Explicitly shut down the build server when SourceKit-LSP is shut down

Sources/BuildSystemIntegration/BuildSystemManager.swift

@@ -430,6 +430,23 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
     }
   }
 
+  /// Explicitly shut down the build server.
+  ///
+  /// The build server is automatically shut down using a background task when `BuildSystemManager` is deallocated.
+  /// This, however, leads to possible race conditions where the shutdown task might not finish before the test is done,
+  /// which could result in the connection being reported as a leak. To avoid this problem, we want to explicitly shut
+  /// down the build server when the `SourceKitLSPServer` gets shut down.
+  package func shutdown() async {
+    guard let buildSystemAdapter = await self.buildSystemAdapterAfterInitialized else {
+      return
+    }
+    await orLog("Sending shutdown request to build server") {
+      _ = try await buildSystemAdapter.send(BuildShutdownRequest())
+      await buildSystemAdapter.send(OnBuildExitNotification())
+    }
+    self.buildSystemAdapter = nil
+  }
+
   deinit {
     // Shut down the build server before closing the connection to it
     Task { [buildSystemAdapter, initializeResult] in

Sources/BuildSystemIntegration/ExternalBuildSystemAdapter.swift

@@ -118,7 +118,7 @@ actor ExternalBuildSystemAdapter {
   }
 
   /// Send a notification to the build server.
-  func send(_ notification: some NotificationType) async {
+  func send(_ notification: some NotificationType) {
     guard let connectionToBuildServer else {
       logger.error("Dropping notification because BSP server has crashed: \(notification.forLogging)")
       return

Sources/SKTestSupport/INPUTS/AbstractBuildServer.py

@@ -38,7 +38,7 @@ def run(self):
 
             try:
                 result = self.handle_message(message)
-                if result:
+                if result is not None:
                     response_message: Dict[str, object] = {
                         "jsonrpc": "2.0",
                         "id": message["id"],
@@ -139,8 +139,8 @@ def textdocument_sourcekitoptions(
             code=-32601, message=f"'textDocument/sourceKitOptions' not implemented"
         )
 
-    def shutdown(self, notification: Dict[str, object]) -> None:
-        pass
+    def shutdown(self, request: Dict[str, object]) -> Dict[str, object]:
+        return {}
 
     def buildtarget_sources(self, request: Dict[str, object]) -> Dict[str, object]:
         raise RequestError(

Sources/SourceKitLSP/SourceKitLSPServer.swift

@@ -1241,6 +1241,16 @@ extension SourceKitLSPServer {
           await service.shutdown()
         }
       }
+      for workspace in workspaces {
+        taskGroup.addTask {
+          await orLog("Shutting down build server") {
+            // If the build server doesn't shut down in 1 second, don't delay SourceKit-LSP's shutdown because of it.
+            try await withTimeout(.seconds(2)) {
+              await workspace.buildSystemManager.shutdown()
+            }
+          }
+        }
+      }
     }
 
     // We have a semantic guarantee that no request or notification should be