fix: add default value for RevokeURL property in external auth config for GitHub (coder#20272)

This PR adds setting default value of `RevokeURL` property of external
auth config for GitHub.

Author: pawbana

coderd/externalauth/externalauth.go

@@ -781,6 +781,9 @@ func applyDefaultsToConfig(config *codersdk.ExternalAuthConfig) {
 
 	// Dynamic defaults
 	switch codersdk.EnhancedExternalAuthProvider(config.Type) {
+	case codersdk.EnhancedExternalAuthProviderGitHub:
+		copyDefaultSettings(config, gitHubDefaults(config))
+		return
 	case codersdk.EnhancedExternalAuthProviderGitLab:
 		copyDefaultSettings(config, gitlabDefaults(config))
 		return
@@ -855,6 +858,29 @@ func copyDefaultSettings(config *codersdk.ExternalAuthConfig, defaults codersdk.
 	}
 }
 
+// gitHubDefaults returns default config values for GitHub.
+// The only dynamic value is the revocation URL which depends on client ID.
+func gitHubDefaults(config *codersdk.ExternalAuthConfig) codersdk.ExternalAuthConfig {
+	defaults := codersdk.ExternalAuthConfig{
+		AuthURL:     xgithub.Endpoint.AuthURL,
+		TokenURL:    xgithub.Endpoint.TokenURL,
+		ValidateURL: "https://api.github.com/user",
+		DisplayName: "GitHub",
+		DisplayIcon: "/icon/github.svg",
+		Regex:       `^(https?://)?github\.com(/.*)?$`,
+		// "workflow" is required for managing GitHub Actions in a repository.
+		Scopes:              []string{"repo", "workflow"},
+		DeviceCodeURL:       "https://github.com/login/device/code",
+		AppInstallationsURL: "https://api.github.com/user/installations",
+	}
+
+	if config.RevokeURL == "" && config.ClientID != "" {
+		defaults.RevokeURL = fmt.Sprintf("https://api.github.com/applications/%s/grant", config.ClientID)
+	}
+
+	return defaults
+}
+
 func bitbucketServerDefaults(config *codersdk.ExternalAuthConfig) codersdk.ExternalAuthConfig {
 	defaults := codersdk.ExternalAuthConfig{
 		DisplayName: "Bitbucket Server",
@@ -1053,18 +1079,6 @@ var staticDefaults = map[codersdk.EnhancedExternalAuthProvider]codersdk.External
 		Regex:       `^(https?://)?bitbucket\.org(/.*)?$`,
 		Scopes:      []string{"account", "repository:write"},
 	},
-	codersdk.EnhancedExternalAuthProviderGitHub: {
-		AuthURL:     xgithub.Endpoint.AuthURL,
-		TokenURL:    xgithub.Endpoint.TokenURL,
-		ValidateURL: "https://api.github.com/user",
-		DisplayName: "GitHub",
-		DisplayIcon: "/icon/github.svg",
-		Regex:       `^(https?://)?github\.com(/.*)?$`,
-		// "workflow" is required for managing GitHub Actions in a repository.
-		Scopes:              []string{"repo", "workflow"},
-		DeviceCodeURL:       "https://github.com/login/device/code",
-		AppInstallationsURL: "https://api.github.com/user/installations",
-	},
 	codersdk.EnhancedExternalAuthProviderSlack: {
 		AuthURL:     "https://slack.com/oauth/v2/authorize",
 		TokenURL:    "https://slack.com/api/oauth.v2.access",