feat: add the single-instance socket

A QLocalServer under the state directory. The first launch listens; a later one
connects, hands over its selectors and exits.

Connect-first ordering, and on Qt 6.11 the probe is the ONLY guard for a live
instance: with UserAccessOption, listen() binds in a private directory and
renames the socket onto the path, which replaces whatever is there, a stale
file and a live socket alike. Measured: with the probe disabled, a second
launch takes the first one's socket and both handover tests fail. The same
rename is what reclaims a stale file after a crash; the removeServer() retry
on AddressInUse is kept for a listen that binds in place.

The server reads each connection asynchronously and parses on disconnect,
rather than blocking in waitForReadyRead() on the UI thread. The client's one
write followed by a disconnect is what marks the payload complete, a reader
past the payload cap is aborted, and a connection that never hangs up is
dropped after two seconds. A connection that writes nothing at all is the other
launch's probe and is ignored without a warning.

A socket that cannot be created does NOT stop the window opening. A read-only
state directory costs single-instance behaviour, which is a degradation; it
must not cost the user their mail client.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 270 insertions, 0 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index ca26933..4a344d5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -54,6 +54,7 @@ add_library(qtmaildir_lib STATIC
     eventpane.cpp
     calendarwindow.cpp
     launchselectors.cpp
+    singleinstance.cpp
 )
 
