create postgres RO connection in the thread where the provider live

python/core/auto_generated/providers/qgsdataprovider.sip.in

@@ -79,6 +79,7 @@ Abstract base class for spatial data provider implementations.
       SkipFullScan,
       ForceReadOnly,
       SkipCredentialsRequest,
+      ParallelThreadLoading,
     };
     typedef QFlags<QgsDataProvider::ReadFlag> ReadFlags;
 

src/core/project/qgsproject.cpp

@@ -1368,6 +1368,7 @@ void QgsProject::preloadProviders( const QVector<QDomNode> &parallelLayerNodes,
 
     // Requesting credential from worker thread could lead to deadlocks because the main thread is waiting for worker thread to fininsh
     layerToLoad.flags.setFlag( QgsDataProvider::SkipCredentialsRequest, true );
+    layerToLoad.flags.setFlag( QgsDataProvider::ParallelThreadLoading, true );
 
     layersToLoad.insert( layerToLoad.layerId, layerToLoad );
   }
@@ -1426,6 +1427,7 @@ void QgsProject::preloadProviders( const QVector<QDomNode> &parallelLayerNodes,
       const LayerToLoad &lay =  it.value();
       QgsDataProvider::ReadFlags providerFlags = lay.flags;
       providerFlags.setFlag( QgsDataProvider::SkipCredentialsRequest, false );
+      providerFlags.setFlag( QgsDataProvider::ParallelThreadLoading, false );
       QgsScopedRuntimeProfile profile( "Create data providers/" + lay.layerId, QStringLiteral( "projectload" ) );
       std::unique_ptr<QgsDataProvider> provider( QgsProviderRegistry::instance()->createProvider( lay.provider, lay.dataSource, lay.options, providerFlags ) );
       i++;

src/core/providers/qgsdataprovider.h

@@ -128,6 +128,7 @@ class CORE_EXPORT QgsDataProvider : public QObject
       SkipFullScan = 1 << 4, //!< Skip expensive full scan on files (i.e. on delimited text) (since QGIS 3.24)
       ForceReadOnly = 1 << 5, //!< Open layer in a read-only mode (since QGIS 3.28)
       SkipCredentialsRequest =  1 << 6, //! Skip credentials if the provided one are not valid, let the provider be invalid, avoiding to block the thread creating the provider if it is not the main thread (since QGIS 3.32).
+      ParallelThreadLoading = 1 << 7, //! Provider is created in a parallel thread than the one where it will live (since QGIS 3.32.1).
     };
     Q_DECLARE_FLAGS( ReadFlags, ReadFlag )
 

src/providers/postgres/qgspostgresprovider.cpp

@@ -303,6 +303,11 @@ QgsPostgresProvider::QgsPostgresProvider( QString const &uri, const ProviderOpti
 
   mLayerMetadata.setType( QStringLiteral( "dataset" ) );
   mLayerMetadata.setCrs( crs() );
+
+  // Constructor is called in another thread than the thread where the provider will live,
+  // so we disconnect the DB, connection will be done later in the provider thread when needed
+  if ( flags.testFlag( QgsDataProvider::ParallelThreadLoading ) )
+    disconnectDb();
 }
 
 QgsPostgresProvider::~QgsPostgresProvider()
@@ -320,7 +325,13 @@ QgsAbstractFeatureSource *QgsPostgresProvider::featureSource() const
 
 QgsPostgresConn *QgsPostgresProvider::connectionRO() const
 {
-  return mTransaction ? mTransaction->connection() : mConnectionRO;
+  if ( mTransaction )
+    return mTransaction->connection();
+
+  if ( !mConnectionRO )
+    mConnectionRO = QgsPostgresConn::connectDb( mUri, true, true, false, !mReadFlags.testFlag( QgsDataProvider::SkipCredentialsRequest ) );
+
+  return mConnectionRO;
 }
 
 void QgsPostgresProvider::setListening( bool isListening )
@@ -3887,7 +3898,7 @@ QgsRectangle QgsPostgresProvider::extent() const
                         quotedValue( mSchemaName ),
                         quotedValue( mTableName ),
                         quotedValue( mGeometryColumn ) );
-            result = mConnectionRO->LoggedPQexec( "QgsPostgresProvider", sql );
+            result = connectionRO()->LoggedPQexec( "QgsPostgresProvider", sql );
             if ( result.PQresultStatus() == PGRES_TUPLES_OK && result.PQntuples() == 1 && !result.PQgetisnull( 0, 0 ) )
             {
               ext = result.PQgetvalue( 0, 0 );

src/providers/postgres/qgspostgresprovider.h

@@ -450,7 +450,7 @@ class QgsPostgresProvider final: public QgsVectorDataProvider
 
     QString paramValue( const QString &fieldvalue, const QString &defaultValue ) const;
 
-    QgsPostgresConn *mConnectionRO = nullptr ; //!< Read-only database connection (initially)
+    mutable QgsPostgresConn *mConnectionRO = nullptr ; //!< Read-only database connection (initially)
     QgsPostgresConn *mConnectionRW = nullptr ; //!< Read-write database connection (on update)
 
     QgsPostgresConn *connectionRO() const;