Change local SetOS to accept only valid values

- Keep as a noop setter for consitency remote
- Compare against default value to fail fast

Signed-off-by: Micah Young <ymicah@vmware.com>

local/local.go

@@ -243,7 +243,9 @@ func (i *Image) SetLabel(key, val string) error {
 }
 
 func (i *Image) SetOS(osVal string) error {
-	i.inspect.Os = osVal
+	if osVal != i.inspect.Os {
+		return fmt.Errorf(`invalid os: must match the daemon: "%s"`, i.inspect.Os)
+	}
 	return nil
 }
 

local/local_test.go

@@ -706,22 +706,46 @@ func testImage(t *testing.T, when spec.G, it spec.S) {
 		})
 	})
 
-	when("#SetOS #SetOSVersion #SetArchitecture", func() {
+	when("#SetOS", func() {
 		var repoName = newTestImageName()
 
 		it.After(func() {
 			h.AssertNil(t, h.DockerRmi(dockerClient, repoName))
 		})
 
-		it("sets the os/arch", func() {
+		it("allows noop sets for values that match the daemon", func() {
 			img, err := local.NewImage(repoName, dockerClient)
 			h.AssertNil(t, err)
 
-			//os has to match daemon
+			err = img.SetOS("fakeos")
+			h.AssertError(t, err, "invalid os: must match the daemon")
+
 			err = img.SetOS(daemonOS)
 			h.AssertNil(t, err)
+
+			h.AssertNil(t, img.Save())
+
+			inspect, _, err := dockerClient.ImageInspectWithRaw(context.TODO(), repoName)
+			h.AssertNil(t, err)
+
+			h.AssertEq(t, inspect.Os, daemonOS)
+		})
+	})
+
+	when("#SetOSVersion #SetArchitecture", func() {
+		var repoName = newTestImageName()
+
+		it.After(func() {
+			h.AssertNil(t, h.DockerRmi(dockerClient, repoName))
+		})
+
+		it("sets the os.version/arch", func() {
+			img, err := local.NewImage(repoName, dockerClient)
+			h.AssertNil(t, err)
+
 			err = img.SetOSVersion("1.2.3.4")
 			h.AssertNil(t, err)
+
 			err = img.SetArchitecture("arm64")
 			h.AssertNil(t, err)
 
@@ -730,7 +754,6 @@ func testImage(t *testing.T, when spec.G, it spec.S) {
 			inspect, _, err := dockerClient.ImageInspectWithRaw(context.TODO(), repoName)
 			h.AssertNil(t, err)
 
-			h.AssertEq(t, inspect.Os, daemonOS)
 			h.AssertEq(t, inspect.OsVersion, "1.2.3.4")
 			h.AssertEq(t, inspect.Architecture, "arm64")
 		})