[WFS provider] Fix excessive memory consumption on big layers (refs #18740)

There was a pseudo memory leak in qgsgml.cpp
And the WFS downloader could also have to process big replies, causing a
lot of features to be instanciated at once.

Was seen on the 'portugal_addresses' layer of http://www.naturalgis.pt/cgi-bin/opendata/mapserv?

Backport of
0addae5
c3c5c97
6cf1c50

Author: rouault

src/core/qgsgml.cpp

@@ -856,7 +856,7 @@ void QgsGmlStreamingParser::endElement( const XML_Char* el )
   const int localNameLen = ( pszSep ) ? ( int )( elLen - nsLen ) - 1 : elLen;
   ParseMode theParseMode( mParseModeStack.isEmpty() ? none : mParseModeStack.top() );
 
-  mDimension = mDimensionStack.isEmpty() ? 0 : mDimensionStack.top() ;
+  mDimension = mDimensionStack.isEmpty() ? 0 : mDimensionStack.pop() ;
 
   const bool isGMLNS = ( nsLen == mGMLNameSpaceURI.size() && mGMLNameSpaceURIPtr && memcmp( el, mGMLNameSpaceURIPtr, nsLen ) == 0 );
 

src/providers/wfs/qgswfsfeatureiterator.cpp

@@ -436,6 +436,7 @@ void QgsWFSFeatureDownloader::run( bool serializeFeatures, int maxFeatures )
   int pagingIter = 1;
   QString gmlIdFirstFeatureFirstIter;
   bool disablePaging = false;
+  // Top level loop to do feature paging in WFS 2
   while ( true )
   {
     success = true;
@@ -456,9 +457,14 @@ void QgsWFSFeatureDownloader::run( bool serializeFeatures, int maxFeatures )
              false /* cache */ );
 
     int featureCountForThisResponse = 0;
+    bool bytesStillAvailableInReply = false;
+    // Loop until there is no data coming from the current request
     while ( true )
     {
-      loop.exec( QEventLoop::ExcludeUserInputEvents );
+      if ( !bytesStillAvailableInReply )
+      {
+        loop.exec( QEventLoop::ExcludeUserInputEvents );
+      }
       if ( mStop )
       {
         interrupted = true;
@@ -475,7 +481,10 @@ void QgsWFSFeatureDownloader::run( bool serializeFeatures, int maxFeatures )
       bool finished = false;
       if ( mReply )
       {
-        data = mReply->readAll();
+        // Limit the number of bytes to process at once, to avoid the GML parser to
+        // create too many objects.
+        data = mReply->read( 10 * 1024 * 1024 );
+        bytesStillAvailableInReply = mReply->bytesAvailable() > 0;
       }
       else
       {

src/providers/wfs/qgswfsrequest.cpp

@@ -24,6 +24,8 @@
 #include <QNetworkCacheMetaData>
 #include <QCryptographicHash> // just for testin file:// fake_qgis_http_endpoint hack
 
+const qint64 READ_BUFFER_SIZE_HINT = 1024 * 1024;
+
 QgsWFSRequest::QgsWFSRequest( const QString& theUri )
     : mUri( theUri )
     , mReply( nullptr )
@@ -116,6 +118,7 @@ bool QgsWFSRequest::sendGET( const QUrl& url, bool synchronous, bool forceRefres
   }
 
   mReply = QgsNetworkAccessManager::instance()->get( request );
+  mReply->setReadBufferSize( READ_BUFFER_SIZE_HINT );
   if ( !mUri.auth().setAuthorizationReply( mReply ) )
   {
     mErrorCode = QgsWFSRequest::NetworkError;
@@ -167,6 +170,7 @@ bool QgsWFSRequest::sendPOST( const QUrl& url, const QString& contentTypeHeader,
   request.setHeader( QNetworkRequest::ContentTypeHeader, contentTypeHeader );
 
   mReply = QgsNetworkAccessManager::instance()->post( request, data );
+  mReply->setReadBufferSize( READ_BUFFER_SIZE_HINT );
   if ( !mUri.auth().setAuthorizationReply( mReply ) )
   {
     mErrorCode = QgsWFSRequest::NetworkError;
@@ -257,6 +261,7 @@ void QgsWFSRequest::replyFinished()
 
           QgsDebugMsg( QString( "redirected: %1 forceRefresh=%2" ).arg( redirect.toString() ).arg( mForceRefresh ) );
           mReply = QgsNetworkAccessManager::instance()->get( request );
+          mReply->setReadBufferSize( READ_BUFFER_SIZE_HINT );
           if ( !mUri.auth().setAuthorizationReply( mReply ) )
           {
             mResponse.clear();