fix(events): avoid recursive loop_uv_run() from vim.ui_attach() shell message

Problem:  vim.ui_attach() msg_show callback runs the risk of a recursive
          loop_uv_run() when trying to display a message from a shell
          command stream.
Solution: Schedule the message callback on the fast_events queue.
(cherry picked from commit fa302037f9)

src/nvim/message.c

@@ -283,7 +283,7 @@ void msg_multiline(String str, int hl_id, bool check_int, bool hist, bool *need_
     if (check_int && got_int) {
       return;
     }
-    if (*s == '\n' || *s == TAB || *s == '\r') {
+    if (*s == '\n' || *s == TAB || *s == '\r' || *s == BELL) {
       // Print all chars before the delimiter
       msg_outtrans_len(chunk, (int)(s - chunk), hl_id, hist);
 
@@ -291,7 +291,11 @@ void msg_multiline(String str, int hl_id, bool check_int, bool hist, bool *need_
         msg_clr_eos();
         *need_clear = false;
       }
-      msg_putchar_hl((uint8_t)(*s), hl_id);
+      if (*s == BELL) {
+        vim_beep(kOptBoFlagShell);
+      } else {
+        msg_putchar_hl((uint8_t)(*s), hl_id);
+      }
       chunk = s + 1;
     }
     s++;

src/nvim/os/shell.c

@@ -7,6 +7,7 @@
 
 #include "auto/config.h"
 #include "klib/kvec.h"
+#include "nvim/api/private/helpers.h"
 #include "nvim/ascii_defs.h"
 #include "nvim/buffer.h"
 #include "nvim/charset.h"
@@ -1119,6 +1120,17 @@ static void out_data_ring(const char *output, size_t size)
   }
 }
 
+static void out_data_event(void **argv)
+{
+  bool need_clear = true;
+  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_append = true;
+  msg_multiline(cbuf_as_string((char *)argv[0], (size_t)argv[1]), hl, false, false, &need_clear);
+  xfree(argv[0]);
+  ui_flush();
+}
+
 /// Continue to append data to last screen line.
 ///
 /// @param output       Data to append to screen lines.
@@ -1129,33 +1141,29 @@ static void out_data_append_to_screen(const char *output, size_t *count, int fd,
 {
   const char *p = output;
   const char *end = output + *count;
-  msg_ext_set_kind(fd == STDERR_FILENO ? "shell_err" : "shell_out");
-  msg_ext_append = true;
+  // Note: this is not 100% precise:
+  // 1. we don't check if received continuation bytes are already invalid
+  //    and we thus do some buffering that could be avoided
+  // 2. we don't compose chars over buffer boundaries, even if we see an
+  //    incomplete UTF-8 sequence that could be composing with the last
+  //    complete sequence.
+  // This will be corrected when we switch to vterm based implementation
   while (p < end) {
-    if (*p == '\n' || *p == '\r' || *p == TAB || *p == BELL) {
-      msg_putchar_hl((uint8_t)(*p), fd == STDERR_FILENO ? HLF_SE : HLF_SO);
-      p++;
-    } else {
-      // Note: this is not 100% precise:
-      // 1. we don't check if received continuation bytes are already invalid
-      //    and we thus do some buffering that could be avoided
-      // 2. we don't compose chars over buffer boundaries, even if we see an
-      //    incomplete UTF-8 sequence that could be composing with the last
-      //    complete sequence.
-      // This will be corrected when we switch to vterm based implementation
-      int i = *p ? utfc_ptr2len_len(p, (int)(end - p)) : 1;
-      if (!eof && i == 1 && utf8len_tab_zero[*(uint8_t *)p] > (end - p)) {
-        *count = (size_t)(p - output);
-        goto end;
-      }
-
-      msg_outtrans_len(p, i, fd == STDERR_FILENO ? HLF_SE : HLF_SO, false);
-      p += i;
+    int i = *p ? utfc_ptr2len_len(p, (int)*count - (int)(p - output)) : 1;
+    if (!eof && i == 1 && utf8len_tab_zero[*(uint8_t *)p] > (end - p)) {
+      *count = (size_t)(p - output);
+      break;
     }
+    p += i;
+  }
+  // Process after uv_run() to avoid recursion in vim.ui_attach() msg_show callback #38664.
+  char *str = xmemdupz(output, *count);
+  if (ui_has(kUIMessages)) {
+    multiqueue_put(main_loop.fast_events, out_data_event,
+                   (void *)str, (void *)*count, (void *)(intptr_t)fd);
+  } else {
+    out_data_event((void *[]){ (void *)str, (void *)*count, (void *)(intptr_t)fd });
   }
-
-end:
-  ui_flush();
 }
 
 static size_t out_data_cb(RStream *stream, const char *ptr, size_t count, void *data, bool eof)