selfupdate: add Check to see the newest release without installing

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

selfupdate/selfupdate.go

@@ -111,21 +111,9 @@ func CheckOnce(ctx context.Context, cfg Config) (string, error) {
 	if !ok {
 		return "", fmt.Errorf("%q is not a release version", cfg.Current)
 	}
-	var rel struct {
-		Tag        string `json:"tag_name"`
-		Prerelease bool   `json:"prerelease"`
-		Draft      bool   `json:"draft"`
-		Assets     []struct {
-			Name string `json:"name"`
-			URL  string `json:"browser_download_url"`
-		} `json:"assets"`
-	}
-	body, err := get(ctx, cfg.HTTP, cfg.Server+"/api/v1/repos/"+cfg.Repo+"/releases/latest", 1<<20)
+	rel, err := latestRelease(ctx, cfg)
 	if err != nil {
-		return "", fmt.Errorf("checking for a new release: %w", err)
-	}
-	if err := json.Unmarshal(body, &rel); err != nil {
-		return "", fmt.Errorf("reading the latest release: %w", err)
+		return "", err
 	}
 	if rel.Prerelease || rel.Draft {
 		return "", nil
@@ -208,6 +196,47 @@ func CheckOnce(ctx context.Context, cfg Config) (string, error) {
 	return rel.Tag, nil
 }
 
+type release struct {
+	Tag        string `json:"tag_name"`
+	Prerelease bool   `json:"prerelease"`
+	Draft      bool   `json:"draft"`
+	Assets     []struct {
+		Name string `json:"name"`
+		URL  string `json:"browser_download_url"`
+	} `json:"assets"`
+}
+
+func latestRelease(ctx context.Context, cfg Config) (release, error) {
+	var rel release
+	body, err := get(ctx, cfg.HTTP, cfg.Server+"/api/v1/repos/"+cfg.Repo+"/releases/latest", 1<<20)
+	if err != nil {
+		return rel, fmt.Errorf("checking for a new release: %w", err)
+	}
+	if err := json.Unmarshal(body, &rel); err != nil {
+		return rel, fmt.Errorf("reading the latest release: %w", err)
+	}
+	return rel, nil
+}
+
+// Check reports the newest full release and whether it is newer than cfg.Current, without
+// installing anything. A development build (a Current that isn't vX.Y.Z) is never "newer".
+func Check(ctx context.Context, cfg Config) (latest string, newer bool, err error) {
+	cfg.defaults()
+	rel, err := latestRelease(ctx, cfg)
+	if err != nil {
+		return "", false, err
+	}
+	if rel.Prerelease || rel.Draft {
+		return "", false, nil
+	}
+	l, ok := parseVersion(rel.Tag)
+	if !ok {
+		return rel.Tag, false, nil
+	}
+	cur, ok := parseVersion(cfg.Current)
+	return rel.Tag, ok && l.newerThan(cur), nil
+}
+
 func get(ctx context.Context, c *http.Client, url string, max int64) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
 	if err != nil {