Debounce filesBuildSettingsChanged calls

Sources/BuildSystemIntegration/BuildSystemManager.swift

@@ -357,6 +357,22 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     }
   }
 
+  /// Debounces calls to `delegate.fileBuildSettingsChanged`.
+  ///
+  /// This helps in the following situation: A build system takes 5s to return build settings for a file and we have 10
+  /// requests for those build settings coming in that time period. Once we get build settings, we get 10 calls to
+  /// `resultReceivedAfterTimeout` in `buildSettings(for:in:language:fallbackAfterTimeout:)`, all for the same document.
+  /// But calling `fileBuildSettingsChanged` once is totally sufficient.
+  ///
+  /// Force-unwrapped optional because initializing it requires access to `self`.
+  private var filesBuildSettingsChangedDebouncer: Debouncer<Set<DocumentURI>>! = nil {
+    didSet {
+      // Must only be set once
+      precondition(oldValue == nil)
+      precondition(filesBuildSettingsChangedDebouncer != nil)
+    }
+  }
+
   private var cachedAdjustedSourceKitOptions = RequestCache<TextDocumentSourceKitOptionsRequest>()
 
   private var cachedBuildTargets = Cache<WorkspaceBuildTargetsRequest, [BuildTargetIdentifier: BuildTargetInfo]>()
@@ -421,6 +437,24 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
       }
     }
 
+    // We don't need a large debounce duration here. It just needs to be big enough to accumulate
+    // `resultReceivedAfterTimeout` calls for the same document (see comment on `filesBuildSettingsChangedDebouncer`).
+    // Since they should all come in at the same time, a couple of milliseconds should be sufficient here, an 20ms be
+    // plenty while still not causing a noticeable delay to the user.
+    self.filesBuildSettingsChangedDebouncer = Debouncer(
+      debounceDuration: .milliseconds(20),
+      combineResults: { $0.union($1) }
+    ) {
+      [weak self] (filesWithChangedBuildSettings) in
+      guard let self, let delegate = await self.delegate else {
+        logger.fault("Not calling fileBuildSettingsChanged because no delegate exists in SwiftPMBuildSystem")
+        return
+      }
+      if !filesWithChangedBuildSettings.isEmpty {
+        await delegate.fileBuildSettingsChanged(filesWithChangedBuildSettings)
+      }
+    }
+
     // TODO: Forward file watch patterns from this initialize request to the client
     // (https://github.com/swiftlang/sourcekit-lsp/issues/1671)
     initializeResult = Task { () -> InitializeBuildResponse? in
@@ -579,7 +613,7 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     self.cachedSourceFilesAndDirectories.clearAll(isolation: self)
 
     await delegate?.buildTargetsChanged(notification.changes)
-    await delegate?.fileBuildSettingsChanged(Set(watchedFiles.keys))
+    await filesBuildSettingsChangedDebouncer.scheduleCall(Set(watchedFiles.keys))
   }
 
   private func logMessage(notification: BuildServerProtocol.OnBuildLogMessageNotification) async {
@@ -837,7 +871,7 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
           try await withTimeout(options.buildSettingsTimeoutOrDefault) {
             return try await self.buildSettingsFromBuildSystem(for: document, in: target, language: language)
           } resultReceivedAfterTimeout: {
-            await self.delegate?.fileBuildSettingsChanged([document])
+            await self.filesBuildSettingsChangedDebouncer.scheduleCall([document])
           }
         } else {
           try await self.buildSettingsFromBuildSystem(for: document, in: target, language: language)
@@ -895,7 +929,7 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
             try await withTimeout(options.buildSettingsTimeoutOrDefault) {
               await self.canonicalTarget(for: mainFile)
             } resultReceivedAfterTimeout: {
-              await self.delegate?.fileBuildSettingsChanged([document])
+              await self.filesBuildSettingsChangedDebouncer.scheduleCall([document])
             }
           }
         var languageForFile: Language
@@ -955,6 +989,10 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     }
     // Handle any messages the build system might have sent us while updating.
     await messageHandlingQueue.async(metadata: .stateChange) {}.valuePropagatingCancellation
+
+    // Ensure that we send out all delegate calls so that everybody is informed about the changes.
+    await filesBuildSettingsChangedDebouncer.flush()
+    await filesDependenciesUpdatedDebouncer.flush()
   }
 
   /// The root targets of the project have depth of 0 and all target dependencies have a greater depth than the target

Sources/SKUtilities/Debouncer.swift

@@ -12,7 +12,7 @@
 
 /// Debounces calls to a function/closure. If multiple calls to the closure are made, it allows aggregating the
 /// parameters.
-package actor Debouncer<Parameter> {
+package actor Debouncer<Parameter: Sendable> {
   /// How long to wait for further `scheduleCall` calls before committing to actually calling `makeCall`.
   private let debounceDuration: Duration
 
@@ -36,7 +36,7 @@ package actor Debouncer<Parameter> {
   /// call can be debounced), the task that would commit the call (unless cancelled), the parameter with which this
   /// call should be made and the time at which the call should be made. Keeping track of the time ensures that we don't
   /// indefinitely debounce if a new `scheduleCall` is made every 0.4s but we debounce for 0.5s.
-  private var inProgressData: (Parameter, ContinuousClock.Instant, Task<Void, Never>)?
+  private var inProgressData: (parameter: Parameter, targetDate: ContinuousClock.Instant, task: Task<Void, Never>)?
 
   package init(
     debounceDuration: Duration,
@@ -66,11 +66,20 @@ package actor Debouncer<Parameter> {
       } catch {
         return
       }
-      inProgressData = nil
-      await makeCall(parameter)
+      await self.flush()
     }
     inProgressData = (parameter, ContinuousClock.now + debounceDuration, task)
   }
+
+  /// If any debounced calls are in progress, make them now, even if the debounce duration hasn't expired yet.
+  package func flush() async {
+    guard let inProgressDataValue = inProgressData else {
+      return
+    }
+    inProgressData = nil
+    let parameter = inProgressDataValue.parameter
+    await makeCall(parameter)
+  }
 }
 
 extension Debouncer<Void> {