Allow zero-task startup (#55)

* Allow zero-task startup

* Add test for zero-task

* Fix Linux build and improve consistency

* Fix sanity

Author: stevapple

Sources/Lifecycle/Lifecycle.swift

@@ -110,7 +110,7 @@ public struct ServiceLifecycle {
             let signalSource = ServiceLifecycle.trap(signal: signal, handler: { signal in
                 self.lifecycle.log("intercepted signal: \(signal)")
                 self.shutdown()
-           })
+            })
             self.lifecycle.shutdownGroup.notify(queue: .global()) {
                 signalSource.cancel()
             }
@@ -357,14 +357,16 @@ public class ComponentLifecycle: LifecycleTask {
     // MARK: - private
 
     private func _start(on queue: DispatchQueue, tasks: [LifecycleTask], callback: @escaping (Error?) -> Void) {
-        precondition(tasks.count > 0, "invalid number of tasks, must be > 0")
         self.stateLock.withLock {
             guard case .idle = self.state else {
                 preconditionFailure("invalid state, \(self.state)")
             }
-            log("starting")
+            self.log("starting")
             self.state = .starting(queue)
         }
+        if tasks.count == 0 {
+            self.log(level: .notice, "no tasks provided")
+        }
         self.startTask(on: queue, tasks: tasks, index: 0) { started, error in
             self.stateLock.lock()
             if error != nil {

Tests/LifecycleTests/ComponentLifecycleTests+XCTest.swift

@@ -40,6 +40,7 @@ extension ComponentLifecycleTests {
             ("testSync", testSync),
             ("testAyncBarrier", testAyncBarrier),
             ("testConcurrency", testConcurrency),
+            ("testZeroTask", testZeroTask),
             ("testRegisterSync", testRegisterSync),
             ("testRegisterShutdownSync", testRegisterShutdownSync),
             ("testRegisterAsync", testRegisterAsync),

Tests/LifecycleTests/ComponentLifecycleTests.swift

@@ -48,7 +48,7 @@ final class ComponentLifecycleTests: XCTestCase {
                                                dispatchPrecondition(condition: .onQueue(.global()))
                                                XCTAssertTrue(startCalls.contains(id))
                                                stopCalls.append(id)
-                                   })
+                                           })
         }
         lifecycle.register(items)
 
@@ -82,7 +82,7 @@ final class ComponentLifecycleTests: XCTestCase {
                                                dispatchPrecondition(condition: .onQueue(testQueue))
                                                XCTAssertTrue(startCalls.contains(id))
                                                stopCalls.append(id)
-                                   })
+                                           })
         }
         lifecycle.register(items)
 
@@ -206,7 +206,7 @@ final class ComponentLifecycleTests: XCTestCase {
                                shutdown: .sync {
                                    XCTAssertTrue(startCalls.contains(id))
                                    stopCalls.append(id)
-                                })
+                               })
         }
         do {
             let id = UUID().uuidString
@@ -217,7 +217,7 @@ final class ComponentLifecycleTests: XCTestCase {
                                shutdown: .sync {
                                    XCTAssertTrue(startCalls.contains(id))
                                    stopCalls.append(id)
-                                })
+                               })
         }
         lifecycle.start { error in
             XCTAssertNil(error)
@@ -470,6 +470,16 @@ final class ComponentLifecycleTests: XCTestCase {
         items.forEach { XCTAssertEqual($0.state, .shutdown, "expected item to be shutdown, but \($0.state)") }
     }
 
+    func testZeroTask() {
+        let lifecycle = ComponentLifecycle(label: "test")
+
+        lifecycle.start { error in
+            XCTAssertNil(error, "not expecting error")
+            lifecycle.shutdown()
+        }
+        lifecycle.wait()
+    }
+
     func testRegisterSync() {
         class Sync {
             var state = State.idle
@@ -811,7 +821,7 @@ final class ComponentLifecycleTests: XCTestCase {
                                    }
                                    callback(nil)
                                }
-                            })
+                           })
 
         lifecycle.start { error in
             XCTAssertNil(error, "not expecting error")