fix: atomic unlink with the PHP last-identity rule

- OAuthIdentitiesDestroy locks the users row (SELECT ... FOR UPDATE) and runs
  the PHP guarded DELETE in one transaction: a self-set password or another
  identity allows the unlink; otherwise 409 with the unchanged body
- the default allow-list adds microsoft, and an OAuthProviderSource's known
  names are used when the host passes no Providers
- TestOAuthIdentitiesDestroyLastIdentityWithPassword (204),
  TestOAuthIdentitiesDestroyConcurrentSmoke; README
- gofmt of the forgotPasswordBody line touched in the previous commit

controllers/api_controller.go

@@ -241,7 +241,7 @@ func oauthProviderSource(app *backpack.App) (classes.OAuthProviderSource, bool)
 	return src, ok && src != nil
 }
 
-const forgotPasswordBody ="If that email exists, a reset link has been sent."
+const forgotPasswordBody = "If that email exists, a reset link has been sent."
 
 // ForgotPassword always returns the same 200 body. A real account also gets a reset mail.
 func ForgotPassword(app *backpack.App) http.HandlerFunc {

controllers/oauth_identities.go

@@ -13,13 +13,27 @@ import (
 )
 
 // OAuthIdentitiesOptions configures host-mounted list/unlink handlers.
-// Providers defaults to google, facebook, github when nil or empty.
+// Providers is the unlink allow-list. When nil or empty, the allow-list is
+// read on every request: the KnownOAuthProviders of a published
+// classes.OAuthProviderSource, else google, facebook, github and microsoft.
 // WriteNotFound is injected by the host so 404 bodies match that app.
 type OAuthIdentitiesOptions struct {
 	Providers     []string
 	WriteNotFound func(http.ResponseWriter, *http.Request)
 }
 
+// defaultOAuthProviders is the unlink allow-list without options and without
+// a published provider source.
+var defaultOAuthProviders = []string{"google", "facebook", "github", "microsoft"}
+
+// oauthGuardedDelete is the PHP unlink rule (User.php unlinkOAuthProvider):
+// the identity goes only when the user has a self-set password or another
+// identity remains.
+const oauthGuardedDelete = `DELETE FROM golem15_user_oauth_identities
+WHERE user_id = ? AND provider = ?
+  AND (EXISTS (SELECT 1 FROM users u WHERE u.id = golem15_user_oauth_identities.user_id AND u.has_self_set_password)
+       OR (SELECT COUNT(*) FROM golem15_user_oauth_identities oi WHERE oi.user_id = golem15_user_oauth_identities.user_id) > 1)`
+
 // OAuthIdentitiesIndex lists the caller's linked OAuth providers.
 func OAuthIdentitiesIndex(app *backpack.App) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
@@ -50,19 +64,18 @@ func OAuthIdentitiesIndex(app *backpack.App) http.HandlerFunc {
 	}
 }
 
-// OAuthIdentitiesDestroy unlinks one provider for the caller.
+// OAuthIdentitiesDestroy unlinks one provider for the caller, under the PHP
+// rule: it succeeds (204) when the user has a self-set password or another
+// identity remains, and answers 409 with the last_method_blocked phrase
+// otherwise. An unknown provider or a missing identity goes to WriteNotFound.
 func OAuthIdentitiesDestroy(app *backpack.App, opts OAuthIdentitiesOptions) http.HandlerFunc {
-	providers := opts.Providers
-	if len(providers) == 0 {
-		providers = []string{"google", "facebook", "github"}
-	}
 	return func(w http.ResponseWriter, r *http.Request) {
 		db, user, ok := oauthIdentityDeps(w, r, app)
 		if !ok {
 			return
 		}
 		provider := r.PathValue("provider")
-		if !slices.Contains(providers, provider) {
+		if !slices.Contains(unlinkProviders(app, opts), provider) {
 			writeOAuthIdentityNotFound(w, r, opts)
 			return
 		}
@@ -78,28 +91,49 @@ func OAuthIdentitiesDestroy(app *backpack.App, opts OAuthIdentitiesOptions) http
 			writeOpaque500(w)
 			return
 		}
-		var n int64
-		if err := db.WithContext(r.Context()).
-			Model(&models.OAuthIdentity{}).
-			Where("user_id = ?", user.ID).
-			Count(&n).Error; err != nil {
+		// D-13: the guarded DELETE alone is not race-safe on Postgres. Under
+		// READ COMMITTED two unlinks of different identities each see the
+		// other's row in their COUNT and do not wait for each other, so both
+		// succeed and a password-less user loses every sign-in method. The
+		// lock on the users row serialises them: the second DELETE runs after
+		// the first commits and sees one identity left.
+		var deleted int64
+		err = db.WithContext(r.Context()).Transaction(func(tx *gorm.DB) error {
+			if err := tx.Exec(`SELECT id FROM users WHERE id = ? FOR UPDATE`, user.ID).Error; err != nil {
+				return err
+			}
+			res := tx.Exec(oauthGuardedDelete, user.ID, provider)
+			if res.Error != nil {
+				return res.Error
+			}
+			deleted = res.RowsAffected
+			return nil
+		})
+		if err != nil {
 			writeOpaque500(w)
 			return
 		}
-		if n == 1 {
+		if deleted == 0 {
 			writeJSON(w, http.StatusConflict, map[string]any{
 				"error": oauthLastMethodBlocked(r, app),
 			})
 			return
 		}
-		if err := db.WithContext(r.Context()).Delete(&row).Error; err != nil {
-			writeOpaque500(w)
-			return
-		}
 		w.WriteHeader(http.StatusNoContent)
 	}
 }
 
+// unlinkProviders is the unlink allow-list for one request.
+func unlinkProviders(app *backpack.App, opts OAuthIdentitiesOptions) []string {
+	if len(opts.Providers) > 0 {
+		return opts.Providers
+	}
+	if src, ok := oauthProviderSource(app); ok {
+		return src.KnownOAuthProviders()
+	}
+	return defaultOAuthProviders
+}
+
 func writeOAuthIdentityNotFound(w http.ResponseWriter, r *http.Request, opts OAuthIdentitiesOptions) {
 	if opts.WriteNotFound != nil {
 		opts.WriteNotFound(w, r)