Eagerly initialize source to target map

The map is used later from notification panel from
the UI thread and current workflow leads to UI freeze
on source to target map initialization

Author: ujohnny

base/src/META-INF/blaze-base.xml

@@ -1,5 +1,5 @@
 <!--
-  ~ Copyright 2016 The Bazel Authors. All rights reserved.
+  ~ Copyright 2025 The Bazel Authors. All rights reserved.
   ~
   ~ Licensed under the Apache License, Version 2.0 (the "License");
   ~ you may not use this file except in compliance with the License.
@@ -625,6 +625,7 @@
 
   <extensions defaultExtensionNs="com.google.idea.blaze">
     <SyncListener implementation="com.google.idea.blaze.base.sync.SyncCache$ClearSyncCache"/>
+    <SyncListener implementation="com.google.idea.blaze.base.sync.SourceToTargetMapInitializer"/>
     <SyncListener implementation="com.google.idea.blaze.base.run.BlazeRunConfigurationSyncListener"/>
     <SyncListener implementation="com.google.idea.blaze.base.sync.status.BlazeSyncStatusListener" order="first"/>
     <SyncListener implementation="com.google.idea.blaze.base.dependencies.ExternalFileProjectManagementHelper$UpdateNotificationsAfterSync"/>

base/src/com/google/idea/blaze/base/sync/SourceToTargetMapInitializer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2025 The Bazel Authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.idea.blaze.base.sync;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.idea.blaze.base.model.BlazeProjectData;
+import com.google.idea.blaze.base.projectview.ProjectViewSet;
+import com.google.idea.blaze.base.scope.BlazeContext;
+import com.google.idea.blaze.base.settings.BlazeImportSettings;
+import com.google.idea.blaze.base.targetmaps.SourceToTargetMap;
+import com.intellij.openapi.project.Project;
+import java.io.File;
+
+public class SourceToTargetMapInitializer implements SyncListener {
+
+  @Override
+  public void onSyncComplete(Project project, BlazeContext context,
+      BlazeImportSettings importSettings, ProjectViewSet projectViewSet,
+      ImmutableSet<Integer> buildIds, BlazeProjectData blazeProjectData, SyncMode syncMode,
+      SyncResult syncResult) {
+    // eagerly instantiate source to target map
+    SourceToTargetMap.getInstance(project).getRulesForSourceFile(new File(""));
+  }
+}

base/src/com/google/idea/blaze/base/targetmaps/AspectSyncSourceToTargetMap.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 The Bazel Authors. All rights reserved.
+ * Copyright 2025 The Bazel Authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@
 import com.google.idea.blaze.base.sync.data.BlazeProjectDataManager;
 import com.google.idea.blaze.base.sync.workspace.ArtifactLocationDecoder;
 import com.intellij.openapi.project.Project;
+import com.intellij.util.concurrency.annotations.RequiresBackgroundThread;
 import java.io.File;
 import java.util.Objects;
 import javax.annotation.Nullable;
@@ -77,6 +78,7 @@ private synchronized ImmutableMultimap<File, TargetKey> getSourceToTargetMap() {
   }
 
   @SuppressWarnings("unused")
+  @RequiresBackgroundThread
   private static ImmutableMultimap<File, TargetKey> computeSourceToTargetMap(
       Project project, BlazeProjectData blazeProjectData) {
     ArtifactLocationDecoder decoder = blazeProjectData.getArtifactLocationDecoder();