fix(folderwatcher): ignore events for stale inotify watch descriptors

src/gui/folderwatcher_linux.cpp

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: GPL-2.0-or-later
  */
 
 #include "config.h"
 
 #include <sys/inotify.h>
 
@@ -168,7 +168,13 @@
             || fileName.startsWith(".sync_")) {
             continue;
         }
-        const QString p = _watchToPath[event->wd] + '/' + fileName;
+        const auto watchPathIt = _watchToPath.constFind(event->wd);
+        if (watchPathIt == _watchToPath.cend()) {
+            qCDebug(lcFolderWatcher) << "Ignoring event for unknown watch descriptor" << event->wd << fileName;
+            continue;
+        }
+
+        const QString p = *watchPathIt + '/' + fileName;
         _parent->changeDetected(p);
 
         if ((event->mask & (IN_MOVED_TO | IN_CREATE))

src/gui/folderwatcher_linux.h

@@ -1,3 +1,3 @@
 /*
  * SPDX-FileCopyrightText: 2020 Nextcloud GmbH and Nextcloud contributors
  * SPDX-FileCopyrightText: 2014 ownCloud GmbH
@@ -7,7 +7,7 @@
 #ifndef MIRALL_FOLDERWATCHER_LINUX_H
 #define MIRALL_FOLDERWATCHER_LINUX_H
 
 #include <QObject>
 #include <QString>
 #include <QSocketNotifier>
 #include <QHash>
@@ -41,6 +41,12 @@
     void slotAddFolderRecursive(const QString &path);
 
 protected:
+    /**
+     * @brief Checks whether an inotify watch descriptor is registered.
+     * @param watchDescriptor The inotify watch descriptor to check.
+     */
+    [[nodiscard]] bool testWatchContains(int watchDescriptor) const { return _watchToPath.contains(watchDescriptor); }
+
     bool findFoldersBelow(const QDir &dir, QStringList &fullList);
     void inotifyRegisterPath(const QString &path);
     void removeFoldersBelow(const QString &path);

test/testinotifywatcher.cpp

@@ -1,3 +1,3 @@
 /*
  * SPDX-FileCopyrightText: 2021 Nextcloud GmbH and Nextcloud contributors
  * SPDX-FileCopyrightText: 2016 ownCloud GmbH
@@ -8,8 +8,13 @@
  * any purpose.
  */
 
 #include <QtTest>
 
+#include <array>
+#include <climits>
+#include <sys/inotify.h>
+#include <unistd.h>
+
 #include "folderwatcher_linux.h"
 #include "common/utility.h"
 
@@ -64,6 +69,30 @@
         QVERIFY2(ok, "findFoldersBelow failed.");
     }
 
+    void testStaleWatchDescriptorIsIgnored()
+    {
+        QVERIFY(!testWatchContains(INT_MAX));
+
+        std::array<int, 2> pipeFds {};
+        QVERIFY(::pipe(pipeFds.data()) == 0);
+
+        QByteArray payload(sizeof(inotify_event) + sizeof("lib"), '\0');
+        const auto event = reinterpret_cast<inotify_event *>(payload.data());
+        event->wd = INT_MAX;
+        event->mask = IN_CREATE;
+        event->cookie = 0;
+        event->len = sizeof("lib");
+        memcpy(event->name, "lib", sizeof("lib"));
+
+        QCOMPARE(::write(pipeFds[1], payload.constData(), payload.size()), payload.size());
+        slotReceivedNotification(pipeFds[0]);
+
+        ::close(pipeFds[0]);
+        ::close(pipeFds[1]);
+
+        QVERIFY(!testWatchContains(INT_MAX));
+    }
+
     void cleanupTestCase() {
         if( _root.startsWith(QDir::tempPath() )) {
            system( QStringLiteral("rm -rf %1").arg(_root).toLocal8Bit() );