[WFS provider] Fix deadlock when adding WFS layer to project from Python console (fixes #17087)

rouault · rouault · commit 75c41f5e18f3 · 2017-11-09T12:42:13.000+01:00
diff --git a/src/providers/wfs/qgswfsfeatureiterator.cpp b/src/providers/wfs/qgswfsfeatureiterator.cpp
@@ -755,11 +755,25 @@ void QgsWFSThreadedFeatureDownloader::stop()
   }
 }
 
+void QgsWFSThreadedFeatureDownloader::startAndWait()
+{
+  start();
+
+  QMutexLocker locker( &mWaitMutex );
+  while ( !mDownloader )
+  {
+    mWaitCond.wait( &mWaitMutex );
+  }
+}
+
 void QgsWFSThreadedFeatureDownloader::run()
 {
   // We need to construct it in the run() method (i.e. in the new thread)
   mDownloader = new QgsWFSFeatureDownloader( mShared );
-  emit ready();
+  {
+    QMutexLocker locker( &mWaitMutex );
+    mWaitCond.wakeOne();
+  }
   mDownloader->run( true, /* serialize features */
                     0 /* user max features */ );
 }
diff --git a/src/providers/wfs/qgswfsfeatureiterator.h b/src/providers/wfs/qgswfsfeatureiterator.h
@@ -25,6 +25,8 @@
 #include <memory>
 #include <QProgressDialog>
 #include <QPushButton>
+#include <QMutex>
+#include <QWaitCondition>
 
 class QgsWFSProvider;
 class QgsWFSSharedData;
@@ -168,20 +170,21 @@ class QgsWFSThreadedFeatureDownloader: public QThread
     //! Return downloader object
     QgsWFSFeatureDownloader *downloader() { return mDownloader; }
 
+    //! Starts thread and wait for it to be started
+    void startAndWait();
+
     //! Stops (synchronously) the download
     void stop();
 
-  signals:
-    //! Emitted when the thread is ready
-    void ready();
-
   protected:
     //! Inherited from QThread. Starts the download
     void run() override;
 
   private:
     QgsWFSSharedData *mShared;  //!< Mutable data shared between provider and feature sources
     QgsWFSFeatureDownloader *mDownloader = nullptr;
+    QWaitCondition mWaitCond;
+    QMutex mWaitMutex;
 };
 
 class QgsWFSFeatureSource;
diff --git a/src/providers/wfs/qgswfsshareddata.cpp b/src/providers/wfs/qgswfsshareddata.cpp
@@ -540,10 +540,7 @@ int QgsWFSSharedData::registerToCache( QgsWFSFeatureIterator *iterator, const Qg
     mDownloadFinished = false;
     mComputedExtent = QgsRectangle();
     mDownloader = new QgsWFSThreadedFeatureDownloader( this );
-    QEventLoop loop;
-    connect( mDownloader, &QgsWFSThreadedFeatureDownloader::ready, &loop, &QEventLoop::quit );
-    mDownloader->start();
-    loop.exec( QEventLoop::ExcludeUserInputEvents );
+    mDownloader->startAndWait();
   }
   if ( mDownloadFinished )
     return -1;
