Fix #420: fiber.cancel() assertion `!(f->flags & (1 << 2))' failed

src/fiber.cc

@@ -133,7 +133,6 @@ void
 fiber_cancel(struct fiber *f)
 {
 	assert(f->fid != 0);
-	assert(!(f->flags & FIBER_CANCEL));
 
 	f->flags |= FIBER_CANCEL;
 
@@ -180,6 +179,12 @@ fiber_is_cancelled()
 void
 fiber_testcancel(void)
 {
+	/*
+	 * Fiber can catch FiberCancelException using try..catch block in C or
+	 * pcall()/xpcall() in Lua. However, FIBER_CANCEL flag is still set
+	 * and the subject fiber will be killed by subsequent unprotected call
+	 * of this function.
+	 */
 	if (fiber_is_cancelled())
 		tnt_raise(FiberCancelException);
 }

test/box/fiber.test

@@ -113,3 +113,8 @@ exec admin "lua f = box.fiber.wrap(y)"
 exec admin "lua box.fiber.kill(f:id())"
 exec admin "lua while box.fiber.status(f) ~= 'dead' do box.fiber.sleep(0.01) end"
 exec admin "lua box.space[0]:truncate()"
+
+exec admin "lua done = false"
+exec admin "lua function test() box.fiber.name('cancel_bug'); local fun, errmsg = loadstring('box.fiber.cancel(box.fiber.self())'); xpcall(fun, function() end); xpcall(fun, function() end); done = true; fun(); end"
+exec admin "lua box.fiber.wrap(test)"
+exec admin "lua done"