Log when the build graph starts being generated and when it finishes

The signposts aren’t easily visible in the log (you need to add `--signpost` to `log show`) and don’t get logged on non-Darwin platforms at all. Add logging for it.

Author: ahoppen

Sources/SemanticIndex/SemanticIndexManager.swift

@@ -186,26 +186,38 @@ public final actor SemanticIndexManager {
   /// This method is intended to initially update the index of a project after it is opened.
   public func scheduleBuildGraphGenerationAndBackgroundIndexAllFiles() async {
     generateBuildGraphTask = Task(priority: .low) {
-      let signposter = Logger(subsystem: LoggingScope.subsystem, category: "preparation").makeSignposter()
-      let signpostID = signposter.makeSignpostID()
-      let state = signposter.beginInterval("Preparing", id: signpostID, "Generating build graph")
-      defer {
-        signposter.endInterval("Preparing", state)
-      }
-      await testHooks.buildGraphGenerationDidStart?()
-      await orLog("Generating build graph") { try await self.buildSystemManager.generateBuildGraph() }
-      await testHooks.buildGraphGenerationDidFinish?()
-      let index = index.checked(for: .modifiedFiles)
-      let filesToIndex = await self.buildSystemManager.sourceFiles().lazy.map(\.uri)
-        .filter { uri in
-          guard let url = uri.fileURL else {
-            // The URI is not a file, so there's nothing we can index.
-            return false
-          }
-          return !index.hasUpToDateUnit(for: url)
+      await withLoggingSubsystemAndScope(
+        subsystem: "org.swift.sourcekit-lsp.indexing",
+        scope: "build-graph-generation"
+      ) {
+        logger.log(
+          "Starting build graph generation with priority \(Task.currentPriority.rawValue, privacy: .public)"
+        )
+        let signposter = logger.makeSignposter()
+        let signpostID = signposter.makeSignpostID()
+        let state = signposter.beginInterval("Preparing", id: signpostID, "Generating build graph")
+        defer {
+          signposter.endInterval("Preparing", state)
         }
-      await scheduleBackgroundIndex(files: filesToIndex)
-      generateBuildGraphTask = nil
+        await testHooks.buildGraphGenerationDidStart?()
+        let startDate = Date()
+        await orLog("Generating build graph") { try await self.buildSystemManager.generateBuildGraph() }
+        await testHooks.buildGraphGenerationDidFinish?()
+        let index = index.checked(for: .modifiedFiles)
+        let filesToIndex = await self.buildSystemManager.sourceFiles().lazy.map(\.uri)
+          .filter { uri in
+            guard let url = uri.fileURL else {
+              // The URI is not a file, so there's nothing we can index.
+              return false
+            }
+            return !index.hasUpToDateUnit(for: url)
+          }
+        await scheduleBackgroundIndex(files: filesToIndex)
+        logger.log(
+          "Finished build graph generation in \(Date().timeIntervalSince(startDate) * 1000, privacy: .public)ms"
+        )
+        generateBuildGraphTask = nil
+      }
     }
     indexStatusDidChange()
   }