Fix updating QgsApplication::libraryPaths; load before any Qt plugin use

Previously, it was happening after setting the default window icon.

For macOS, refactor libraryPaths update to ensure both QT_PLUGIN_PATH
and qt.conf are properly honored (fixes 4.5 year old bug), with
prioritization of qgis libs or libs shipped with .app bundle.

Author: dakcarto

src/app/main.cpp

@@ -867,6 +867,90 @@ int main( int argc, char *argv[] )
     }
   }
 
+  // Redefine QgsApplication::libraryPaths as necessary.
+  // IMPORTANT: Do *after* QgsApplication myApp(...), but *before* Qt uses any plugins,
+  //            e.g. loading splash screen, setting window icon, etc.
+  //            Always honor QT_PLUGIN_PATH env var or qt.conf, which will
+  //            be part of libraryPaths just after QgsApplication creation.
+#ifdef Q_OS_WIN
+  // For non static builds on win (static builds are not supported)
+  // we need to be sure we can find the qt image plugins.
+  QCoreApplication::addLibraryPath( QApplication::applicationDirPath()
+                                    + QDir::separator() + "qtplugins" );
+#endif
+#ifdef Q_OS_MAC
+  // Resulting libraryPaths has critical QGIS plugin paths first, then any Qt plugin paths, then
+  // any dev-defined paths (in app's qt.conf) and/or user-defined paths (QT_PLUGIN_PATH env var).
+  //
+  // NOTE: Minimizes, though does not fully protect against, crashes due to dev/user-defined libs
+  //       built against a different Qt/QGIS, while still allowing custom C++ plugins to load.
+  QStringList libPaths( QCoreApplication::libraryPaths() );
+
+  QgsDebugMsgLevel( QStringLiteral( "Initial macOS QCoreApplication::libraryPaths: %1" )
+                    .arg( libPaths.join( " " ) ), 4 );
+
+  // Strip all critical paths that should always be prepended
+  if ( libPaths.removeAll( QDir::cleanPath( QgsApplication::pluginPath() ) ) )
+  {
+    QgsDebugMsgLevel( QStringLiteral( "QgsApplication::pluginPath removed from initial libraryPaths" ), 4 );
+  }
+  if ( libPaths.removeAll( QCoreApplication::applicationDirPath() ) )
+  {
+    QgsDebugMsgLevel( QStringLiteral( "QCoreApplication::applicationDirPath removed from initial libraryPaths" ), 4 );
+  }
+  // Prepend path, so a standard Qt bundle directory is parsed
+  QgsDebugMsgLevel( QStringLiteral( "Prepending QCoreApplication::applicationDirPath to libraryPaths" ), 4 );
+  libPaths.prepend( QCoreApplication::applicationDirPath() );
+
+  // Check if we are running in a 'release' app bundle, i.e. contains copied-in
+  // standard Qt-specific plugin subdirectories (ones never created by QGIS, e.g. 'sqldrivers' is).
+  // Note: bundleclicked(...) is inadequate to determine which *type* of bundle was opened, e.g. release or build dir.
+  // An app bundled with QGIS_MACAPP_BUNDLE > 0 is considered a release bundle.
+  QString  relLibPath( QDir::cleanPath( QCoreApplication::applicationDirPath().append( "/../PlugIns" ) ) );
+  // Note: relLibPath becomes the defacto QT_PLUGINS_DIR of a release app bundle
+  if ( QFile::exists( relLibPath + QStringLiteral( "/imageformats" ) )
+       && QFile::exists( relLibPath + QStringLiteral( "/codecs" ) ) )
+  {
+    // We are in a release app bundle.
+    // Strip QT_PLUGINS_DIR because it will crash a launched release app bundle, since
+    // the appropriate Qt frameworks and plugins have been copied into the bundle.
+    if ( libPaths.removeAll( QT_PLUGINS_DIR ) )
+    {
+      QgsDebugMsgLevel( QStringLiteral( "QT_PLUGINS_DIR removed from initial libraryPaths" ), 4 );
+    }
+    // Prepend the Plugins path, so copied-in Qt plugin bundle directories are parsed.
+    QgsDebugMsgLevel( QStringLiteral( "Prepending <bundle>/Plugins to libraryPaths" ), 4 );
+    libPaths.prepend( relLibPath );
+
+    // TODO: see if this or another method can be used to avoid QCA's install prefix plugins
+    //       from being parsed and loaded (causes multi-Qt-loaded errors when bundled Qt should
+    //       be the only one loaded). QCA core (> v2.1.3) needs an update first.
+    //setenv( "QCA_PLUGIN_PATH", relLibPath.toUtf8().constData(), 1 );
+  }
+  else
+  {
+    // We are either running from build dir bundle, or launching Mach-O binary directly.
+    // Add system Qt plugins, since they are not bundled, and not always referenced by default.
+    // An app bundled with QGIS_MACAPP_BUNDLE = 0 will still have Plugins/qgis in it.
+    // Note: Don't always prepend.
+    //       User may have already defined it in QT_PLUGIN_PATH in a specific order.
+    if ( !libPaths.contains( QT_PLUGINS_DIR ) )
+    {
+      QgsDebugMsgLevel( QStringLiteral( "Prepending QT_PLUGINS_DIR to libraryPaths" ), 4 );
+      libPaths.prepend( QT_PLUGINS_DIR );
+    }
+  }
+
+  QgsDebugMsgLevel( QStringLiteral( "Prepending QgsApplication::pluginPath to libraryPaths" ), 4 );
+  libPaths.prepend( QDir::cleanPath( QgsApplication::pluginPath() ) );
+
+  // Redefine library search paths.
+  QCoreApplication::setLibraryPaths( libPaths );
+
+  QgsDebugMsgLevel( QStringLiteral( "Rewritten macOS QCoreApplication::libraryPaths: %1" )
+                    .arg( QCoreApplication::libraryPaths().join( " " ) ), 4 );
+#endif
+
 #ifdef Q_OS_MAC
   // Set hidpi icons; use SVG icons, as PNGs will be relatively too small
   QCoreApplication::setAttribute( Qt::AA_UseHighDpiPixmaps );
@@ -1100,36 +1184,6 @@ int main( int argc, char *argv[] )
     }
   }
 
