From ecbeedfd33df161660133a4b65c3a95b4f921dc6 Mon Sep 17 00:00:00 2001
From: Adrian Vovk <adrianvovk@gmail.com>
Date: Thu, 13 Aug 2026 14:45:53 -0400
Subject: [PATCH] gdm/userVerifier: Fix mechanisms-changed race

Starting with GDM e760c8e4, GDM will return an error from
BeginVerification() whenever `gdm-switchable-auth` is unavailable.

This would cause AuthServicesSSSDSwitchable to emit `mechanisms-changed`
during ShellUserVerifier.begin(). The signal would cause a chain of
events that leads to ShellUserVerifier.cancel() being called inside of
that same call stack. In response to this, GDM will send us a reset
signal. Meanwhile, the call stack would move on and try to call
BeginVerification() for AuthServicesLegacy.

In the unhappy case, we'd receive the reset signal before
BeginVerification() is done, which completely de-syncs gnome-shell's and
GDM's views of what's going on. Ultimately the password box would never
appear. gnome-shell would kick the user right back into the user list,
but GDM would think that gdm-password is currently ongoing.

The fix is to delay the propagation of `mechanisms-changed` until we're
done with BeginVerification() for all available auth services.

Closes: https://gitlab.gnome.org/GNOME/gdm/-/work_items/1089
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4336>
---
 js/gdm/userVerifier.js | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/js/gdm/userVerifier.js b/js/gdm/userVerifier.js
index 7428450da5..e2d2e352f2 100644
--- a/js/gdm/userVerifier.js
+++ b/js/gdm/userVerifier.js
@@ -63,6 +63,9 @@ export class ShellUserVerifier extends Signals.EventEmitter {
         this._authServices = [];
         this._driverService = null;
 
+        this._beginPromise = null;
+        this._redistributingRoles = false;
+
         this._messageQueue = [];
         this._messageQueueTimeoutId = 0;
 
@@ -87,6 +90,9 @@ export class ShellUserVerifier extends Signals.EventEmitter {
         this._cancellable?.cancel();
         this._cancellable = new Gio.Cancellable();
 
+        const {promise, resolve} = Promise.withResolvers();
+        this._beginPromise = promise;
+
         try {
             const proxies = await this._getUserVerifierProxies(userName, this._cancellable);
             this._setUserVerifier(proxies.userVerifier);
@@ -101,6 +107,9 @@ export class ShellUserVerifier extends Signals.EventEmitter {
                 logErrorUnlessCancelled(e);
         }
 
+        resolve();
+        this._beginPromise = null;
+
         hold?.release();
     }
 
@@ -408,7 +417,7 @@ export class ShellUserVerifier extends Signals.EventEmitter {
                 'reset', (_, args) => this.emit('reset', args),
                 'show-choice-list', (_, args) => this.emit('show-choice-list', args),
                 'show-button', (_, args) => this.emit('show-button', args),
-                'mechanisms-changed', () => this._onMechanismsChanged(),
+                'mechanisms-changed', () => this._onMechanismsChanged().catch(logError),
                 'web-login', (_, args) => this.emit('web-login', args),
                 this);
         });
@@ -442,10 +451,12 @@ export class ShellUserVerifier extends Signals.EventEmitter {
         this._redistributingRoles = false;
     }
 
-    _onMechanismsChanged() {
+    async _onMechanismsChanged() {
         if (this._redistributingRoles)
             return;
 
+        await this._beginPromise;
+
         this._redistributeRoles();
 
         // Collect mechanisms from all authServices in priority order,
-- 
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