libutil: explicitly declare and document our reserved signals

Change-Id: Ia27cce0d3577219b7476f7ce6dade4387ba727b2

lix/libmain/shared.cc

@@ -130,11 +130,7 @@ static void sigHandler(int signo) { }
 
 void initNix()
 {
-    // kj needs a signal for internal use. no system lix habitually runs on causes
-    // kj to actually *use* this signal, but better safe than sorry—and since some
-    // OSes (*cough* macos) don't support realtime signals we must use SIGUSR2 for
-    // this, thus "consuming" both USR signals. at some point we will change this.
-    kj::UnixEventPort::setReservedSignal(SIGUSR2);
+    kj::UnixEventPort::setReservedSignal(KJ_RESERVED_SIGNAL);
 
     registerCrashHandler();
 
@@ -155,9 +151,11 @@ void initNix()
     if (sigaction(SIGCHLD, &act, 0))
         throw SysError("resetting SIGCHLD");
 
-    /* Install a dummy SIGUSR1 handler for use with pthread_kill(). */
+    /* Install a dummy INTERRUPT_NOTIFY_SIGNAL handler for use with pthread_kill(). */
     act.sa_handler = sigHandler;
-    if (sigaction(SIGUSR1, &act, 0)) throw SysError("handling SIGUSR1");
+    if (sigaction(INTERRUPT_NOTIFY_SIGNAL, &act, 0)) {
+        throw SysError("handling interrupt notify signal %i", INTERRUPT_NOTIFY_SIGNAL);
+    }
 
 #if __APPLE__
     /* HACK: on darwin, we need can’t use sigprocmask with SIGWINCH.

lix/libstore/pathlocks.cc

@@ -48,8 +48,8 @@ void lockFile(int fd, LockType lockType, NeverAsync)
 
 static kj::Promise<Result<void>> lockFileAsyncInner(int fd, LockType lockType)
 try {
-    // start a thread to lock the file synchronously, waiting for SIGUSR1 to signal
-    // that the call was canceled. SIGUSR1 is already set aside for such signaling.
+    // start a thread to lock the file synchronously, waiting for an
+    // INTERRUPT_NOTIFY_SIGNAL to signal that the call was canceled.
 
     int type = convertLockType(lockType);
     auto pfp = kj::newPromiseAndCrossThreadFulfiller<Result<void>>();
@@ -64,7 +64,7 @@ try {
         pfp.fulfiller->fulfill(result::success());
     });
     auto cancel = kj::defer([&] {
-        pthread_kill(locker.native_handle(), SIGUSR1);
+        pthread_kill(locker.native_handle(), INTERRUPT_NOTIFY_SIGNAL);
         locker.join();
     });
 

lix/libutil/signals.hh

@@ -35,6 +35,16 @@
 
 namespace nix {
 
+/// reserved signal used to notify threads of interruption requests, e.g. users
+/// pressing Control-C on the terminal. we purposely do not use SIGINT handlers
+/// provided by the OS to allow for more orderly cleanup of running operations.
+static inline constexpr int INTERRUPT_NOTIFY_SIGNAL = SIGUSR1;
+/// kj needs a signal for internal use. no system lix habitually runs on causes
+/// kj to actually *use* this signal, but better safe than sorry—and since some
+/// OSes (*cough* macos) don't support realtime signals we must use SIGUSR2 for
+/// this, thus "consuming" both USR signals. at some point we will change this.
+static inline constexpr int KJ_RESERVED_SIGNAL = SIGUSR2;
+
 /* User interruption. */
 
 class Interrupted;
@@ -124,7 +134,7 @@ kj::Promise<Result<T>> makeInterruptible(kj::Promise<Result<T>> p)
 void triggerInterrupt();
 
 /**
- * A RAII class that causes the current thread to receive SIGUSR1 when
+ * A RAII class that causes the current thread to receive `INTERRUPT_NOTIFY_SIGNAL` when
  * the signal handler thread receives SIGINT. That is, this allows
  * SIGINT to be multiplexed to multiple threads.
  */
@@ -135,8 +145,9 @@ struct ReceiveInterrupts
 
     ReceiveInterrupts()
         : target(pthread_self())
-        , callback(createInterruptCallback([&]() { pthread_kill(target, SIGUSR1); }))
-    { }
+        , callback(createInterruptCallback([&] { pthread_kill(target, INTERRUPT_NOTIFY_SIGNAL); }))
+    {
+    }
 };
 
 };