 target_include_directories(qtmaildir_lib
diff --git a/src/singleinstance.cpp b/src/singleinstance.cpp
new file mode 100644
index 0000000..dbc53ed
--- /dev/null
+++ b/src/singleinstance.cpp
@@ -0,0 +1,188 @@
+/*
+ * 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.
+ */
+
+#include "singleinstance.h"
+
+#include <QDebug>
+#include <QFileInfo>
+#include <QLocalServer>
+#include <QLocalSocket>
+#include <QTimer>
+
+namespace {
+
+/// How long a client waits for the running instance, and how long the server
+/// keeps a connection open waiting for its payload. Short: both processes are
+/// on this machine and the peer is either there or it is not. A long wait
+/// would stall a launch behind a wedged instance, which is worse than opening
+/// a window.
+constexpr int kTimeoutMs = 2000;
+
+}  // namespace
+
+SingleInstance::SingleInstance(const QString &socketPath, QObject *parent)
+    : QObject(parent), m_socketPath(socketPath)
+{
+    // Registered here rather than at a call site so any connection carrying
+    // this type works, including a queued one a future caller might add. A
+    // Q_DECLARE_METATYPE alone gives the type a metatype but does not register
+    // it under the name a queued invoke resolves, which is the trap AGENTS.md
+    // records for Q_ENUM.
+    qRegisterMetaType<LaunchSelectors>();
+}
+
+SingleInstance::~SingleInstance()
+{
+    if (m_server) {
+        m_server->close();
+        // Removes the filesystem entry, so an orderly exit leaves nothing for
+        // the next launch to reclaim.
+        QLocalServer::removeServer(m_socketPath);
+    }
+}
+
+bool SingleInstance::isServer() const
+{
+    return m_server != nullptr && m_server->isListening();
+}
+
+bool SingleInstance::tryBecomeServer()
+{
+    if (isServer())
+        return true;
+
+    // CONNECT FIRST, and the order is the design. A successful connection means
+    // a live instance owns this socket and this process is the messenger. A
+    // refused connection on an EXISTING file means the file is stale, left by a
+    // crash, and can be removed; doing it in this order is what stops a live
+    // instance being removed out from under itself.
+    {
+        QLocalSocket probe;
+        probe.connectToServer(m_socketPath);
+        if (probe.waitForConnected(kTimeoutMs)) {
+            probe.disconnectFromServer();
+            return false;
+        }
+    }
+
+    auto *server = new QLocalServer(this);
+    // The socket is the user's own, in their own state directory. Nothing else
+    // has any business connecting to it.
+    //
+    // With this option Qt binds in a private temporary directory and RENAMES
+    // the socket onto the path, and a rename replaces whatever is there: a
+    // stale file and a LIVE instance's socket alike (measured on Qt 6.11). So
+    // listen() does not refuse a taken path, and the probe above is the only
+    // thing standing between a second launch and the first one's socket.
+    server->setSocketOptions(QLocalServer::UserAccessOption);
+
+    if (!server->listen(m_socketPath)) {
+        if (server->serverError() == QAbstractSocket::AddressInUseError
+            && QFileInfo::exists(m_socketPath)) {
+            // Nothing answered the probe above, so this file is stale. Not
+            // reached on Qt 6.11, for the reason above; kept because a listen
+            // that binds in place (no socket options, or a future Qt) fails
+            // here with AddressInUse on a stale file.
+            QLocalServer::removeServer(m_socketPath);
+            server->listen(m_socketPath);
+        }
+    }
+
+    if (!server->isListening()) {
+        // A read-only state directory, a filesystem that has no unix sockets,
+        // or a path whose parent does not exist. Report and carry on: the
+        // window must still open. Losing single-instance behaviour is a
+        // degradation; losing the mail client is not acceptable.
+        qWarning() << "qtmaildir: cannot create the single-instance socket at"
+                   << m_socketPath << ":" << server->errorString()
+                   << "- continuing without it";
+        delete server;
+        return false;
+    }
+
+    m_server = server;
+    connect(m_server, &QLocalServer::newConnection, this, [this]() {
+        while (QLocalSocket *socket = m_server->nextPendingConnection()) {
+            // Read ASYNCHRONOUSLY and parse on disconnect. The client's whole
+            // life is one write followed by a disconnect, so the disconnect is
+            // what says the payload is complete; a readAll() after the first
+            // readyRead could see only part of it. And no waitForReadyRead():
+            // this runs on the UI thread, and a peer that connects and writes
+            // nothing (the probe in tryBecomeServer() is exactly that) must
+            // not freeze the window.
+            auto *buffer = new QByteArray;
+            connect(socket, &QLocalSocket::readyRead, socket, [socket, buffer]() {
+                buffer->append(socket->readAll());
+                // A peer still writing past the cap is not a launch payload.
+                if (buffer->size() > LaunchSelectors::kMaxPayloadBytes)
+                    socket->abort();
+            });
+            connect(socket, &QLocalSocket::disconnected, this,
+                    [this, socket, buffer]() {
+                buffer->append(socket->readAll());
+                const QByteArray payload = *buffer;
+                delete buffer;
+                socket->deleteLater();
+
+                // Nothing written at all is the other launch's connect-first
+                // probe, not a request. A real request is never empty, even
+                // with no selectors, because the payload carries a version.
+                if (payload.isEmpty())
+                    return;
+
+                QString error;
+                const LaunchSelectors selectors =
+                    LaunchSelectors::fromPayload(payload, &error);
+                if (!error.isEmpty()) {
+                    qWarning() << "qtmaildir: ignoring a launch payload:"
+                               << error;
+                    return;
+                }
+
+                // Emitted even when empty: a bare launch means "raise
+                // yourself".
+                emit selectorsReceived(selectors);
+            });
+            // A peer that connects and never hangs up is dropped rather than
+            // held open for the life of the window.
+            QTimer::singleShot(kTimeoutMs, socket, [socket]() {
+                socket->abort();
+            });
+        }
+    });
+
+    return true;
+}
+
+bool SingleInstance::sendToRunningInstance(const LaunchSelectors &selectors)
+{
+    QLocalSocket socket;
+    socket.connectToServer(m_socketPath);
+    if (!socket.waitForConnected(kTimeoutMs))
+        return false;
+
+    socket.write(selectors.toPayload());
+    // Flushed before returning, because the caller exits immediately
+    // afterwards and an unflushed write would be lost with the process.
+    if (!socket.waitForBytesWritten(kTimeoutMs))
+        return false;
+
+    // The disconnect is what tells the server the payload is complete.
+    socket.disconnectFromServer();
+    return true;
+}
diff --git a/src/singleinstance.h b/src/singleinstance.h
new file mode 100644
index 0000000..3f63bfd
--- /dev/null
+++ b/src/singleinstance.h
@@ -0,0 +1,81 @@
+/*
+ * 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 "launchselectors.h"
+
+class QLocalServer;
+
+/// Makes a launch either the running instance or a messenger to it (item 200).
+///
+/// **This is not network protocol work.** A QLocalServer is a unix domain
+/// socket between two copies of this program, owned by the user, in the user's
+/// own state directory. The rule in AGENTS.md is about IMAP and SMTP.
+///
+/// Knows nothing about queries, accounts or mail: it carries a LaunchSelectors
+/// from one process to another and emits what arrived.
+class SingleInstance : public QObject
+{
+    Q_OBJECT
+
+public:
+    /// \p socketPath is a filesystem path, so a test can point it inside a
+    /// QTemporaryDir and never touch the user's real one.
+    explicit SingleInstance(const QString &socketPath,
+                            QObject *parent = nullptr);
+    ~SingleInstance() override;
+
+    /// Tries to become the instance others talk to.
+    ///
+    /// Returns true when this process is now listening, false when another
+    /// instance already is OR when no socket could be created at all. The
+    /// caller treats both falses the same way for the second case: **a socket
+    /// that cannot be created must not stop the window opening**, or a
+    /// read-only state directory costs the user their mail client.
+    ///
+    /// Handles the stale socket file, which is the ordinary aftermath of a
+    /// crash: it attempts a CONNECTION first, and a refused connection on an
+    /// existing file proves nothing is serving it, so the file may be
+    /// replaced. Connecting first is what stops a live instance being removed
+    /// out from under itself, and it is the ONLY guard: listen() replaces a
+    /// taken path rather than refusing it (see the .cpp).
+    bool tryBecomeServer();
+
+    /// True when tryBecomeServer() succeeded and this process is listening.
+    bool isServer() const;
+
+    /// Sends \p selectors to the running instance. Returns false when there is
+    /// none, or when the write could not be completed.
+    ///
+    /// An EMPTY selector set is still sent: a bare `qtmaildir` against a
+    /// running window means "raise yourself", which is a request and not a
+    /// no-op.
+    bool sendToRunningInstance(const LaunchSelectors &selectors);
+
+signals:
+    /// A later launch handed these over. Emitted on the server side only.
+    void selectorsReceived(const LaunchSelectors &selectors);
+
+private:
+    QString m_socketPath;
+    QLocalServer *m_server = nullptr;
+};