Don't use uninitialized sigset_t.

If signal handlers haven't been established, then it's
useless to try to block them, especially since the sigset_t
used for blocking hasn't been initialized yet.

src/xz/signals.c

@@ -27,6 +27,11 @@ static volatile sig_atomic_t exit_signal = 0;
 /// user_abort to true.
 static sigset_t hooked_signals;
 
+/// True once signals_init() has finished. This is used to skip blocking
+/// signals (with uninitialized hooked_signals) if signals_block() and
+/// signals_unblock() are called before signals_init() has been called.
+static bool signals_are_initialized = false;
+
 /// signals_block() and signals_unblock() can be called recursively.
 static size_t signals_block_count = 0;
 
@@ -91,6 +96,8 @@ signals_init(void)
 			message_signal_handler();
 	}
 
+	signals_are_initialized = true;
+
 	return;
 }
 
@@ -99,11 +106,13 @@ signals_init(void)
 extern void
 signals_block(void)
 {
+	if (signals_are_initialized) {
 		if (signals_block_count++ == 0) {
 			const int saved_errno = errno;
 			mythread_sigmask(SIG_BLOCK, &hooked_signals, NULL);
 			errno = saved_errno;
 		}
+	}
 
 	return;
 }
@@ -112,6 +121,7 @@ signals_block(void)
 extern void
 signals_unblock(void)
 {
+	if (signals_are_initialized) {
 		assert(signals_block_count > 0);
 
 		if (--signals_block_count == 0) {
@@ -119,6 +129,7 @@ signals_unblock(void)
 			mythread_sigmask(SIG_UNBLOCK, &hooked_signals, NULL);
 			errno = saved_errno;
 		}
+	}
 
 	return;
 }
@@ -157,6 +168,9 @@ signal_handler(DWORD type lzma_attribute((unused)))
 	// Since we don't get a signal number which we could raise() at
 	// signals_exit() like on POSIX, just set the exit status to
 	// indicate an error, so that we cannot return with zero exit status.
+	//
+	// FIXME: Since this function runs in its own thread,
+	// set_exit_status() should have a mutex.
 	set_exit_status(E_ERROR);
 	user_abort = true;
 	return TRUE;