Nucleic: Gitea Runner macOS VM Support

Sources/gitea-macos-runner/CommandDaemon.swift

@@ -110,9 +110,10 @@ struct DaemonCommand: AsyncParsableCommand {
                     // Expected on shutdown.
                 } catch {
                     logger.critical("daemon stopped", metadata: ["error": .string("\(error)")])
-                    // Fully qualified: inside a ParsableCommand a bare `exit`
-                    // resolves to ParsableCommand.exit(withError:).
-                    await MainActor.run { Foundation.exit(1) }
+                    // Not `MainActor.run`: the main actor is parked inside
+                    // `app.run()` for the life of the process, so hopping onto
+                    // it to exit is its own deadlock. See `VZAppRuntime.run`.
+                    VZAppRuntime.flushAndExit(1)
                 }
             }
         )
@@ -123,18 +124,40 @@ struct DaemonCommand: AsyncParsableCommand {
 /// run loop it requires, while the real work runs in a `Task`.
 ///
 /// Shared by `daemon` and `vm boot`: any command that starts a VM needs this.
-@MainActor
 enum VZAppRuntime {
     /// Signal sources have to outlive the call that creates them or they are
     /// cancelled on deinit and the signals go nowhere.
-    private static var signalSources: [DispatchSourceSignal] = []
-    private static var isTerminating = false
+    private nonisolated(unsafe) static var signalSources: [DispatchSourceSignal] = []
+    private nonisolated(unsafe) static var isTerminating = false
+    private static let stateLock = NSLock()
+
+    /// Signals land here rather than on `.main`. See ``run(onSignal:body:)``.
+    private static let signalQueue = DispatchQueue(
+        label: "xyz.blakeslee.gitea-macos-runner.signals")
 
     /// Starts the run loop and runs `body` alongside it. Never returns.
     ///
+    /// ## Nothing here may touch the main queue
+    ///
+    /// This is reached from Swift's async `main`, so the frame that calls
+    /// `app.run()` is *itself* a block executing on the main dispatch queue —
+    /// and it never returns. libdispatch will not re-enter a serial queue that
+    /// already has a block in flight, so from this moment the main queue is
+    /// closed for business: a plain `Task { }` inheriting a `@MainActor`
+    /// context, a `DispatchSource` handler on `.main`, or an
+    /// `await MainActor.run { … }` all enqueue work that can never be drained.
+    ///
+    /// The symptom is exact and was reported as a hang in `image build`: a live
+    /// run loop, zero CPU, and no output past the last line printed before this
+    /// call — `body` had been enqueued behind `app.run()` and never got a first
+    /// tick. Hence `Task.detached`, a private signal queue, and ``exit(_:)``
+    /// called straight from whichever thread reaches it. A normal AppKit app
+    /// does not hit this because its `main()` is not a main-queue block.
+    ///
     /// - Parameters:
     ///   - onSignal: Cleanup to perform on `SIGINT`/`SIGTERM` before exiting.
     ///   - body: The work to run. When it returns, the process exits zero.
+    @MainActor
     static func run(
         onSignal: @escaping @Sendable () async -> Void,
         body: @escaping @Sendable () async -> Void
@@ -149,30 +172,48 @@ enum VZAppRuntime {
             // DispatchSourceSignal only observes; the default disposition still
             // kills the process unless it is ignored first.
             signal(signalNumber, SIG_IGN)
-            let source = DispatchSource.makeSignalSource(signal: signalNumber, queue: .main)
+            let source = DispatchSource.makeSignalSource(signal: signalNumber, queue: signalQueue)
             source.setEventHandler {
-                Task { @MainActor in
-                    guard !isTerminating else { return }
-                    isTerminating = true
+                guard beginTerminating() else { return }
+                Task.detached {
                     CLI.note("received signal; shutting down…")
                     await onSignal()
-                    NSApp.terminate(nil)
-                    exit(0)
+                    flushAndExit(0)
                 }
             }
             source.resume()
+            stateLock.lock()
             signalSources.append(source)
+            stateLock.unlock()
         }
 
-        Task {
+        // Detached on purpose: an inheriting `Task { }` would be queued behind
+        // the `app.run()` below and never start. See the note above.
+        Task.detached {
             await body()
-            await MainActor.run {
-                NSApp.terminate(nil)
-                exit(0)
-            }
+            flushAndExit(0)
         }
 
         app.run()
-        exit(0)
+        flushAndExit(0)
+    }
+
+    /// Wins the race to shut down, exactly once.
+    private static func beginTerminating() -> Bool {
+        stateLock.lock()
+        defer { stateLock.unlock() }
+        guard !isTerminating else { return false }
+        isTerminating = true
+        return true
+    }
+
+    /// Exits from any thread, without hopping to the unusable main actor.
+    ///
+    /// `NSApp.terminate(nil)` is deliberately not called: it requires the main
+    /// actor, which is exactly what is not available here.
+    nonisolated static func flushAndExit(_ code: Int32) -> Never {
+        fflush(stdout)
+        fflush(stderr)
+        exit(code)
     }
 }