feat(server): add ExtAuth logout URL configuration (#5074)

* feat(server): add ExtAuth logout URL configuration (#4467)

When external authentication (reverse proxy auth) is active, the Logout
button is hidden because authentication is managed externally. Many
external auth services (Authelia, Authentik, Keycloak) provide a logout
URL that can terminate the session.

Add `ExtAuth.LogoutURL` config option that, when set, shows the Logout
button in the UI and redirects the user to the external auth provider's
logout endpoint instead of the Navidrome login page.

* feat(server): add validation for ExtAuth logout URL configuration

* feat(server): refactor ExtAuth logout URL validation to a reusable function

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

conf/configuration.go

@@ -250,6 +250,7 @@ type pluginsOptions struct {
 type extAuthOptions struct {
 	TrustedSources string
 	UserHeader     string
+	LogoutURL      string
 }
 
 type searchOptions struct {
@@ -345,6 +346,7 @@ func Load(noConfigDump bool) {
 		validateBackupSchedule,
 		validatePlaylistsPath,
 		validatePurgeMissingOption,
+		validateURL("ExtAuth.LogoutURL", Server.ExtAuth.LogoutURL),
 	)
 	if err != nil {
 		os.Exit(1)
@@ -548,6 +550,33 @@ func validateSchedule(schedule, field string) (string, error) {
 	return schedule, err
 }
 
+// validateURL checks if the provided URL is valid and has either http or https scheme.
+// It returns a function that can be used as a hook to validate URLs in the config.
+func validateURL(optionName, optionURL string) func() error {
+	return func() error {
+		if optionURL == "" {
+			return nil
+		}
+		u, err := url.Parse(optionURL)
+		if err != nil {
+			log.Error(fmt.Sprintf("Invalid %s: it could not be parsed", optionName), "url", optionURL, "err", err)
+			return err
+		}
+		if u.Scheme != "http" && u.Scheme != "https" {
+			err := fmt.Errorf("invalid scheme for %s: '%s'. Only 'http' and 'https' are allowed", optionName, u.Scheme)
+			log.Error(err.Error())
+			return err
+		}
+		// Require an absolute URL with a non-empty host and no opaque component.
+		if u.Host == "" || u.Opaque != "" {
+			err := fmt.Errorf("invalid %s: '%s'. A full http(s) URL with a non-empty host is required", optionName, optionURL)
+			log.Error(err.Error())
+			return err
+		}
+		return nil
+	}
+}
+
 func normalizeSearchBackend(value string) string {
 	v := strings.ToLower(strings.TrimSpace(value))
 	switch v {
@@ -641,6 +670,7 @@ func setViperDefaults() {
 	viper.SetDefault("passwordencryptionkey", "")
 	viper.SetDefault("extauth.userheader", "Remote-User")
 	viper.SetDefault("extauth.trustedsources", "")
+	viper.SetDefault("extauth.logouturl", "")
 	viper.SetDefault("prometheus.enabled", false)
 	viper.SetDefault("prometheus.metricspath", consts.PrometheusDefaultPath)
 	viper.SetDefault("prometheus.password", "")

conf/configuration_test.go

@@ -52,6 +52,48 @@ var _ = Describe("Configuration", func() {
 		})
 	})
 
+	Describe("ValidateURL", func() {
+		It("accepts a valid http URL", func() {
+			fn := conf.ValidateURL("TestOption", "http://example.com/path")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("accepts a valid https URL", func() {
+			fn := conf.ValidateURL("TestOption", "https://example.com/path")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("rejects a URL with no scheme", func() {
+			fn := conf.ValidateURL("TestOption", "example.com/path")
+			Expect(fn()).To(MatchError(ContainSubstring("invalid scheme")))
+		})
+
+		It("rejects a URL with an unsupported scheme", func() {
+			fn := conf.ValidateURL("TestOption", "javascript://example.com/path")
+			Expect(fn()).To(MatchError(ContainSubstring("invalid scheme")))
+		})
+
+		It("accepts an empty URL (optional config)", func() {
+			fn := conf.ValidateURL("TestOption", "")
+			Expect(fn()).To(Succeed())
+		})
+
+		It("includes the option name in the error message", func() {
+			fn := conf.ValidateURL("MyOption", "ftp://example.com")
+			Expect(fn()).To(MatchError(ContainSubstring("MyOption")))
+		})
+
+		It("rejects a URL that cannot be parsed", func() {
+			fn := conf.ValidateURL("TestOption", "://invalid")
+			Expect(fn()).To(HaveOccurred())
+		})
+
+		It("rejects a URL without a host", func() {
+			fn := conf.ValidateURL("TestOption", "http:///path")
+			Expect(fn()).To(MatchError(ContainSubstring("non-empty host is required")))
+		})
+	})
+
 	DescribeTable("NormalizeSearchBackend",
 		func(input, expected string) {
 			Expect(conf.NormalizeSearchBackend(input)).To(Equal(expected))

conf/export_test.go

@@ -8,4 +8,6 @@ var SetViperDefaults = setViperDefaults
 
 var ParseLanguages = parseLanguages
 
+var ValidateURL = validateURL
+
 var NormalizeSearchBackend = normalizeSearchBackend