uloop: fix corner cases with recursive uloop_run calls

With multiple recursive calls to uloop_run, the callback for the same fd
can be run multiple times from different levels in the stack.
Prevent this by tracking the stack of uloop_fd callbacks and buffering new
incoming events for fds already on the stack.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

uloop.c

@@ -43,6 +43,14 @@ struct uloop_fd_event {
 	unsigned int events;
 };
 
+struct uloop_fd_stack {
+	struct uloop_fd_stack *next;
+	struct uloop_fd *fd;
+	unsigned int events;
+};
+
+static struct uloop_fd_stack *fd_stack = NULL;
+
 #define ULOOP_MAX_EVENTS 10
 
 static struct list_head timeouts = LIST_HEAD_INIT(timeouts);
@@ -285,6 +293,32 @@ static int uloop_fetch_events(int timeout)
 
 #endif
 
+static bool uloop_fd_stack_event(struct uloop_fd *fd, int events)
+{
+	struct uloop_fd_stack *cur;
+
+	/*
+	 * Do not buffer events for level-triggered fds, they will keep firing.
+	 * Caller needs to take care of recursion issues.
+	 */
+	if (!(fd->flags & ULOOP_EDGE_TRIGGER))
+		return false;
+
+	for (cur = fd_stack; cur; cur = cur->next) {
+		if (cur->fd != fd)
+			continue;
+
+		if (events < 0)
+			cur->fd = NULL;
+		else
+			cur->events |= events | ULOOP_EVENT_BUFFERED;
+
+		return true;
+	}
+
+	return false;
+}
+
 static void uloop_run_events(int timeout)
 {
 	struct uloop_fd_event *cur;
@@ -298,17 +332,33 @@ static void uloop_run_events(int timeout)
 	}
 
 	while (cur_nfds > 0) {
+		struct uloop_fd_stack stack_cur;
+		unsigned int events;
+
 		cur = &cur_fds[cur_fd++];
 		cur_nfds--;
 
 		fd = cur->fd;
+		events = cur->events;
 		if (!fd)
 			continue;
 
 		if (!fd->cb)
 			continue;
 
-		fd->cb(fd, cur->events);
+		if (uloop_fd_stack_event(fd, cur->events))
+			continue;
+
+		stack_cur.next = fd_stack;
+		stack_cur.fd = fd;
+		fd_stack = &stack_cur;
+		do {
+			stack_cur.events = 0;
+			fd->cb(fd, events);
+			events = stack_cur.events & ULOOP_EVENT_MASK;
+		} while (stack_cur.fd && events);
+		fd_stack = stack_cur.next;
+
 		return;
 	}
 }
@@ -352,6 +402,7 @@ int uloop_fd_delete(struct uloop_fd *fd)
 		cur_fds[cur_fd + i].fd = NULL;
 	}
 	fd->registered = false;
+	uloop_fd_stack_event(fd, -1);
 	return __uloop_fd_delete(fd);
 }
 

uloop.h

@@ -44,8 +44,13 @@ typedef void (*uloop_process_handler)(struct uloop_process *c, int ret);
 #define ULOOP_WRITE		(1 << 1)
 #define ULOOP_EDGE_TRIGGER	(1 << 2)
 #define ULOOP_BLOCKING		(1 << 3)
+
+#define ULOOP_EVENT_MASK	(ULOOP_READ | ULOOP_WRITE)
+
+/* internal flags */
+#define ULOOP_EVENT_BUFFERED	(1 << 4)
 #ifdef USE_KQUEUE
-#define ULOOP_EDGE_DEFER	(1 << 4)
+#define ULOOP_EDGE_DEFER	(1 << 5)
 #endif
 
 struct uloop_fd