.Digest, err error) {
+	if algorithm.String() == "" {
+		algorithm = engine.Algorithm
+	}
+	digester := algorithm.Digester()
 
 	file, err := ioutil.TempFile(engine.temp, "blob-")
 	if err != nil {

dir/dir_test.go

@@ -85,15 +85,10 @@ func TestGood(t *testing.T) {
 	}
 	defer engine.Close(ctx)
 
-	dirEngine, ok := engine.(*Engine)
-	if !ok {
-		t.Fatalf("template.New() did not return a *template.Engine")
-	}
-
 	var digestSha256 digest.Digest
 	bodyIn := "Hello, World!"
 	t.Run("put default algorithm", func(t *testing.T) {
-		digestSha256, err = engine.Put(ctx, strings.NewReader(bodyIn))
+		digestSha256, err = engine.Put(ctx, "", strings.NewReader(bodyIn))
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -107,12 +102,7 @@ func TestGood(t *testing.T) {
 
 	var digestSha512 digest.Digest
 	t.Run("put SHA-512", func(t *testing.T) {
-		dirEngine.Algorithm = digest.SHA512
-		defer func() {
-			dirEngine.Algorithm = digest.SHA256
-		}()
-
-		digestSha512, err = engine.Put(ctx, strings.NewReader(bodyIn))
+		digestSha512, err = engine.Put(ctx, digest.SHA512, strings.NewReader(bodyIn))
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -215,7 +205,7 @@ func TestGood(t *testing.T) {
 
 	t.Run("digests", func(t *testing.T) {
 		// Inject sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
-		_, err = engine.Put(ctx, strings.NewReader(""))
+		_, err = engine.Put(ctx, "", strings.NewReader(""))
 		if err != nil {
 			t.Fatal(err)
 		}

interface.go

@@ -152,7 +152,12 @@ type Writer interface {
 	// Put adds a new blob to the store.  The action is idempotent; a
 	// nil return means "that content is stored at DIGEST" without
 	// implying "because of your Put()".
-	Put(ctx context.Context, reader io.Reader) (digest digest.Digest, err error)
+	//
+
+	// The algorithm argument allows you to require a particular digest
+	// algorithm.  Set to the empty string to allow the Writer to use
+	// its preferred algorithm.
+	Put(ctx context.Context, algorithm digest.Algorithm, reader io.Reader) (digest digest.Digest, err error)
 }
 
 // Deleter represents a content-addressable storage engine deleter.