Discuss alternatives to polling all the recent files every 2 seconds.

Somebody asked about repeated SMB traffic when running the Qt Wireshark:

https://ask.wireshark.org/questions/45036/permanent-smb2-traffic-with-1997

and that's due to said polling.  On the main desktop platforms (Windows,
OS X, Linux) we can use QFileSystemWatcher plus, on Windows and OS X,
some additional stuff to check for unmounts (and get FreeBSD as a
freebie, as it works the same as OS X), and avoid timeout-based polling.

Change-Id: I3a1d0110fedcdb442ed09c16dab4bf0cb06ca331
Reviewed-on: https://code.wireshark.org/review/10038
Reviewed-by: Guy Harris <guy@alum.mit.edu>

ui/qt/wireshark_application.cpp

@@ -515,6 +515,68 @@ WiresharkApplication::WiresharkApplication(int &argc,  char **argv) :
         capture_icon_.addFile(icon_path);
     }
 
+    //
+    // XXX - this means we try to check for the existence of all files
+    // in the recent list every 2 seconds; that causes noticeable network
+    // traffic if any of them are stored on file servers.
+    //
+    // QFileSystemWatcher should allow us to watch for files being
+    // removed or renamed.  It uses kqueues and EVFILT_VNODE on FreeBSD,
+    // NetBSD, FSEvents on OS X, inotify on Linux if available, and
+    // FindFirstChagneNotification() on Windows.
+    //
+    // For unmounts, however:
+    //
+    // OS X and FreeBSD also support EVFILT_FS events, which notify you
+    // of file system mounts and, more importantly for this case,
+    // *un*mounts.  We'd need to add our own kqueue for that, if
+    // we can check those with QSocketNotifier.
+    //
+    // On Linux, inotify:
+    //
+    //    http://man7.org/linux/man-pages/man7/inotify.7.html
+    //
+    // appears to deliver "filesystem containing watched object was
+    // unmounted" events.  It looks as if Qt turns them into "changed"
+    // events.
+    //
+    // On Windows, I think our main loop should receive a WM_DEVICECHANGE
+    // Windows message with DBT_DEVICEREMOVECOMPLETE if an unmount occurs
+    // - even for network devices.  We can use the winEvent method of the
+    // QWidget for the top-level window to get Windows messages.
+    //
+    // For platforms that have file change notification mechanisms not
+    // supported by QFileSystemWatcher:
+    //
+    // Solaris 11 has File Events Notification on event ports:
+    //
+    //    https://blogs.oracle.com/praks/entry/file_events_notification
+    //
+    // and unmount events are delivered (see port_associate(3C)).  I'm
+    // guessing that select()/poll() work on event port file descriptors.
+    //
+    // On AIX 6.1 or later, we could use modDir events, in the AIX Event
+    // Infrastructure, on the containing directories for the files; it
+    // uses file descriptors, so the main loop should be able to handle
+    // it.  There's no indication that unmount events are delivered,
+    // however.
+    //
+    // For dead UN*Xes, IRIX has the imon(7m) device and the file access
+    // monitoring daemon that uses it, but it doesn't appear to
+    // deliver rename or unmount events.  I don't know what Tru64
+    // UNIX has, if anything.
+    //
+    // So we should probably use QFileSystemWatcher, in combination
+    // with EVFILT_FS events on systems with EVFILT_FS defined in a
+    // <sys/event.h> header, and in combination with WM_DEVICECHANGE
+    // message on Windows.  On other platforms, Qt will use polling
+    // in QFileSystemWatcher.  We couldn't use anything else on AIX
+    // or HP-UX, as they don't have a mechanism to get unmount events
+    // (and HP-UX doesn't appear to have any file change event
+    // mechanism); it'd be nice if Qt supported Solaris 11 event ports,
+    // but we wouldn't *lose* any functionality on Solaris with this
+    // change.
+    //
     recent_timer_.setParent(this);
     connect(&recent_timer_, SIGNAL(timeout()), this, SLOT(refreshRecentFiles()));
     recent_timer_.start(2000);