QmlJS: Destroy local snapshot before modifying its original

If we keep the copy of the snapshot around while it's being modified, we
trigger the copy-on-write mechanism. That is expensive, and destroying
the snapshot afterwards is also expensive.

Task-number: QTCREATORBUG-25899
Change-Id: I9b7e26baf63a4b47c85457e5657fee971a6ce132
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/qmljs/qmljsmodelmanagerinterface.cpp

@@ -977,16 +977,21 @@ void ModelManagerInterface::parseLoop(QSet<QString> &scannedPaths,
             }
         }
 #endif
-        // update snapshot. requires synchronization, but significantly reduces amount of file
-        // system queries for library imports because queries are cached in libraryInfo
-        const Snapshot snapshot = modelManager->snapshot();
-
         // get list of referenced files not yet in snapshot or in directories already scanned
         QStringList importedFiles;
+
+        // update snapshot. requires synchronization, but significantly reduces amount of file
+        // system queries for library imports because queries are cached in libraryInfo
+        {
+            // Make sure the snapshot is destroyed before updateDocument, so that we don't trigger
+            // the copy-on-write mechanism on its internals.
+            const Snapshot snapshot = modelManager->snapshot();
+
             findNewImplicitImports(doc, snapshot, &importedFiles, &scannedPaths);
             findNewFileImports(doc, snapshot, &importedFiles, &scannedPaths);
             findNewLibraryImports(doc, snapshot, modelManager, &importedFiles, &scannedPaths,
                                   &newLibraries);
+        }
 
         // add new files to parse list
         for (const QString &file : qAsConst(importedFiles)) {