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

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 

src/nvim/lua/executor.c

@@ -223,6 +223,9 @@ static int nlua_state_init(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   // schedule
   lua_pushcfunction(lstate, &nlua_schedule);
   lua_setfield(lstate, -2, "schedule");
+  // in_fast_event
+  lua_pushcfunction(lstate, &nlua_in_fast_event);
+  lua_setfield(lstate, -2, "in_fast_event");
 
   // vim.loop
   luv_set_loop(lstate, &main_loop.uv);
@@ -457,6 +460,12 @@ int nlua_debug(lua_State *lstate)
   return 0;
 }
 
+int nlua_in_fast_event(lua_State *lstate)
+{
+  lua_pushboolean(lstate, in_fast_callback > 0);
+  return 1;
+}
+
 #ifdef WIN32
 /// os.getenv: override os.getenv to maintain coherency. #9681
 ///

test/functional/lua/loop_spec.lua

@@ -75,6 +75,7 @@ describe('vim.loop', function()
     exec_lua([[
       local timer = vim.loop.new_timer()
       timer:start(20, 0, function ()
+        _G.is_fast = vim.in_fast_event()
         timer:close()
         vim.api.nvim_set_var("valid", true)
         vim.api.nvim_command("echomsg 'howdy'")
@@ -89,18 +90,20 @@ describe('vim.loop', function()
       {1:~                                                 }|
       {2:                                                  }|
       {3:Error executing luv callback:}                     |
-      {3:[string "<nvim>"]:4: E5560: nvim_set_var must not }|
+      {3:[string "<nvim>"]:5: E5560: nvim_set_var must not }|
       {3:be called in a lua loop callback}                  |
       {4:Press ENTER or type command to continue}^           |
     ]])
     feed('<cr>')
     eq(false, eval("get(g:, 'valid', v:false)"))
+    eq(true, exec_lua("return _G.is_fast"))
 
     -- callbacks can be scheduled to be executed in the main event loop
     -- where the entire API is available
     exec_lua([[
       local timer = vim.loop.new_timer()
       timer:start(20, 0, vim.schedule_wrap(function ()
+        _G.is_fast = vim.in_fast_event()
         timer:close()
         vim.api.nvim_set_var("valid", true)
         vim.api.nvim_command("echomsg 'howdy'")
@@ -120,6 +123,7 @@ describe('vim.loop', function()
       howdy                                             |
     ]])
     eq(true, eval("get(g:, 'valid', v:false)"))
+    eq(false, exec_lua("return _G.is_fast"))
 
     -- fast (not deferred) API functions are allowed to be called directly
     exec_lua([[