Fix QFileSystemWatcher::removePath after move operations

Foreword:
- During a file or directory move the inotify id for an
  entity is not changed.
- QFileSystemWatcher implementation uses a QMultiHash for
  mapping an id to its path.

Suppose this filesystem hypothetical directory structure
- A
|--> B
and user watches both A and B directories.

Suppose that the B directory gets moved by calling "mv B B1".
The user receives a directoryChanged event for parent directory
A and scan filesystem for changes. During this scan the
user notices:
- a new directory B1
- a deleted directory B
The user simply invoke QFileSystemWatcher::addPath(B1) and
QFileSystemWatcher::removePath(B).

With the actual implementation the second operation could fail:
- The call QFileSystemWatcher::addPath(B1) insert a duplicated
  records in the QFileSystemWatcher::idToPath
  multihash ( {0, "A"}, {1, "A/B"} {1, "A/B1"}
- The call QFileSystemWatcher::removePath(B) fails because
  - it first retrieves the the id for path B ---> pathToId("A/B")  <-- return 1
  - Then it calls idToPath.take with the id obtain in the
    previous step <--- idToPath.take(1)
This last operation could take the record {1, "A/B1"} instead
of the {1, "A/B"} (because both have the same key) meaning that
the next check "x != path" evaluates to true (making the
removePath function fail).

This patch fixes the problem in the following way:
- Use idToPath.equal_range in order to obtain all paths with
  the given id
- Remove the correct record that match (id, path)
- Prevent the removal of the inotify watch if another record
  with the same id is still present in the multihash (like a
  simple reference counting).

Change-Id: I9c8480b2a869d91e500af5c4aded596b9aa53b46
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/io/qfilesystemwatcher_inotify.cpp

@@ -330,13 +330,24 @@ QStringList QInotifyFileSystemWatcherEngine::removePaths(const QStringList &path
     while (it.hasNext()) {
         QString path = it.next();
         int id = pathToID.take(path);
-        QString x = idToPath.take(id);
-        if (x.isEmpty() || x != path)
+
+        // Multiple paths could be associated to the same watch descriptor
+        // when a file is moved and added with the new name.
+        // So we should find and delete the correct one by using
+        // both id and path
+        auto path_range = idToPath.equal_range(id);
+        auto path_it = std::find(path_range.first, path_range.second, path);
+        if (path_it == idToPath.end())
             continue;
 
+        const ssize_t num_elements = std::distance(path_range.first, path_range.second);
+        idToPath.erase(path_it);
+
+        // If there was only one path associated to the given id we should remove the watch
+        if (num_elements == 1) {
             int wd = id < 0 ? -id : id;
-        // qDebug() << "removing watch for path" << path << "wd" << wd;
             inotify_rm_watch(inotifyFd, wd);
+        }
 
         it.remove();
         if (id < 0) {