executor: apply label to only final stage

In #4673 we made a change were
we were applying labels to end of each stage, which is different than
what we were doing before i.e applying label at the end of the each
step.

However buildkit does not adds label to any stage or steps it only adds
label at the end of final stage so lets do that.

Closes: #4804

Signed-off-by: Aditya R <arajan@redhat.com>

imagebuildah/executor.go

@@ -470,33 +470,33 @@ func (b *Executor) buildStage(ctx context.Context, cleanupStages map[int]*StageE
 	output := ""
 	if stageIndex == len(stages)-1 {
 		output = b.output
-	}
-	// Check if any labels were passed in via the API, and add a final line
-	// to the Dockerfile that would provide the same result.
-	// Reason: Docker adds label modification as a last step which can be
-	// processed like regular steps, and if no modification is done to
-	// layers, its easier to re-use cached layers.
-	if len(b.labels) > 0 {
-		var labelLine string
-		labels := append([]string{}, b.labels...)
-		for _, labelSpec := range labels {
-			label := strings.SplitN(labelSpec, "=", 2)
-			key := label[0]
-			value := ""
-			if len(label) > 1 {
-				value = label[1]
-			}
-			// check only for an empty key since docker allows empty values
-			if key != "" {
-				labelLine += fmt.Sprintf(" %q=%q", key, value)
+		// Check if any labels were passed in via the API, and add a final line
+		// to the Dockerfile that would provide the same result.
+		// Reason: Docker adds label modification as a last step which can be
+		// processed like regular steps, and if no modification is done to
+		// layers, its easier to re-use cached layers.
+		if len(b.labels) > 0 {
+			var labelLine string
+			labels := append([]string{}, b.labels...)
+			for _, labelSpec := range labels {
+				label := strings.SplitN(labelSpec, "=", 2)
+				key := label[0]
+				value := ""
+				if len(label) > 1 {
+					value = label[1]
+				}
+				// check only for an empty key since docker allows empty values
+				if key != "" {
+					labelLine += fmt.Sprintf(" %q=%q", key, value)
+				}
 			}
-		}
-		if len(labelLine) > 0 {
-			additionalNode, err := imagebuilder.ParseDockerfile(strings.NewReader("LABEL" + labelLine + "\n"))
-			if err != nil {
-				return "", nil, fmt.Errorf("while adding additional LABEL step: %w", err)
+			if len(labelLine) > 0 {
+				additionalNode, err := imagebuilder.ParseDockerfile(strings.NewReader("LABEL" + labelLine + "\n"))
+				if err != nil {
+					return "", nil, fmt.Errorf("while adding additional LABEL step: %w", err)
+				}
+				stage.Node.Children = append(stage.Node.Children, additionalNode.Children...)
 			}
-			stage.Node.Children = append(stage.Node.Children, additionalNode.Children...)
 		}
 	}
 

tests/bud.bats

@@ -1965,6 +1965,17 @@ _EOF
   done
   run_buildah inspect --format '{{ index .Docker.Config.Labels "somefancylabel"}}' imagetwo
   expect_output ""
+
+  # build another multi-stage image with different label, it should use stages from cache from previous build
+  run_buildah build --layers $WITH_POLICY_JSON --label anotherfancylabel=true -t imagethree -f Dockerfile.name $BUDFILES/multi-stage-builds
+  # must use all steps from cache but should not contain label
+  for i in 2 6;do
+      expect_output --substring --from="${lines[$i]}" "Using cache"
+  done
+  run_buildah inspect --format '{{ index .Docker.Config.Labels "somefancylabel"}}' imagetwo
+  expect_output ""
+  run_buildah inspect --format '{{ index .Docker.Config.Labels "anotherfancylabel"}}' imagethree
+  expect_output "true"
 }
 
 @test "bud-multi-stage-builds" {