Add support for HUP, USR1, USR2 signals (#86)

Motivation:

Adding support for capturing HUP too in addition to USR1/USR2.

Modifications:

* Added HUP signal, added descriptive strings for new signals.
* Optionally allow to let signal handler run multiple times, useful for USR signals

Result:
HUP and USR signals available.

Author: hassila

Sources/Lifecycle/Lifecycle.swift

@@ -158,7 +158,7 @@ public struct ServiceLifecycle {
             let signalSource = ServiceLifecycle.trap(signal: signal, handler: { signal in
                 self.log("intercepted signal: \(signal)")
                 self.shutdown()
-            })
+            }, cancelAfterTrap: true)
             self.underlying.shutdownGroup.notify(queue: .global()) {
                 signalSource.cancel()
             }
@@ -176,12 +176,16 @@ extension ServiceLifecycle {
     /// - parameters:
     ///    - signal: The signal to trap.
     ///    - handler: closure to invoke when the signal is captured.
+    ///    - on: DispatchQueue to run the signal handler on (default global dispatch queue)
+    ///    - 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()) -> DispatchSourceSignal {
+    public static func trap(signal sig: Signal, handler: @escaping (Signal) -> Void, on queue: DispatchQueue = .global(), cancelAfterTrap: Bool = false) -> DispatchSourceSignal {
         let signalSource = DispatchSource.makeSignalSource(signal: sig.rawValue, queue: queue)
         signal(sig.rawValue, SIG_IGN)
         signalSource.setEventHandler(handler: {
-            signalSource.cancel()
+            if cancelAfterTrap {
+                signalSource.cancel()
+            }
             handler(sig)
         })
         signalSource.resume()
@@ -194,6 +198,10 @@ extension ServiceLifecycle {
 
         public static let TERM = Signal(rawValue: SIGTERM)
         public static let INT = Signal(rawValue: SIGINT)
+        public static let USR1 = Signal(rawValue: SIGUSR1)
+        public static let USR2 = Signal(rawValue: SIGUSR2)
+        public static let HUP = Signal(rawValue: SIGHUP)
+
         // for testing
         internal static let ALRM = Signal(rawValue: SIGALRM)
 
@@ -203,6 +211,9 @@ extension ServiceLifecycle {
             case Signal.TERM: result += "TERM, "
             case Signal.INT: result += "INT, "
             case Signal.ALRM: result += "ALRM, "
+            case Signal.USR1: result += "USR1, "
+            case Signal.USR2: result += "USR2, "
+            case Signal.HUP: result += "HUP, "
             default: () // ok to ignore
             }
             result += "rawValue: \(self.rawValue))"