nit: output snapshot ID when --json is used (#921)

snapshot/manager.go

@@ -123,6 +123,10 @@ func SaveSnapshot(ctx context.Context, rep repo.RepositoryWriter, man *Manifest)
 		return "", errors.New("missing path")
 	}
 
+	// clear manifest ID in case it was set, since we'll be generating a new one and we don't want
+	// to write previous ID in JSON.
+	man.ID = ""
+
 	id, err := rep.PutManifest(ctx, sourceInfoToLabels(man.Source), man)
 	if err != nil {
 		return "", errors.Wrap(err, "error putting manifest")

snapshot/manifest.go

@@ -15,7 +15,7 @@ import (
 
 // Manifest represents information about a single point-in-time filesystem snapshot.
 type Manifest struct {
-	ID     manifest.ID `json:"-"`
+	ID     manifest.ID `json:"id"`
 	Source SourceInfo  `json:"source"`
 
 	Description string    `json:"description"`

tests/end_to_end_test/snapshot_create_test.go

@@ -46,10 +46,22 @@ func TestSnapshotCreate(t *testing.T) {
 	mustParseJSONLines(t, e.RunAndExpectSuccess(t, "snapshot", "create", sharedTestDataDir2, "--json"), &man1)
 	mustParseJSONLines(t, e.RunAndExpectSuccess(t, "snapshot", "create", sharedTestDataDir2, "--json"), &man2)
 
+	if man1.ID == "" {
+		t.Fatalf("missing root id")
+	}
+
+	if man2.ID == "" {
+		t.Fatalf("missing root id")
+	}
+
 	if man1.RootEntry.ObjectID == "" {
 		t.Fatalf("missing root id")
 	}
 
+	if man1.ID == man2.ID {
+		t.Fatalf("unexpectedly identical snapshot IDs: %v", man1.ID)
+	}
+
 	if man1.RootEntry.ObjectID != man2.RootEntry.ObjectID {
 		t.Fatalf("unexpected difference in root objects %v vs %v", man1.RootEntry.ObjectID, man2.RootEntry.ObjectID)
 	}