Watch for changes to compile_commands.json and compile_flags.txt

Similar to how we reload the package if Package.swift is changed, we also watch `compilation_database.json` and `compile_flags.txt` and notify the build system delegate that build settings changed if these files are modified.

rdar://92388223  [#486]

Sources/SKCore/CompilationDatabaseBuildSystem.swift

@@ -24,30 +24,58 @@ import struct Foundation.URL
 /// one compilation database, located at the project root.
 public final class CompilationDatabaseBuildSystem {
 
+  /// Queue guarding the following properties:
+  /// - `compdb`
+  /// - `watchedFiles`
+  /// - `_indexStorePath`
+  let queue: DispatchQueue = .init(label: "CompilationDatabaseBuildSystem.queue", qos: .userInitiated)
+
   /// The compilation database.
-  var compdb: CompilationDatabase? = nil
+  var compdb: CompilationDatabase? = nil {
+    didSet {
+      dispatchPrecondition(condition: .onQueue(queue))
+      // Build settings have changed and thus the index store path might have changed.
+      // Recompute it on demand.
+      _indexStorePath = nil
+    }
+  }
 
   /// Delegate to handle any build system events.
   public weak var delegate: BuildSystemDelegate? = nil
 
+  let projectRoot: AbsolutePath?
+
   let fileSystem: FileSystem
 
-  public lazy var indexStorePath: AbsolutePath? = {
-    if let allCommands = self.compdb?.allCommands {
-      for command in allCommands {
-        let args = command.commandLine
-        for i in args.indices.reversed() {
-          if args[i] == "-index-store-path" && i != args.endIndex - 1 {
-            return try? AbsolutePath(validating: args[i+1])
+  /// The URIs for which the delegate has registered for change notifications,
+  /// mapped to the language the delegate specified when registering for change notifications.
+  var watchedFiles: [DocumentURI: Language] = [:]
+
+  private var _indexStorePath: AbsolutePath?
+  public var indexStorePath: AbsolutePath? {
+    return queue.sync {
+      if let indexStorePath = _indexStorePath {
+        return indexStorePath
+      }
+
+      if let allCommands = self.compdb?.allCommands {
+        for command in allCommands {
+          let args = command.commandLine
+          for i in args.indices.reversed() {
+            if args[i] == "-index-store-path" && i != args.endIndex - 1 {
+              _indexStorePath = try? AbsolutePath(validating: args[i+1])
+              return _indexStorePath
+            }
           }
         }
       }
+      return nil
     }
-    return nil
-  }()
+  }
 
   public init(projectRoot: AbsolutePath? = nil, fileSystem: FileSystem = localFileSystem) {
     self.fileSystem = fileSystem
+    self.projectRoot = projectRoot
     if let path = projectRoot {
       self.compdb = tryLoadCompilationDatabase(directory: path, fileSystem)
     }
@@ -61,16 +89,22 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
   }
 
   public func registerForChangeNotifications(for uri: DocumentURI, language: Language) {
-    guard let delegate = self.delegate else { return }
+    queue.async {
+      self.watchedFiles[uri] = language
 
-    let settings = self._settings(for: uri)
-    DispatchQueue.global().async {
+      guard let delegate = self.delegate else { return }
+
+      let settings = self.settings(for: uri)
       delegate.fileBuildSettingsChanged([uri: FileBuildSettingsChange(settings)])
     }
   }
 
   /// We don't support change watching.
-  public func unregisterForChangeNotifications(for: DocumentURI) {}
+  public func unregisterForChangeNotifications(for uri: DocumentURI) {
+    queue.async {
+      self.watchedFiles[uri] = nil
+    }
+  }
 
   public func buildTargets(reply: @escaping (LSPResult<[BuildTarget]>) -> Void) {
     reply(.failure(buildTargetsNotSupported))
@@ -84,14 +118,18 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     reply(.failure(buildTargetsNotSupported))
   }
 
-  func database(for url: URL) -> CompilationDatabase? {
+  /// Must be invoked on `queue`.
+  private func database(for url: URL) -> CompilationDatabase? {
+    dispatchPrecondition(condition: .onQueue(queue))
     if let path = try? AbsolutePath(validating: url.path) {
       return database(for: path)
     }
     return compdb
   }
 
-  func database(for path: AbsolutePath) -> CompilationDatabase? {
+  /// Must be invoked on `queue`.
+  private func database(for path: AbsolutePath) -> CompilationDatabase? {
+    dispatchPrecondition(condition: .onQueue(queue))
     if compdb == nil {
       var dir = path
       while !dir.isRoot {
@@ -110,12 +148,51 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     return compdb
   }
 
-  public func filesDidChange(_ events: [FileEvent]) {}
+  private func fileEventShouldTriggerCompilationDatabaseReload(event: FileEvent) -> Bool {
+    switch event.uri.fileURL?.lastPathComponent {
+    case "compile_commands.json", "compile_flags.txt":
+      return true
+    default:
+      return false
+    }
+  }
+
+  /// The compilation database has been changed on disk.
+  /// Reload it and notify the delegate about build setting changes.
+  /// Must be called on `queue`.
+  private func reloadCompilationDatabase() {
+    dispatchPrecondition(condition: .onQueue(queue))
+
+    guard let projectRoot = self.projectRoot else { return }
+
+    self.compdb = tryLoadCompilationDatabase(directory: projectRoot, self.fileSystem)
+
+    if let delegate = self.delegate {
+      var changedFiles: [DocumentURI: FileBuildSettingsChange] = [:]
+      for (uri, _) in self.watchedFiles {
+        if let settings = self.settings(for: uri) {
+          changedFiles[uri] = FileBuildSettingsChange(settings)
+        } else {
+          changedFiles[uri] = .removedOrUnavailable
+        }
+      }
+      delegate.fileBuildSettingsChanged(changedFiles)
+    }
+  }
+
+  public func filesDidChange(_ events: [FileEvent]) {
+    queue.async {
+      if events.contains(where: { self.fileEventShouldTriggerCompilationDatabaseReload(event: $0) }) {
+        self.reloadCompilationDatabase()
+      }
+    }
+  }
 }
 
 extension CompilationDatabaseBuildSystem {
-  /// Exposed for *testing*.
-  public func _settings(for uri: DocumentURI) -> FileBuildSettings? {
+  /// Must be invoked on `queue`.
+  private func settings(for uri: DocumentURI) -> FileBuildSettings? {
+    dispatchPrecondition(condition: .onQueue(queue))
     guard let url = uri.fileURL else {
       // We can't determine build settings for non-file URIs.
       return nil
@@ -126,4 +203,11 @@ extension CompilationDatabaseBuildSystem {
       compilerArguments: Array(cmd.commandLine.dropFirst()),
       workingDirectory: cmd.directory)
   }
+
+  /// Exposed for *testing*.
+  public func _settings(for uri: DocumentURI) -> FileBuildSettings? {
+    return queue.sync {
+      return self.settings(for: uri)
+    }
+  }
 }