[LyX/2.3.x] Fix segfault after deleting monitor

[Scott Kostyshak]

commit a8ad4002a06a42d395de2595c5c3254256bccd10
Author: Guillaume MM <g...@lyx.org>
Date:   Tue Nov 13 23:10:35 2018 +0100

    Fix segfault after deleting monitor
    
    The boost signal was sent synchronously, and so made the Qt signal to be 
posted
    in FileMonitor::changed after the boost signal returned, so after the 
sender was
    possibly destroyed.
    
    The solution is to make the boost signal asynchronous using the Qt event 
loop.
    
    Thanks to Scott Kostyshak for the report and MWE.
    
    (cherry picked from commit 131f4b92bac3ecb75b47c266dfa3d8543bd4d578)
---
 src/insets/RenderPreview.cpp |    1 +
 src/support/FileMonitor.cpp  |   17 ++++++++---------
 src/support/FileMonitor.h    |    7 -------
 3 files changed, 9 insertions(+), 16 deletions(-)

diff --git a/src/insets/RenderPreview.cpp b/src/insets/RenderPreview.cpp
index df0ac64..a216250 100644
--- a/src/insets/RenderPreview.cpp
+++ b/src/insets/RenderPreview.cpp
@@ -304,6 +304,7 @@ void RenderMonitoredPreview::startMonitoring() const
 {
        if (!monitoring()) {
                monitor_ = FileSystemWatcher::activeMonitor(filename_);
+               // Disconnected at the same time as this is destroyed.
                monitor_->connect([this](bool /* exists */){ changed_(); });
        }
 }
diff --git a/src/support/FileMonitor.cpp b/src/support/FileMonitor.cpp
index c703d2b..a3b17ad 100644
--- a/src/support/FileMonitor.cpp
+++ b/src/support/FileMonitor.cpp
@@ -176,7 +176,14 @@ FileMonitor::FileMonitor(std::shared_ptr<FileMonitorGuard> 
monitor)
 
 void FileMonitor::connectToFileMonitorGuard()
 {
+       // Connections need to be asynchronous because the receiver can own this
+       // object and therefore is allowed to delete it.
+       // Qt signal:
        QObject::connect(monitor_.get(), SIGNAL(fileChanged(bool)),
+                        this, SIGNAL(fileChanged(bool)),
+                        Qt::QueuedConnection);
+       // Boost signal:
+       QObject::connect(this, SIGNAL(fileChanged(bool)),
                         this, SLOT(changed(bool)));
 }
 
@@ -187,18 +194,10 @@ signals2::connection FileMonitor::connect(slot const & 
slot)
 }
 
 
-void FileMonitor::disconnect()
-{
-       fileChanged_.disconnect_all_slots();
-       QObject::disconnect(this, SIGNAL(fileChanged(bool)));
-}
-
-
 void FileMonitor::changed(bool const exists)
 {
        // emit boost signal
        fileChanged_(exists);
-       Q_EMIT fileChanged(exists);
 }
 
 
@@ -244,7 +243,7 @@ void ActiveFileMonitor::checkModified()
                }
        }
        if (changed)
-               FileMonitor::changed(exists);
+               Q_EMIT FileMonitor::fileChanged(exists);
        QTimer::singleShot(interval_, this, SLOT(clearCooldown()));
 }
 
diff --git a/src/support/FileMonitor.h b/src/support/FileMonitor.h
index 14aa834..07a8dc5 100644
--- a/src/support/FileMonitor.h
+++ b/src/support/FileMonitor.h
@@ -60,10 +60,6 @@ typedef std::unique_ptr<ActiveFileMonitor> 
ActiveFileMonitorPtr;
 ///   monitor.connect(...);
 /// (stops watching the first)
 ///
-/// Reset connections:
-///   monitor.disconnect();
-///   or the disconnect method of the connection object for the boost signal.
-///
 class FileSystemWatcher
 {
 public:
@@ -137,9 +133,6 @@ public:
        typedef sig::slot_type slot;
        /// Connect and you'll be informed when the file has changed.
        signals2::connection connect(slot const &);
-       /// disconnect all slots connected to the boost signal fileChanged_ or 
to
-       /// the qt signal fileChanged()
-       void disconnect();
        /// absolute path being tracked
        std::string const & filename() { return monitor_->filename(); }
        /// Make sure the good file is being monitored, after e.g. a move or a