fix(messages): fast context for for nvim_echo({kind}) callback #39755

Problem:  vim.ui_attach() callback for nvim_echo() call that spoofs an
          internal message kind is executed in fast context.

Solution: Set msg_show callback |api-fast| context dynamically at
          external message callsites, and for internal list_cmd",
          "progress" and "shell*" messages.

src/nvim/api/deprecated.c

@@ -924,6 +924,7 @@ static void write_msg(String message, bool to_err, bool writeln)
   static StringBuilder err_line_buf = KV_INITIAL_VALUE;
   StringBuilder *line_buf = to_err ? &err_line_buf : &out_line_buf;
 
+  msg_ext_no_fast();
 #define PUSH_CHAR(c) \
   if (kv_max(*line_buf) == 0) { \
     kv_resize(*line_buf, LINE_BUFFER_MIN_SIZE); \

src/nvim/api/vim.c

@@ -906,6 +906,7 @@ Union(Integer, String) nvim_echo(ArrayOf(Tuple(String, *HLGroupID)) chunks, Bool
     msg_didany = true;
     msg_no_more = true;
   }
+  msg_ext_no_fast();
   id = msg_multihl(opts->id, hl_msg, kind, history, opts->err, &msg_data, &needs_clear);
   if (opts->_truncate) {
     msg_no_more = false;

src/nvim/eval.c

@@ -6163,6 +6163,7 @@ void ex_echo(exarg_T *eap)
       if (atstart) {
         atstart = false;
         msg_ext_set_append(eap->cmdidx == CMD_echon);
+        msg_ext_no_fast();
         msg_ext_set_kind("echo");
         // Call msg_start() after eval1(), evaluating the expression
         // may cause a message to appear.
@@ -6263,11 +6264,13 @@ void ex_execute(exarg_T *eap)
 
   if (ret != FAIL && ga.ga_data != NULL) {
     if (eap->cmdidx == CMD_echomsg) {
+      msg_ext_no_fast();
       msg_ext_set_kind("echomsg");
       msg(ga.ga_data, echo_hl_id);
     } else if (eap->cmdidx == CMD_echoerr) {
       // We don't want to abort following commands, restore did_emsg.
       int save_did_emsg = did_emsg;
+      msg_ext_no_fast();
       emsg_multiline(ga.ga_data, "echoerr", HLF_E, true);
       if (!force_abort) {
         did_emsg = save_did_emsg;

src/nvim/ex_cmds.c

@@ -1219,6 +1219,7 @@ void do_bang(int addr_count, exarg_T *eap, bool forceit, bool do_in, bool do_out
   if (addr_count == 0) {                // :!
     // echo the command
     msg_start();
+    msg_ext_no_fast();
     msg_ext_set_kind("shell_cmd");
     msg_putchar(':');
     msg_putchar('!');

src/nvim/lua/executor.c

@@ -306,6 +306,7 @@ void nlua_error(lua_State *const lstate, const char *const msg)
     fprintf(stderr, msg, (int)len, str);
     fprintf(stderr, "\n");
   } else {
+    msg_ext_no_fast();
     semsg_multiline("lua_error", msg, (int)len, str);
   }
 
@@ -342,6 +343,7 @@ static void nlua_luv_error_event(void **argv)
   luv_err_t type = (luv_err_t)(intptr_t)argv[1];
   switch (type) {
   case kCallback:
+    msg_ext_no_fast();
     semsg_multiline("lua_error", "Lua callback:\n%s", error);
     break;
   case kThread:
@@ -1049,6 +1051,7 @@ static void nlua_print_event(void **argv)
   HlMessageChunk chunk = { { .data = argv[0], .size = (size_t)(intptr_t)argv[1] - 1 }, 0 };
   kv_push(msg, chunk);
   bool needs_clear = false;
+  msg_ext_no_fast();
   msg_multihl(NIL, msg, "lua_print", true, false, NULL, &needs_clear);
 }
 

src/nvim/message.c

@@ -1111,6 +1111,7 @@ char *msg_progress(char *s, char *id, char *status, int hl_id, bool hist, bool t
   };
   HlMessage chunks = KV_INITIAL_VALUE;
   kv_push(chunks, ((HlMessageChunk){ cstr_as_string(s), hl_id }));
+  msg_ext_no_fast();
   msg_multihl(CSTR_AS_OBJ(id), chunks, "progress", false, false, &data, &clear);
   kv_destroy(chunks);
   ui_flush();
@@ -1705,6 +1706,10 @@ void msg_ext_set_kind(const char *msg_kind)
   // the kind but this is called more consistently at the start of a message
   // than msg_start() at this point.
   redir_col = msg_ext_append ? redir_col : 0;
+
+  if (strcmp("list_cmd", msg_kind) == 0) {
+    msg_ext_no_fast();
+  }
 }
 
 void msg_ext_set_append(bool append)
@@ -1719,6 +1724,13 @@ void msg_ext_set_trigger(const char *trigger)
   msg_ext_trigger = trigger;
 }
 
