lws_sul: make it clear NULL no longer valid in lws_service_fd

READMEs/README.lws_sul.md

@@ -0,0 +1,62 @@
+# `lws_sul` scheduler api
+
+Since v3.2 lws no longer requires periodic checking for timeouts and
+other events.  A new system was refactored in where future events are
+scheduled on to a single, unified, sorted linked-list in time order,
+with everything at us resolution.
+
+This makes it very cheap to know when the next scheduled event is
+coming and restrict the poll wait to match, or for event libraries
+set a timer to wake at the earliest event when returning to the
+event loop.
+
+Everything that was checked periodically was converted to use `lws_sul`
+and schedule its own later event.  The end result is when lws is idle,
+it will stay asleep in the poll wait until a network event or the next
+scheduled `lws_sul` event happens, which is optimal for power.
+
+# Side effect for older code
+
+If your older code uses `lws_service_fd()`, it used to be necessary
+to call this with a NULL pollfd periodically to indicate you wanted
+to let the background checks happen.  `lws_sul` eliminates the whole
+concept of periodic checking and NULL is no longer a valid pollfd
+value for this and related apis.
+
+# Using `lws_sul` in user code
+
+See `minimal-http-client-multi` for an example of using the `lws_sul`
+scheduler from your own code; it uses it to spread out connection
+attempts so they are staggered in time.  You must create an
+`lws_sorted_usec_list_t` object somewhere, eg, in you own existing object.
+
+```
+static lws_sorted_usec_list_t sul_stagger;
+```
+
+Create your own callback for the event... the argument points to the sul object
+used when the callback was scheduled.  You can use pointer arithmetic to translate
+that to your own struct when the `lws_sorted_usec_list_t` was a member of the
+same struct.
+
+```
+static void
+stagger_cb(lws_sorted_usec_list_t *sul)
+{
+...
+}
+```
+
+When you want to schedule the callback, use `lws_sul_schedule()`... this will call
+it 10ms in the future
+
+```
+	lws_sul_schedule(context, 0, &sul_stagger, stagger_cb, 10 * LWS_US_PER_MS);
+```
+
+In the case you destroy your object and need to cancel the scheduled callback, use
+
+```
+	lws_sul_schedule(context, 0, &sul_stagger, NULL, LWS_SET_TIMER_USEC_CANCEL);
+```
+

include/libwebsockets/lws-service.h

@@ -44,7 +44,7 @@
  *	2) Call the receive callback for incoming frame data received by
  *	    server or client connections.
  *
- *  Since v4.0 internally the timeout wait is ignored, the lws scheduler is
+ *  Since v3.2 internally the timeout wait is ignored, the lws scheduler is
  *  smart enough to stay asleep until an event is queued.
  */
 LWS_VISIBLE LWS_EXTERN int
@@ -93,7 +93,7 @@ lws_cancel_service(struct lws_context *context);
  * lws_service_fd() - Service polled socket with something waiting
  * \param context:	Websocket context
  * \param pollfd:	The pollfd entry describing the socket fd and which events
- *		happened, or NULL to tell lws to do only timeout servicing.
+ *		happened
  *
  * This function takes a pollfd that has POLLIN or POLLOUT activity and
  * services it according to the state of the associated
@@ -110,6 +110,10 @@ lws_cancel_service(struct lws_context *context);
  * If the socket is foreign to lws, it leaves revents alone.  So you can
  * see if you should service yourself by checking the pollfd revents
  * after letting lws try to service it.
+ *
+ * lws before v3.2 allowed pollfd to be NULL, to indicate that background
+ * periodic processing should be done.  Since v3.2, lws schedules any items
+ * that need handling in the future using lws_sul and NULL is no longer valid.
  */
 LWS_VISIBLE LWS_EXTERN int
 lws_service_fd(struct lws_context *context, struct lws_pollfd *pollfd);