-  // For non static builds on mac and win (static builds are not supported)
-  // we need to be sure we can find the qt image
-  // plugins. In mac be sure to look in the
-  // application bundle...
-#ifdef Q_OS_WIN
-  QCoreApplication::addLibraryPath( QApplication::applicationDirPath()
-                                    + QDir::separator() + "qtplugins" );
-#endif
-#ifdef Q_OS_MACX
-  // IMPORTANT: do before Qt uses any plugins, e.g. before loading splash screen
-  QString  myPath( QCoreApplication::applicationDirPath().append( "/../PlugIns" ) );
-  // Check if it contains a standard Qt-specific plugin subdirectory
-  if ( !QFile::exists( myPath + "/imageformats" ) )
-  {
-    // We are either running from build dir bundle, or launching binary directly.
-    // Use system Qt plugins, since they are not bundled.
-    // An app bundled with QGIS_MACAPP_BUNDLE=0 will still have Plugins/qgis in it
-    myPath = QT_PLUGINS_DIR;
-  }
-
-  // First clear the plugin search paths so we can be sure only plugins we define
-  // are being used. Note: this strips QgsApplication::pluginPath()
-  QStringList myPathList;
-  QCoreApplication::setLibraryPaths( myPathList );
-
-  QgsDebugMsg( QString( "Adding Mac QGIS and Qt plugins dirs to search path: %1" ).arg( myPath ) );
-  QCoreApplication::addLibraryPath( QgsApplication::pluginPath() );
-  QCoreApplication::addLibraryPath( myPath );
-#endif
-
   // set authentication database directory
   if ( !authdbdirectory.isEmpty() )
   {