+// Should be executed at all callsites emitting non-internal messages.
+void msg_ext_no_fast(void)
+{
+  msg_ext_ui_flush();
+  msg_ext_fast = false;
+}
+
 /// Prepare for outputting characters in the command line.
 void msg_start(void)
 {
@@ -2366,7 +2378,7 @@ void msg_puts_len(const char *const str, const ptrdiff_t len, int hl_id, bool hi
   // Don't print anything when using ":silent cmd" or empty message.
   if (msg_silent != 0 || *str == NUL) {
     if (*str == NUL && ui_has(kUIMessages)) {
-      msg_ext_ui_flush();  // ensure messages until now are emitted
+      msg_ext_no_fast();
       ui_call_msg_show(cstr_as_string("empty"), (Array)ARRAY_DICT_INIT, false, false, false,
                        INTEGER_OBJ(-1), (String)STRING_INIT);
       cmdline_was_last_drawn = false;
@@ -3421,6 +3433,7 @@ void msg_ext_ui_flush(void)
     msg_ext_overwrite = false;
     msg_ext_history = false;
     msg_ext_append = false;
+    msg_ext_fast = true;
     msg_ext_kind = NULL;
     msg_id_next += (msg_ext_id.data.integer == msg_id_next);
     msg_ext_id = INTEGER_OBJ(msg_id_next);

src/nvim/message.h

@@ -37,6 +37,8 @@ EXTERN bool msg_ext_skip_flush INIT( = false);
 EXTERN bool msg_ext_overwrite INIT( = false);
 /// Set to true to avoid setting "verbose" kind for "last set" messages.
 EXTERN bool msg_ext_skip_verbose INIT( = false);
+/// Set to false for non-internal messages to determine UI callback |api-fast| context.
+EXTERN bool msg_ext_fast INIT( = true);
 
 /// allocated grid for messages. Used unless ext_messages is active.
 /// See also the description at msg_scroll_flush()

src/nvim/os/shell.c

@@ -701,6 +701,7 @@ int os_call_shell(char *cmd, int opts, char *extra_args)
 
   if (!emsg_silent && exitcode != 0 && !(opts & kShellOptSilent)) {
     msg_ext_set_kind("shell_ret");
+    msg_ext_no_fast();
     if (!ui_has(kUIMessages)) {
       msg_putchar('\n');
     }
@@ -1126,6 +1127,7 @@ static void out_data_event(void **argv)
   int hl = (int)(intptr_t)argv[2] == STDERR_FILENO ? HLF_SE : HLF_SO;
   msg_ext_set_kind((int)(intptr_t)argv[2] == STDERR_FILENO ? "shell_err" : "shell_out");
   msg_ext_set_append(true);
+  msg_ext_no_fast();
   msg_multiline(cbuf_as_string((char *)argv[0], (size_t)argv[1]), hl, false, false, &need_clear);
   xfree(argv[0]);
   ui_flush();

src/nvim/ui.c

@@ -777,28 +777,6 @@ static void ui_attach_error(uint32_t ns_id, const char *name, const char *msg)
 
 void ui_call_event(char *name, Array args)
 {
-  // Internal messages are considered unsafe and are executed in fast context.
-  bool fast = strcmp(name, "msg_show") == 0;
-  const char *not_fast[] = {
-    "empty",
-    "echo",
-    "echomsg",
-    "echoerr",
-    "list_cmd",
-    "lua_error",
-    "lua_print",
-    "progress",
-    "shell_cmd",
-    "shell_err",
-    "shell_out",
-    "shell_ret",
-    NULL,
-  };
-
-  for (int i = 0; fast && not_fast[i]; i++) {
-    fast = !strequal(not_fast[i], args.items[0].data.string.data);
-  }
-
   // Don't impose textlock restrictions upon UI event handlers.
   int save_expr_map_lock = expr_map_lock;
   int save_textlock = textlock;
@@ -806,6 +784,7 @@ void ui_call_event(char *name, Array args)
   textlock = 0;
 
   bool handled = false;
+  bool fast = msg_ext_fast && strcmp("msg_show", name) == 0;
   UIEventCallback *event_cb;
   map_foreach(&ui_event_cbs, ui_event_ns_id, event_cb, {
     Error err = ERROR_INIT;

test/functional/ui/messages2_spec.lua

@@ -691,6 +691,19 @@ describe('messages2', function()
       {1:~                                                    }|*12
       foo                                                  |
     ]])
+    feed('<CR>')
+    -- Fast context is not determined by message kind #39666
+    exec_lua(function()
+      vim.schedule(function()
+        vim.api.nvim_echo({ { 'bar' } }, false, { kind = 'search_cmd' })
+        vim.fn.getchar()
+      end)
+    end)
+    screen:expect([[
+      ^                                                     |
+      {1:~                                                    }|*12
+      bar                                                  |
+    ]])
   end)
 
   it('properly formatted carriage return messages', function()