fix(lua): disallow vim.wait() in fast contexts

`vim.wait()` cannot be called in a fast callback since the main loop
cannot be run in that context as it is not reentrant

Fixes #26122

6 files changed, 29 insertions, 8 deletions

diff --git a/runtime/doc/lua.txt b/runtime/doc/lua.txt
index a79b26dfb0..a35d70cae8 100644
--- a/runtime/doc/lua.txt
+++ b/runtime/doc/lua.txt
@@ -1143,6 +1143,8 @@ vim.wait({time}, {callback}, {interval}, {fast_only})             *vim.wait()*
     milliseconds (default 200). Nvim still processes other events during this
     time.
 
+    Cannot be called while in an |api-fast| event.
+
     Examples: >lua
         ---
         -- Wait for 100 ms, allowing other events to process
@@ -1173,8 +1175,7 @@ vim.wait({time}, {callback}, {interval}, {fast_only})             *vim.wait()*
       • {interval}   (integer|nil) (Approximate) number of milliseconds to
                      wait between polls
       • {fast_only}  (boolean|nil) If true, only |api-fast| events will be
-                     processed. If called from while in an |api-fast| event,
-                     will automatically be set to `true`.
+                     processed.
 
     Return: ~
         boolean, nil|-1|-2
@@ -1828,7 +1829,8 @@ vim.system({cmd}, {opts}, {on_exit})                            *vim.system()*
         • pid (integer) Process ID
         • wait (fun(timeout: integer|nil): SystemCompleted) Wait for the
           process to complete. Upon timeout the process is sent the KILL
-          signal (9) and the exit code is set to 124.
+          signal (9) and the exit code is set to 124. Cannot be called in
+          |api-fast|.
           • SystemCompleted is an object with the fields:
             • code: (integer)
             • signal: (integer)
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index b5bef13325..825e5ba41f 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -325,6 +325,8 @@ The following changes to existing APIs or features add new behavior.
   NOTE: the regexp engine still has a hard-coded limit of considering
   6 composing chars only.
 
+• |vim.wait()| is no longer allowed to be called in |api-fast|.
+
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*
 
diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index 12b632075d..6cccbe8313 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -124,7 +124,8 @@ vim.log = {
 --- @return vim.SystemObj Object with the fields:
 ---   - pid (integer) Process ID
 ---   - wait (fun(timeout: integer|nil): SystemCompleted) Wait for the process to complete. Upon
----     timeout the process is sent the KILL signal (9) and the exit code is set to 124.
+---     timeout the process is sent the KILL signal (9) and the exit code is set to 124. Cannot
+---     be called in |api-fast|.
 ---     - SystemCompleted is an object with the fields:
 ---      - code: (integer)
 ---      - signal: (integer)
diff --git a/runtime/lua/vim/_meta/builtin.lua b/runtime/lua/vim/_meta/builtin.lua
index 92c23f7764..eeba356672 100644
--- a/runtime/lua/vim/_meta/builtin.lua
+++ b/runtime/lua/vim/_meta/builtin.lua
@@ -205,6 +205,8 @@ function vim.schedule(fn) end
 --- milliseconds (default 200). Nvim still processes other events during
 --- this time.
 ---
+--- Cannot be called while in an |api-fast| event.
+---
 --- Examples:
 ---
 --- ```lua
@@ -235,8 +237,6 @@ function vim.schedule(fn) end
 --- @param callback? fun(): boolean Optional callback. Waits until {callback} returns true
 --- @param interval? integer (Approximate) number of milliseconds to wait between polls
 --- @param fast_only? boolean If true, only |api-fast| events will be processed.
----                           If called from while in an |api-fast| event, will
----                           automatically be set to `true`.
 --- @return boolean, nil|-1|-2
 ---     - If {callback} returns `true` during the {time}: `true, nil`
 ---     - If {callback} never returns `true` during the {time}: `false, -1`
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index ce2a247a6f..04eda99d17 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -411,6 +411,10 @@ static bool nlua_wait_condition(lua_State *lstate, int *status, bool *callback_r
 static int nlua_wait(lua_State *lstate)
   FUNC_ATTR_NONNULL_ALL
 {
+  if (in_fast_callback) {
+    return luaL_error(lstate, e_luv_api_disabled, "vim.wait");
+  }
+
   intptr_t timeout = luaL_checkinteger(lstate, 1);
   if (timeout < 0) {
     return luaL_error(lstate, "timeout must be >= 0");
@@ -449,8 +453,7 @@ static int nlua_wait(lua_State *lstate)
     fast_only = lua_toboolean(lstate, 4);
   }
 
-  MultiQueue *loop_events = fast_only || in_fast_callback > 0
-                            ? main_loop.fast_events : main_loop.events;
+  MultiQueue *loop_events = fast_only ? main_loop.fast_events : main_loop.events;
 
   TimeWatcher *tw = xmalloc(sizeof(TimeWatcher));
 
diff --git a/test/functional/lua/vim_spec.lua b/test/functional/lua/vim_spec.lua
index 1533a1823f..61fa16f59d 100644
--- a/test/functional/lua/vim_spec.lua
+++ b/test/functional/lua/vim_spec.lua
@@ -2769,6 +2769,19 @@ describe('lua stdlib', function()
         eq({'notification', 'wait', {-2}}, next_msg(500))
       end)
     end)
+
+    it('should not run in fast callbacks #26122', function()
+      exec_lua([[
+        vim.uv.new_timer():start(0, 100, function()
+          local count = 0
+          vim.wait(100, function()
+            count = count + 1
+            return count == 10
+          end, 100)
+        end)
+      ]])
+      assert_alive()
+    end)
   end)
 
   it('vim.notify_once', function()