Don’t poll the index for unit changes on every filesDidChange call

`pollForUnitChangesAndWait` is a costly operation since it iterates through all the unit files on the file system. We should only run it during initial indexing (where we might need to build the initial indexstore-db) but not for any subsequent indexing calls.

Author: ahoppen

Sources/SemanticIndex/SemanticIndexManager.swift

@@ -54,7 +54,7 @@ private struct OpaqueQueuedIndexTask: Equatable {
 
 private enum InProgressIndexStore {
   /// We are waiting for preparation of the file's target to be scheduled. The next step is that we wait for
-  /// prepration to finish before we can update the index store for this file.
+  /// preparation to finish before we can update the index store for this file.
   ///
   /// `preparationTaskID` identifies the preparation task so that we can transition a file's index state to
   /// `updatingIndexStore` when its preparation task has finished.
@@ -273,18 +273,25 @@ package final actor SemanticIndexManager {
   /// build system that don't currently have a unit with a timestamp that matches the mtime of the file.
   ///
   /// If `filesToIndex` is `nil`, all files in the build system with out-of-date units are indexed.
+  ///
+  /// If `ensureAllUnitsRegisteredInIndex` is `true`, ensure that all units are registered in the index before
+  /// triggering the indexing. This is a costly operation since it iterates through all the unit files on the file
+  /// system but if existing unit files are not known to the index, we might re-index those files even if they are
+  /// up-to-date. Generally this should be set to `true` during the initial indexing (in which case we might be need to
+  /// build the indexstore-db) and `false` for all subsequent indexing.
   package func scheduleBuildGraphGenerationAndBackgroundIndexAllFiles(
     filesToIndex: [DocumentURI]?,
+    ensureAllUnitsRegisteredInIndex: Bool,
     indexFilesWithUpToDateUnit: Bool
   ) async {
     let taskId = UUID()
     let generateBuildGraphTask = Task(priority: .low) {
       await withLoggingSubsystemAndScope(subsystem: indexLoggingSubsystem, scope: "build-graph-generation") {
         await hooks.buildGraphGenerationDidStart?()
         await self.buildSystemManager.waitForUpToDateBuildGraph()
-        // Ensure that we have an up-to-date indexstore-db. Waiting for the indexstore-db to be updated is cheaper than
-        // potentially not knowing about unit files, which causes the corresponding source files to be re-indexed.
-        index.pollForUnitChangesAndWait()
+        if ensureAllUnitsRegisteredInIndex {
+          index.pollForUnitChangesAndWait()
+        }
         await hooks.buildGraphGenerationDidFinish?()
         // TODO: Ideally this would be a type like any Collection<DocumentURI> & Sendable but that doesn't work due to
         // https://github.com/swiftlang/swift/issues/75602
@@ -324,7 +331,11 @@ package final actor SemanticIndexManager {
   package func scheduleReindex() async {
     await indexStoreUpToDateTracker.markAllKnownOutOfDate()
     await preparationUpToDateTracker.markAllKnownOutOfDate()
-    await scheduleBuildGraphGenerationAndBackgroundIndexAllFiles(filesToIndex: nil, indexFilesWithUpToDateUnit: true)
+    await scheduleBuildGraphGenerationAndBackgroundIndexAllFiles(
+      filesToIndex: nil,
+      ensureAllUnitsRegisteredInIndex: false,
+      indexFilesWithUpToDateUnit: true
+    )
   }
 
   private func waitForBuildGraphGenerationTasks() async {
@@ -412,6 +423,7 @@ package final actor SemanticIndexManager {
 
     await scheduleBuildGraphGenerationAndBackgroundIndexAllFiles(
       filesToIndex: changedFiles,
+      ensureAllUnitsRegisteredInIndex: false,
       indexFilesWithUpToDateUnit: false
     )
   }

Sources/SourceKitLSP/Workspace.swift

@@ -190,6 +190,7 @@ package final class Workspace: Sendable, BuildSystemManagerDelegate {
     if let semanticIndexManager {
       await semanticIndexManager.scheduleBuildGraphGenerationAndBackgroundIndexAllFiles(
         filesToIndex: nil,
+        ensureAllUnitsRegisteredInIndex: true,
         indexFilesWithUpToDateUnit: false
       )
     }