fix(ui): no fast context for prompt message kinds #31224

Problem:  No longer able to show prompt messages with vim.ui_attach().
Solution: Do not execute callback in fast context for prompt message
          kinds. These events must be safe to show the incoming message
          so the event itself serves to indicate that the message
          should be shown immediately.

4 files changed, 33 insertions, 4 deletions

diff --git a/runtime/doc/lua.txt b/runtime/doc/lua.txt
index e6f7e62e02..243c907180 100644
--- a/runtime/doc/lua.txt
+++ b/runtime/doc/lua.txt
@@ -1083,7 +1083,9 @@ vim.ui_attach({ns}, {options}, {callback})                   *vim.ui_attach()*
     |ui-popupmenu| and the sections below for event format for respective
     events.
 
-    Callbacks for `msg_show` events are executed in |api-fast| context.
+    Callbacks for `msg_show` events are executed in |api-fast| context unless
+    Nvim will wait for input, in which case messages should be shown
+    immediately.
 
     Excessive errors inside the callback will result in forced detachment.
 
diff --git a/runtime/lua/vim/_meta/builtin.lua b/runtime/lua/vim/_meta/builtin.lua
index dd6ef69eb8..b8779b66fe 100644
--- a/runtime/lua/vim/_meta/builtin.lua
+++ b/runtime/lua/vim/_meta/builtin.lua
@@ -233,7 +233,9 @@ function vim.wait(time, callback, interval, fast_only) end
 --- {callback} receives event name plus additional parameters. See |ui-popupmenu|
 --- and the sections below for event format for respective events.
 ---
---- Callbacks for `msg_show` events are executed in |api-fast| context.
+--- Callbacks for `msg_show` events are executed in |api-fast| context unless
+--- Nvim will wait for input, in which case messages should be shown
+--- immediately.
 ---
 --- Excessive errors inside the callback will result in forced detachment.
 ---
diff --git a/src/nvim/ui.c b/src/nvim/ui.c
index d50747e63f..7c81110ae9 100644
--- a/src/nvim/ui.c
+++ b/src/nvim/ui.c
@@ -717,6 +717,13 @@ void ui_call_event(char *name, bool fast, Array args)
 {
   bool handled = false;
   UIEventCallback *event_cb;
+
+  // Prompt messages should be shown immediately so must be safe
+  if (strcmp(name, "msg_show") == 0) {
+    char *kind = args.items[0].data.string.data;
+    fast = !kind || (strncmp(kind, "confirm", 7) != 0 && strcmp(kind, "return_prompt") != 0);
+  }
+
   map_foreach(&ui_event_cbs, ui_event_ns_id, event_cb, {
     Error err = ERROR_INIT;
     uint32_t ns_id = ui_event_ns_id;
diff --git a/test/functional/lua/ui_event_spec.lua b/test/functional/lua/ui_event_spec.lua
index f78fced14e..0580442d0a 100644
--- a/test/functional/lua/ui_event_spec.lua
+++ b/test/functional/lua/ui_event_spec.lua
@@ -237,20 +237,21 @@ describe('vim.ui_attach', function()
     })
   end)
 
-  it('aborts :function on error with ext_messages', function()
+  it('msg_show in fast context', function()
     exec_lua([[
     vim.ui_attach(ns, { ext_messages = true }, function(event, _, content)
       if event == "msg_show" then
-        -- "fast-api" does not prevent aborting :function
         vim.api.nvim_get_runtime_file("foo", false)
         -- non-"fast-api" is not allowed in msg_show callback and should be scheduled
         local _, err = pcall(vim.api.nvim_buf_set_lines, 0, -2, -1, false, { content[1][2] })
+        pcall(vim.api.nvim__redraw, { flush = true })
         vim.schedule(function()
           vim.api.nvim_buf_set_lines(0, -2, -1, false, { content[1][2], err })
         end)
       end
     end)
     ]])
+    -- "fast-api" does not prevent aborting :function
     feed(':func Foo()<cr>bar<cr>endf<cr>:func Foo()<cr>')
     screen:expect({
       grid = [[
@@ -267,6 +268,23 @@ describe('vim.ui_attach', function()
         },
       },
     })
+    -- No fast context for prompt message kinds
+    feed(':%s/Function/Replacement/c<cr>')
+    screen:expect({
+      grid = [[
+        ^E122: {10:Function} Foo already exists, add !|
+         to replace it                          |
+        replace with Replacement (y/n/a/q/l/^E/^|
+        Y)?                                     |
+        {1:~                                       }|
+      ]],
+      messages = {
+        {
+          content = { { 'replace with Replacement (y/n/a/q/l/^E/^Y)?', 6, 19 } },
+          kind = 'confirm_sub',
+        },
+      },
+    })
   end)
 
   it('detaches after excessive errors', function()