integration: parse runtime image overrides via the shared helper

tsic's websocket-tagged override and tsric's prebuilt image both did their
own strings.Cut into repo:tag. Route them through ParseImageRef alongside
PrebuiltImage so there is one repository:tag parser, and drop tsic's
now-unused errInvalidTailscaleImageFormat.

Author: kradalby

integration/integrationutil/util.go

@@ -39,12 +39,25 @@ func PrebuiltImage(envVar string) (string, string, bool, error) {
 		return "", "", false, nil
 	}
 
+	repo, tag, err := ParseImageRef(image)
+	if err != nil {
+		return "", "", false, fmt.Errorf("%s=%w", envVar, err)
+	}
+
+	return repo, tag, true, nil
+}
+
+// ParseImageRef splits an already-resolved "repository:tag" image string into
+// its parts. Use it where the image comes from somewhere other than a single
+// knob (e.g. a websocket-tagged override chosen at runtime); for the common
+// read-a-knob case use [PrebuiltImage].
+func ParseImageRef(image string) (string, string, error) {
 	repo, tag, found := strings.Cut(image, ":")
 	if !found {
-		return "", "", false, fmt.Errorf("%s=%q: %w", envVar, image, errInvalidImageFormat)
+		return "", "", fmt.Errorf("%q: %w", image, errInvalidImageFormat)
 	}
 
-	return repo, tag, true, nil
+	return repo, tag, nil
 }
 
 // PeerSyncTimeout returns the timeout for peer synchronization based on environment:

integration/tsic/tsic.go

@@ -66,7 +66,6 @@ var (
 	errTailscaleWrongPeerCount         = errors.New("wrong peer count")
 	errTailscaleCannotUpWithoutAuthkey = errors.New("cannot up without authkey")
 	errInvalidClientConfig             = errors.New("verifiably invalid client config requested")
-	errInvalidTailscaleImageFormat     = errors.New("invalid HEADSCALE_INTEGRATION_TAILSCALE_IMAGE format, expected repository:tag")
 	errTailscaleImageRequiredInCI      = errors.New("HEADSCALE_INTEGRATION_TAILSCALE_IMAGE must be set in CI for HEAD version")
 	errContainerNotInitialized         = errors.New("container not initialized")
 	errFQDNNotYetAvailable             = errors.New("FQDN not yet available")
@@ -455,10 +454,9 @@ func New(
 		if prebuiltImage != "" {
 			log.Printf("Using pre-built tailscale image: %s", prebuiltImage) //nolint:gosec // G706: integration-only log of trusted env value
 
-			// Parse image into repository and tag
-			repo, tag, ok := strings.Cut(prebuiltImage, ":")
-			if !ok {
-				return nil, errInvalidTailscaleImageFormat
+			repo, tag, err := integrationutil.ParseImageRef(prebuiltImage)
+			if err != nil {
+				return nil, err
 			}
 
 			tailscaleOptions.Repository = repo

integration/tsric/tsric.go

@@ -210,9 +210,9 @@ func New(
 	if prebuiltImage := getPrebuiltImage(); prebuiltImage != "" {
 		log.Printf("Using pre-built tailscale-rs image: %s", prebuiltImage)
 
-		repo, tag, ok := strings.Cut(prebuiltImage, ":")
-		if !ok {
-			return nil, fmt.Errorf("tsric: invalid image format %q, expected repository:tag", prebuiltImage) //nolint:err113
+		repo, tag, err := integrationutil.ParseImageRef(prebuiltImage)
+		if err != nil {
+			return nil, fmt.Errorf("tsric: %w", err)
 		}
 
 		runOptions.Repository = repo