MAINT-5357: Add llcoro::yield() function to yield for one frame.

indra/llcommon/lleventcoro.cpp

@@ -145,6 +145,14 @@ void storeToLLSDPath(LLSD& dest, const LLSD& rawPath, const LLSD& value)
 
 } // anonymous
 
+void llcoro::yield()
+{
+    // By viewer convention, we post an event on the "mainloop" LLEventPump
+    // each iteration of the main event-handling loop. So waiting for a single
+    // event on "mainloop" gives us a one-frame yield.
+    waitForEventOn("mainloop");
+}
+
 LLSD llcoro::postAndWait(const LLSD& event, const LLEventPumpOrPumpName& requestPump,
                          const LLEventPumpOrPumpName& replyPump, const LLSD& replyPumpNamePath)
 {

indra/llcommon/lleventcoro.h

@@ -101,6 +101,13 @@ VoidListener<LISTENER> voidlistener(const LISTENER& listener)
     return VoidListener<LISTENER>(listener);
 }
 
+/**
+ * Yield control from a coroutine for one "mainloop" tick. If your coroutine
+ * runs without suspending for nontrivial time, sprinkle in calls to this
+ * function to avoid stalling the rest of the viewer processing.
+ */
+void yield();
+
 /**
  * Post specified LLSD event on the specified LLEventPump, then wait for a
  * response on specified other LLEventPump. This is more than mere