ex_getln: Fix “echoerr msg not shown” problem

This also attempted to fix problem with cancelling input() on error by avoiding 
standard error printing facilities (assumed thrown error message is the 
problem), but with no luck so far.

src/nvim/ex_getln.c

@@ -64,6 +64,7 @@
 #include "nvim/os/time.h"
 #include "nvim/lib/kvec.h"
 #include "nvim/api/private/helpers.h"
+#include "nvim/highlight_defs.h"
 
 /*
  * Variables shared between getcmdline(), redrawcmdline() and others.
@@ -2202,6 +2203,13 @@ enum { MAX_CB_ERRORS = 1 };
 ///         to do.
 static bool color_cmdline(void)
 {
+  bool printed_errmsg = false;
+#define PRINT_ERRMSG(...) \
+  do { \
+    msg_putchar('\n'); \
+    msg_printf_attr(hl_attr(HLF_E)|MSG_HIST, __VA_ARGS__); \
+    printed_errmsg = true; \
+  } while (0)
   bool ret = true;
   kv_size(ccline_colors) = 0;
 
@@ -2274,7 +2282,7 @@ static bool color_cmdline(void)
     goto color_cmdline_error;
   }
   if (tv.v_type != VAR_LIST) {
-    emsgf(_("E5400: Callback should return list"));
+    PRINT_ERRMSG(_("E5400: Callback should return list"));
     goto color_cmdline_error;
   }
   if (tv.vval.v_list == NULL) {
@@ -2285,13 +2293,13 @@ static bool color_cmdline(void)
   for (const listitem_T *li = tv.vval.v_list->lv_first;
        li != NULL; li = li->li_next, i++) {
     if (li->li_tv.v_type != VAR_LIST) {
-      emsgf(_("E5401: List item %i is not a List"), i);
+      PRINT_ERRMSG(_("E5401: List item %i is not a List"), i);
       goto color_cmdline_error;
     }
     const list_T *const l = li->li_tv.vval.v_list;
     if (tv_list_len(l) != 3) {
-      emsgf(_("E5402: List item %i has incorrect length: %li /= 3"),
+      PRINT_ERRMSG(_("E5402: List item %i has incorrect length: %li /= 3"),
-            i, tv_list_len(l));
+                   i, tv_list_len(l));
       goto color_cmdline_error;
     }
     bool error = false;
@@ -2299,13 +2307,13 @@ static bool color_cmdline(void)
     if (error) {
       goto color_cmdline_error;
     } else if (!(prev_end <= start && start < ccline.cmdlen)) {
-      emsgf(_("E5403: Chunk %i start %" PRIdVARNUMBER " not in range "
+      PRINT_ERRMSG(_("E5403: Chunk %i start %" PRIdVARNUMBER " not in range "
-              "[%" PRIdVARNUMBER ", %i)"),
+                     "[%" PRIdVARNUMBER ", %i)"),
-            i, start, prev_end, ccline.cmdlen);
+                   i, start, prev_end, ccline.cmdlen);
       goto color_cmdline_error;
     } else if (utf8len_tab_zero[(uint8_t)ccline.cmdbuff[start]] == 0) {
-      emsgf(_("E5405: Chunk %i start %" PRIdVARNUMBER " splits multibyte "
+      PRINT_ERRMSG(_("E5405: Chunk %i start %" PRIdVARNUMBER " splits "
-              "character"), i, start);
+                     "multibyte character"), i, start);
       goto color_cmdline_error;
     }
     if (start != prev_end) {
@@ -2320,14 +2328,14 @@ static bool color_cmdline(void)
     if (error) {
       goto color_cmdline_error;
     } else if (!(start < end && end <= ccline.cmdlen)) {
-      emsgf(_("E5404: Chunk %i end %" PRIdVARNUMBER " not in range "
+      PRINT_ERRMSG(_("E5404: Chunk %i end %" PRIdVARNUMBER " not in range "
-              "(%" PRIdVARNUMBER ", %i]"),
+                     "(%" PRIdVARNUMBER ", %i]"),
-            i, end, start, ccline.cmdlen);
+                   i, end, start, ccline.cmdlen);
       goto color_cmdline_error;
     } else if (end < ccline.cmdlen
                && utf8len_tab_zero[(uint8_t)ccline.cmdbuff[end]] == 0) {
-      emsgf(_("E5406: Chunk %i end %" PRIdVARNUMBER " splits multibyte "
+      PRINT_ERRMSG(_("E5406: Chunk %i end %" PRIdVARNUMBER " splits multibyte "
-              "character"), i, end);
+                     "character"), i, end);
       goto color_cmdline_error;
     }
     prev_end = end;
@@ -2364,19 +2372,19 @@ color_cmdline_end:
   return ret;
 color_cmdline_error:
   if (ERROR_SET(&err)) {
-    emsgf(_("E5407: Callback has thrown an exception: %s"), err.msg);
+    PRINT_ERRMSG(_("E5407: Callback has thrown an exception: %s"), err.msg);
     api_clear_error(&err);
   }
+  assert(printed_errmsg);
+
   prev_prompt_errors++;
   kv_size(ccline_colors) = 0;
-  if (did_emsg) {
+  prev_prompt_errors += MAX_CB_ERRORS;
-    did_emsg = false;
+  redrawcmdline();
-    prev_prompt_errors += MAX_CB_ERRORS;
+  prev_prompt_errors -= MAX_CB_ERRORS;
-    redrawcmdline();
+  ret = false;
-    prev_prompt_errors -= MAX_CB_ERRORS;
-    ret = false;
-  }
   goto color_cmdline_end;
+#undef PRINT_ERRMSG
 }
 
 /*

src/nvim/message.c

@@ -1611,6 +1611,27 @@ void msg_puts_attr_len(const char *const str, const ptrdiff_t len, int attr)
   }
 }
 
+/// Print a formatted message
+///
+/// Message printed is limited by #IOSIZE. Must not be used from inside
+/// msg_puts_attr().
+///
+/// @param[in]  attr  Highlight attributes.
+/// @param[in]  fmt  Format string.
+void msg_printf_attr(const int attr, const char *const fmt, ...)
+  FUNC_ATTR_NONNULL_ALL
+{
+  static char msgbuf[IOSIZE];
+
+  va_list ap;
+  va_start(ap, fmt);
+  const size_t len = vim_vsnprintf(msgbuf, sizeof(msgbuf), fmt, ap, NULL);
+  va_end(ap);
+
+  msg_scroll = true;
+  msg_puts_attr_len(msgbuf, (ptrdiff_t)len, attr);
+}
+
 /*
  * The display part of msg_puts_attr_len().
  * May be called recursively to display scroll-back text.

test/functional/ui/cmdline_highlight_spec.lua

@@ -289,7 +289,16 @@ describe('Command-line coloring', function()
   it('does the right thing when errorring', function()
     set_color_cb('Echoerring')
     start_prompt('e')
-    -- FIXME Does not work well with :echoerr: error message not shown.
+    screen:expect([[
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      {EOB:~                                       }|
+      :                                       |
+      {ERR:E5407: Callback has thrown an exception:}|
+      {ERR: Vim(echoerr):HERE}                      |
+      :e^                                      |
+    ]])
   end)
   it('does the right thing when throwing', function()
     set_color_cb('Throwing')