Merge branch 'multiselect-discoverability'

Multi-select discoverability (items 24, 25) and awareness of syncs this
window did not start (item 27).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 106 insertions, 0 deletions

diff --git a/src/syncmonitor.h b/src/syncmonitor.h
new file mode 100644
index 0000000..3aca520
--- /dev/null
+++ b/src/syncmonitor.h
@@ -0,0 +1,106 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include <QObject>
+#include <QString>
+#include <QTimer>
+
+/// Notices syncs this process did not start.
+///
+/// The user's cron runs mailsync.sh every ten minutes, so mail can appear and
+/// tags can change while the window sits idle. The script holds an flock for
+/// the whole run, which is already the signal: no status file is needed, and a
+/// kernel lock cannot go stale because it dies with the process holding it.
+///
+/// **Read the lock, never take it.** Three ways to observe an flock look
+/// plausible and two are wrong, both verified on Slackware, Linux 6.18:
+///
+///   - `flock -n` acquires in order to test. Polling every two seconds would
+///     open a window every two seconds in which a starting mailsync.sh is
+///     refused the lock and exits 75. It would cause the very skips the sync
+///     script reports.
+///   - `fcntl(F_OFD_GETLK)` never acquires, and looks ideal, but reports
+///     UNLOCKED against a lock held by flock(2): the two are separate lock
+///     namespaces in the kernel and cannot see each other. A silent false
+///     negative, which is the worst failure available here.
+///   - /proc/locks is a pure read. It observes flock(2) entries correctly and
+///     cannot acquire, steal, or contend, so it can also never disturb the
+///     Xapian write lock notmuch new holds during the same run.
+///
+/// Do not "simplify" this to flock -n.
+class SyncMonitor : public QObject
+{
+    Q_OBJECT
+public:
+    enum class State {
+        Unknown,   ///< The lock table cannot be read; claim nothing.
+        Idle,      ///< Readable, and nothing holds the lock.
+        Running,   ///< Something holds the lock: a sync is in progress.
+    };
+    Q_ENUM(State)
+
+    /// @param lockPath  the file mailsync.sh flocks, /tmp/mbsync.lock.
+    /// @param locksPath the kernel lock table; injectable so tests can drive
+    ///                  transitions without holding real locks.
+    explicit SyncMonitor(const QString &lockPath,
+                         const QString &locksPath = QStringLiteral("/proc/locks"),
+                         QObject *parent = nullptr);
+
+    State state() const { return m_state; }
+
+    /// True only for State::Running. Unknown is deliberately not "running":
+    /// callers use this to decide whether to wait, and waiting forever on a
+    /// platform with no /proc/locks would be worse than not noticing a sync.
+    bool isRunning() const { return m_state == State::Running; }
+
+    void setInterval(int ms);
+    void start();
+    void stop();
+
+    /// One observation. Public so tests can step it without a running timer.
+    void poll();
+
+    /// Whether @p content holds an flock(2) entry for @p inode.
+    ///
+    /// Static and content-based: this is the part worth testing, and it is
+    /// testable only while it is separate from reading the file.
+    static bool lockHeldIn(const QString &content, qint64 inode);
+
+    /// The inode of @p path, or -1 when it does not exist.
+    static qint64 inodeOf(const QString &path);
+
+    /// The lock file assets/mailsync.sh takes, and the only one worth watching.
+    ///
+    /// Hardcoded to match LOCKFILE in that script. Two sources of truth is the
+    /// standing hazard here: change one and the monitor silently reports Idle
+    /// forever, since a missing lock file is a legitimate "no sync running".
+    static QString defaultLockPath();
+
+signals:
+    /// Emitted only when the state actually changes, never once per poll: the
+    /// status bar must not be repainted every two seconds forever.
+    void stateChanged(SyncMonitor::State state);
+
+private:
+    QString m_lockPath;
+    QString m_locksPath;
+    State m_state = State::Unknown;
+    QTimer m_timer;
+};