fix(api): avoid double hit-enter prompt with nvim_err_writeln (#22879)

2 files changed, 59 insertions, 12 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 9812313b7b..343c3b6e40 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -737,7 +737,7 @@ error:
 void nvim_out_write(String str)
   FUNC_API_SINCE(1)
 {
-  write_msg(str, false);
+  write_msg(str, false, false);
 }
 
 /// Writes a message to the Vim error buffer. Does not append "\n", the
@@ -747,7 +747,7 @@ void nvim_out_write(String str)
 void nvim_err_write(String str)
   FUNC_API_SINCE(1)
 {
-  write_msg(str, true);
+  write_msg(str, true, false);
 }
 
 /// Writes a message to the Vim error buffer. Appends "\n", so the buffer is
@@ -758,8 +758,7 @@ void nvim_err_write(String str)
 void nvim_err_writeln(String str)
   FUNC_API_SINCE(1)
 {
-  nvim_err_write(str);
-  nvim_err_write((String) { .data = "\n", .size = 1 });
+  write_msg(str, true, true);
 }
 
 /// Gets the current list of buffer handles
@@ -1672,23 +1671,24 @@ theend:
 ///
 /// @param message  Message to write
 /// @param to_err   true: message is an error (uses `emsg` instead of `msg`)
-static void write_msg(String message, bool to_err)
+/// @param writeln  Append a trailing newline
+static void write_msg(String message, bool to_err, bool writeln)
 {
   static StringBuilder out_line_buf = KV_INITIAL_VALUE;
   static StringBuilder err_line_buf = KV_INITIAL_VALUE;
 
-#define PUSH_CHAR(i, line_buf, msg) \
+#define PUSH_CHAR(c, line_buf, msg) \
   if (kv_max(line_buf) == 0) { \
     kv_resize(line_buf, LINE_BUFFER_MIN_SIZE); \
   } \
-  if (message.data[i] == NL) { \
+  if (c == NL) { \
     kv_push(line_buf, NUL); \
     msg(line_buf.items); \
     kv_drop(line_buf, kv_size(line_buf)); \
     kv_resize(line_buf, LINE_BUFFER_MIN_SIZE); \
-    continue; \
-  } \
-  kv_push(line_buf, message.data[i]);
+  } else { \
+    kv_push(line_buf, c); \
+  }
 
   no_wait_return++;
   for (uint32_t i = 0; i < message.size; i++) {
@@ -1696,9 +1696,16 @@ static void write_msg(String message, bool to_err)
       break;
     }
     if (to_err) {
-      PUSH_CHAR(i, err_line_buf, emsg);
+      PUSH_CHAR(message.data[i], err_line_buf, emsg);
+    } else {
+      PUSH_CHAR(message.data[i], out_line_buf, msg);
+    }
+  }
+  if (writeln) {
+    if (to_err) {
+      PUSH_CHAR(NL, err_line_buf, emsg);
     } else {
-      PUSH_CHAR(i, out_line_buf, msg);
+      PUSH_CHAR(NL, out_line_buf, msg);
     }
   }
   no_wait_return--;
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index 9d5a0c4b4e..83347a499b 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -2080,6 +2080,46 @@ describe('API', function()
     end)
   end)
 
+  describe('nvim_err_writeln', function()
+    local screen
+
+    before_each(function()
+      screen = Screen.new(40, 8)
+      screen:attach()
+      screen:set_default_attr_ids({
+        [0] = {bold=true, foreground=Screen.colors.Blue},
+        [1] = {foreground = Screen.colors.White, background = Screen.colors.Red},
+        [2] = {bold = true, foreground = Screen.colors.SeaGreen},
+        [3] = {bold = true, reverse = true},
+      })
+    end)
+
+    it('shows only one return prompt after all lines are shown', function()
+      nvim_async('err_writeln', 'FAILURE\nERROR\nEXCEPTION\nTRACEBACK')
+      screen:expect([[
+                                                |
+        {0:~                                       }|
+        {3:                                        }|
+        {1:FAILURE}                                 |
+        {1:ERROR}                                   |
+        {1:EXCEPTION}                               |
+        {1:TRACEBACK}                               |
+        {2:Press ENTER or type command to continue}^ |
+      ]])
+      feed('<CR>')
+      screen:expect([[
+        ^                                        |
+        {0:~                                       }|
+        {0:~                                       }|
+        {0:~                                       }|
+        {0:~                                       }|
+        {0:~                                       }|
+        {0:~                                       }|
+                                                |
+      ]])
+    end)
+  end)
+
   describe('nvim_list_chans, nvim_get_chan_info', function()
     before_each(function()
       command('autocmd ChanOpen * let g:opened_event = deepcopy(v:event)')