Fix a nondeterministic test failure of testDontStackTargetPreparationForEditorFunctionality

We fulfilled the `allDocumentsOpened` expectation directly after dispatching the notification to open LibD.swift. This left a very short window in which LibB’s preparation could finish and LibC’s expectation could start before the open notification for LibD.swift was handled, causing an unexpected preparation of LibC.

This fixes the issue first reported as rdar://129698768 and for which I added logging in #1478.

# Conflicts:
#	Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

Author: ahoppen

Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

@@ -692,7 +692,16 @@ final class BackgroundIndexingTests: XCTestCase {
       DidChangeWatchedFilesNotification(changes: [FileEvent(uri: try project.uri(for: "LibA.swift"), type: .changed)])
     )
 
-    // Quickly flip through all files
+    // Quickly flip through all files. The way the test is designed to work is as follows:
+    //  - LibB.swift gets opened and prepared. Preparation is simulated to take a long time until both LibC.swift and
+    //    LibD.swift have been opened.
+    //  - LibC.swift gets opened. This queues preparation of LibC but doesn't cancel preparation of LibB because we
+    //    don't cancel in-progress preparation tasks to guarantee forward progress (see comment at the end of
+    //    `SemanticIndexManager.prepare`).
+    //  - Now LibD.swift gets opened. This cancels preparation of LibC which actually cancels LibC's preparation for
+    //    real because LibC's preparation hasn't started yet (it's only queued).
+    // Thus, the only targets that are being prepared are LibB and LibD, which is checked by the
+    // `ExpectedIndexTaskTracker`.
     _ = try project.openDocument("LibB.swift")
     try await self.fulfillmentOfOrThrow([libBStartedPreparation])
 
@@ -704,6 +713,9 @@ final class BackgroundIndexingTests: XCTestCase {
     _ = try await project.testClient.send(BarrierRequest())
     _ = try project.openDocument("LibD.swift")
 
+    // Send a barrier request to ensure we have finished opening LibD before allowing the preparation of LibB to finish.
+    _ = try await project.testClient.send(BarrierRequest())
+
     allDocumentsOpened.fulfill()
     try await self.fulfillmentOfOrThrow([libDPreparedForEditing])
   }