[mssql] Fix iteration failure from background threads

We need to defer the database retrieval until iteration begins,
because otherwise we are creating the database in the main thread
and then using it in another thread (blocked in Qt 5.11, dangerous
in Qt < 5.11).

Author: nyalldawson

src/providers/mssql/qgsmssqlfeatureiterator.cpp

@@ -51,21 +51,11 @@ QgsMssqlFeatureIterator::QgsMssqlFeatureIterator( QgsMssqlFeatureSource *source,
 
   BuildStatement( request );
 
-  // connect to the database
-  mDatabase = QgsMssqlProvider::GetDatabase( mSource->mService, mSource->mHost, mSource->mDatabaseName, mSource->mUserName, mSource->mPassword );
-
-  if ( !mDatabase.open() )
-  {
-    QgsDebugMsg( "Failed to open database" );
-    QgsDebugMsg( mDatabase.lastError().text() );
-    return;
-  }
-
-  // create sql query
-  mQuery.reset( new QSqlQuery( mDatabase ) );
-
-  // start selection
-  rewind();
+  // WARNING - we can't obtain the database connection now, as this method should be
+  // run from the main thread, yet iteration can be done in a different thread.
+  // This would result in failure, because QSqlDatabase instances cannot be used
+  // from a different thread where they were created. Instead, we defer creation
+  // of the database until the first feature is fetched.
 }
 
 
@@ -294,6 +284,27 @@ bool QgsMssqlFeatureIterator::fetchFeature( QgsFeature &feature )
 {
   feature.setValid( false );
 
+  if ( !mDatabase.isValid() )
+  {
+    // No existing connection, so set it up now. It's safe to do here as we're now in
+    // the thread were iteration is actually occurring.
+    mDatabase = QgsMssqlProvider::GetDatabase( mSource->mService, mSource->mHost, mSource->mDatabaseName, mSource->mUserName, mSource->mPassword );
+
+    if ( !mDatabase.open() )
+    {
+      QgsDebugMsg( "Failed to open database" );
+      QgsDebugMsg( mDatabase.lastError().text() );
+      return false;
+    }
+
+    // create sql query
+    mQuery.reset( new QSqlQuery( mDatabase ) );
+
+    // start selection
+    if ( !rewind() )
+      return false;
+  }
+
   if ( !mQuery )
     return false;