fix repeated shutdown issue on linux (#104)

motivation: repeatedly calling shutdown in linux was broken due to multiple calls to singal() which work on darwin but not linux

changes:
* conditionalize the call to signal to once per process
* add test

Author: tomerd

Sources/Lifecycle/Lifecycle.swift

@@ -333,6 +333,9 @@ public struct ServiceLifecycle {
 }
 
 extension ServiceLifecycle {
+    private static var trapped: Set<Int32> = []
+    private static let trappedLock = Lock()
+
     /// Setup a signal trap.
     ///
     /// - parameters:
@@ -342,14 +345,20 @@ extension ServiceLifecycle {
     ///    - cancelAfterTrap: Defaults to false, which means the signal handler can be run multiple times. If true, the DispatchSignalSource will be cancelled after being trapped once.
     /// - returns: a `DispatchSourceSignal` for the given trap. The source must be cancelled by the caller.
     public static func trap(signal sig: Signal, handler: @escaping (Signal) -> Void, on queue: DispatchQueue = .global(), cancelAfterTrap: Bool = false) -> DispatchSourceSignal {
+        // on linux, we can call singal() once per process
+        self.trappedLock.withLockVoid {
+            if !trapped.contains(sig.rawValue) {
+                signal(sig.rawValue, SIG_IGN)
+                trapped.insert(sig.rawValue)
+            }
+        }
         let signalSource = DispatchSource.makeSignalSource(signal: sig.rawValue, queue: queue)
-        signal(sig.rawValue, SIG_IGN)
-        signalSource.setEventHandler(handler: {
+        signalSource.setEventHandler {
             if cancelAfterTrap {
                 signalSource.cancel()
             }
             handler(sig)
-        })
+        }
         signalSource.resume()
         return signalSource
     }

Tests/LifecycleTests/ServiceLifecycleTests+XCTest.swift

@@ -34,6 +34,7 @@ extension ServiceLifecycleTests {
             ("testNesting2", testNesting2),
             ("testSignalDescription", testSignalDescription),
             ("testBacktracesInstalledOnce", testBacktracesInstalledOnce),
+            ("testRepeatShutdown", testRepeatShutdown),
         ]
     }
 }

Tests/LifecycleTests/ServiceLifecycleTests.swift

@@ -233,4 +233,42 @@ final class ServiceLifecycleTests: XCTestCase {
         _ = ServiceLifecycle(configuration: config)
         _ = ServiceLifecycle(configuration: config)
     }
+
+    func testRepeatShutdown() {
+        if ProcessInfo.processInfo.environment["SKIP_SIGNAL_TEST"].flatMap(Bool.init) ?? false {
+            print("skipping testRepeatShutdown")
+            return
+        }
+
+        var count = 0
+
+        struct Service {
+            static let signal = ServiceLifecycle.Signal.ALRM
+
+            let lifecycle: ServiceLifecycle
+
+            init() {
+                self.lifecycle = ServiceLifecycle(configuration: .init(shutdownSignal: [Service.signal]))
+                self.lifecycle.register(GoodItem())
+            }
+        }
+
+        func gracefulShutdown() {
+            let service = Service()
+            service.lifecycle.start { error in
+                XCTAssertNil(error, "not expecting error")
+                kill(getpid(), Service.signal.rawValue)
+            }
+
+            service.lifecycle.wait()
+            count = count + 1 // not thread safe but fine for this purpose
+        }
+
+        let attempts = Int.random(in: 2 ..< 5)
+        for _ in 0 ..< attempts {
+            gracefulShutdown()
+        }
+
+        XCTAssertEqual(attempts, count)
+    }
 }