lua: add vim.in_fast_event() to check if we are in a luv callback

1 files changed, 10 insertions, 1 deletions

diff --git a/runtime/doc/if_lua.txt b/runtime/doc/if_lua.txt
index f886fd28c5..7ddcb6cc92 100644
--- a/runtime/doc/if_lua.txt
+++ b/runtime/doc/if_lua.txt
@@ -400,7 +400,8 @@ is safe to execute API methods. >
     end))
 
 A subset of the API is available in direct luv callbacks ("fast" callbacks),
-most notably |nvim_get_mode()| and |nvim_input()|.
+most notably |nvim_get_mode()| and |nvim_input()|.  It is possible to
+check whether code is running in this context using |vim.in_fast_event()|.
 
 
 Example: repeating timer
@@ -462,6 +463,14 @@ vim.schedule({callback})				*vim.schedule()*
         Schedules {callback} to be invoked soon by the main event-loop. Useful
         to avoid |textlock| or other temporary restrictions.
 
+vim.in_fast_event()					*vim.in_fast_event()*
+        Returns true if the code is executing as part of a "fast" event
+        handler, where most of the API is disabled. These are low-level event
+        such as luv callbacks |lua-loop-callbacks|, which can be invoked at
+        any time nvim polls for input. When this returns `false` most API
+        functions are callable, but can be subjected to other restrictions,
+        such as |textlock|.
+
 vim.type_idx						*vim.type_idx*
 	Type index for use in |lua-special-tbl|.  Specifying one of the 
 	values from |vim.types| allows typing the empty table (it is