CNB command-line should only be rewritten if changed (#4176)

pkg/skaffold/debug/cnb.go

@@ -69,10 +69,13 @@ func updateForCNBImage(container *v1.Container, ic imageConfiguration, transform
 		return ContainerDebugConfiguration{}, "", fmt.Errorf("buildpacks metadata has no processes")
 	}
 
-	// the buildpacks launcher is retained as the entrypoint
+	// The buildpacks launcher is retained as the entrypoint
 	ic, rewriter := adjustCommandLine(m, ic)
 	c, img, err := transformer(container, ic)
-	if err == nil && rewriter != nil {
+
+	// Only rewrite the container.Args if set: some transforms only alter env vars,
+	// and the image's arguments are not changed.
+	if err == nil && container.Args != nil && rewriter != nil {
 		container.Args = rewriter(container.Args)
 	}
 	return c, img, err

pkg/skaffold/debug/cnb_test.go

@@ -110,15 +110,30 @@ func TestUpdateForCNBImage(t *testing.T) {
 		},
 	}
 	for _, test := range tests {
-		testutil.Run(t, test.description, func(t *testutil.T) {
-			dummyTransform := func(c *v1.Container, ic imageConfiguration) (ContainerDebugConfiguration, string, error) {
+		// Test that when a transform modifies the command-line arguments, then
+		// the changes are reflected to the launcher command-line
+		testutil.Run(t, test.description+" (args changed)", func(t *testutil.T) {
+			argsChangedTransform := func(c *v1.Container, ic imageConfiguration) (ContainerDebugConfiguration, string, error) {
 				c.Args = ic.arguments
 				return ContainerDebugConfiguration{}, "", nil
 			}
-
 			copy := v1.Container{}
-			_, _, err := updateForCNBImage(&copy, test.input, dummyTransform)
+			_, _, err := updateForCNBImage(&copy, test.input, argsChangedTransform)
 			t.CheckErrorAndDeepEqual(test.shouldErr, err, test.expected, copy)
 		})
+
+		// Test that when the arguments are left unchanged, that the container is unchanged
+		testutil.Run(t, test.description+" (args unchanged)", func(t *testutil.T) {
+			argsUnchangedTransform := func(c *v1.Container, ic imageConfiguration) (ContainerDebugConfiguration, string, error) {
+				return ContainerDebugConfiguration{}, "", nil
+			}
+
+			copy := v1.Container{}
+			_, _, err := updateForCNBImage(&copy, test.input, argsUnchangedTransform)
+			t.CheckError(test.shouldErr, err)
+			if copy.Args != nil {
+				t.Errorf("args not nil: %v", copy.Args)
+			}
+		})
 	}
 }