Don’t block the generation of a build system by build graph generation

We currently load the entire package before generating a `SwiftPMBuildSystem`. That means that the initialize request to SourceKit-LSP is blocked until the package has been loaded, preventing us from offering any sort of functionality, including syntactic functionality like formatting.

Decouple build system creation and build graph generation (aka. package loading for SwiftPM). We can operate with fallback build settings until the build graph has been loaded and reopen the document once the proper build settings are available.

rdar://126644596

Author: ahoppen

Sources/BuildSystemIntegration/BuildServerBuildSystem.swift

@@ -281,7 +281,9 @@ extension BuildServerBuildSystem: BuildSystem {
     return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
   }
 
-  package func generateBuildGraph(allowFileSystemWrites: Bool) {}
+  package func generateBuildGraph() {}
+
+  package func waitForUpToDateBuildGraph() async {}
 
   package func topologicalSort(of targets: [ConfiguredTarget]) async -> [ConfiguredTarget]? {
     return nil

Sources/BuildSystemIntegration/BuildSystem.swift

@@ -138,13 +138,10 @@ package protocol BuildSystem: AnyObject, Sendable {
   func configuredTargets(for document: DocumentURI) async -> [ConfiguredTarget]
 
   /// Re-generate the build graph.
-  ///
-  /// If `allowFileSystemWrites` is `true`, this should include all the tasks that are necessary for building the entire
-  /// build graph, like resolving package versions.
-  ///
-  /// If `allowFileSystemWrites` is `false`, no files must be written to disk. This mode is used to determine whether
-  /// the build system can handle a source file, and decide whether a workspace should be opened with this build system
-  func generateBuildGraph(allowFileSystemWrites: Bool) async throws
+  func generateBuildGraph() async throws
+
+  /// Wait until the build graph has been loaded.
+  func waitForUpToDateBuildGraph() async
 
   /// Sort the targets so that low-level targets occur before high-level targets.
   ///

Sources/BuildSystemIntegration/BuildSystemManager.swift

@@ -265,8 +265,12 @@ extension BuildSystemManager {
     return settings
   }
 
-  package func generateBuildGraph(allowFileSystemWrites: Bool) async throws {
-    try await self.buildSystem?.generateBuildGraph(allowFileSystemWrites: allowFileSystemWrites)
+  package func generateBuildGraph() async throws {
+    try await self.buildSystem?.generateBuildGraph()
+  }
+
+  package func waitForUpToDateBuildGraph() async {
+    await self.buildSystem?.waitForUpToDateBuildGraph()
   }
 
   package func topologicalSort(of targets: [ConfiguredTarget]) async throws -> [ConfiguredTarget]? {

Sources/BuildSystemIntegration/CompilationDatabaseBuildSystem.swift

@@ -133,7 +133,9 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     throw PrepareNotSupportedError()
   }
 
-  package func generateBuildGraph(allowFileSystemWrites: Bool) {}
+  package func generateBuildGraph() {}
+
+  package func waitForUpToDateBuildGraph() async {}
 
   package func topologicalSort(of targets: [ConfiguredTarget]) -> [ConfiguredTarget]? {
     return nil

Sources/BuildSystemIntegration/SwiftPMBuildSystem.swift

@@ -129,6 +129,10 @@ package actor SwiftPMBuildSystem {
 
   private let testHooks: SwiftPMTestHooks
 
+  /// The queue on which we reload the package to ensure we don't reload it multiple times concurrently, which can cause
+  /// issues in SwiftPM.
+  private let packageLoadingQueue = AsyncQueue<Serial>()
+
   /// Delegate to handle any build system events.
   package weak var delegate: BuildSystemIntegration.BuildSystemDelegate? = nil
 
@@ -370,7 +374,14 @@ package actor SwiftPMBuildSystem {
 extension SwiftPMBuildSystem {
   /// (Re-)load the package settings by parsing the manifest and resolving all the targets and
   /// dependencies.
-  package func reloadPackage(forceResolvedVersions: Bool) async throws {
+  package func reloadPackage() async throws {
+    try await packageLoadingQueue.asyncThrowing {
+      try await self.reloadPackageImpl()
+    }.valuePropagatingCancellation
+  }
+
+  /// - Important: Must only be called on `packageLoadingQueue`.
+  private func reloadPackageImpl() async throws {
     await reloadPackageStatusCallback(.start)
     await testHooks.reloadPackageDidStart?()
     defer {
@@ -382,7 +393,7 @@ extension SwiftPMBuildSystem {
 
     let modulesGraph = try await self.workspace.loadPackageGraph(
       rootInput: PackageGraphRootInput(packages: [AbsolutePath(projectRoot)]),
-      forceResolvedVersions: forceResolvedVersions,
+      forceResolvedVersions: !isForIndexBuild,
       observabilityScope: observabilitySystem.topScope
     )
 
@@ -547,8 +558,12 @@ extension SwiftPMBuildSystem: BuildSystemIntegration.BuildSystem {
     return []
   }
 
-  package func generateBuildGraph(allowFileSystemWrites: Bool) async throws {
-    try await self.reloadPackage(forceResolvedVersions: !isForIndexBuild || !allowFileSystemWrites)
+  package func generateBuildGraph() async throws {
+    try await self.reloadPackage()
+  }
+
+  package func waitForUpToDateBuildGraph() async {
+    await self.packageLoadingQueue.async {}.valuePropagatingCancellation
   }
 
   package func topologicalSort(of targets: [ConfiguredTarget]) -> [ConfiguredTarget]? {

Sources/SemanticIndex/SemanticIndexManager.swift

@@ -266,9 +266,7 @@ package final actor SemanticIndexManager {
           signposter.endInterval("Preparing", state)
         }
         await testHooks.buildGraphGenerationDidStart?()
-        await orLog("Generating build graph") {
-          try await self.buildSystemManager.generateBuildGraph(allowFileSystemWrites: true)
-        }
+        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()