fix(prompt): clear undo when changing/appending prompt

Problem: undoing after the prompt is changed breaks it (and causes init_prompt
to abort it and append a new one), as the undo history contains the old prompt.

Solution: like submitting, clear the undo buffer. Don't do it in init_prompt if
the line was empty; that may not result in a new prompt, and causes commands
like "S" to lose the history.

As u_save, etc. wasn't being called by prompt_setprompt, undoing after it fixes
the prompt usually gave undesirable results anyway.

Remove the added undo_spec.lua test, as its approach no longer works as a repro,
and finding a new one seems fiddly.

src/nvim/edit.c

@@ -1615,6 +1615,8 @@ static void init_prompt(int cmdchar_todo)
       ml_append(lnum, prompt, 0, false);
       appended_lines_mark(lnum, 1);
       curbuf->b_prompt_start.mark.lnum = curbuf->b_ml.ml_line_count;
+      // Like submitting, undo history was relevant to the old prompt.
+      u_clearallandblockfree(curbuf);
     }
     curbuf->b_prompt_start.mark.col = prompt_len;
     curwin->w_cursor.lnum = curbuf->b_ml.ml_line_count;

src/nvim/eval/buffer.c

@@ -792,7 +792,7 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
       // If for some odd reason the old prompt is missing,
       // replace prompt line with new-prompt (discards user-input).
       ml_replace_buf(buf, prompt_lno, (char *)new_prompt, true, false);
-      extmark_splice_cols(buf, prompt_lno - 1, 0, old_line_len, new_prompt_len, kExtmarkUndo);
+      extmark_splice_cols(buf, prompt_lno - 1, 0, old_line_len, new_prompt_len, kExtmarkNoUndo);
       cursor_col = new_prompt_len;
     } else {
       // Replace prev-prompt + user-input with new-prompt + user-input
@@ -801,7 +801,7 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
         xfree(new_line);
       }
       extmark_splice_cols(buf, prompt_lno - 1, 0, buf->b_prompt_start.mark.col, new_prompt_len,
-                          kExtmarkUndo);
+                          kExtmarkNoUndo);
       cursor_col += new_prompt_len - buf->b_prompt_start.mark.col;
     }
 
@@ -810,6 +810,8 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
       check_cursor_col(curwin);
     }
     changed_lines(buf, prompt_lno, 0, prompt_lno + 1, 0, true);
+    // Undo history contains the old prompt.
+    u_clearallandblockfree(buf);
   }
 
   // Clear old prompt text and replace with the new one

test/functional/editor/undo_spec.lua

@@ -225,22 +225,3 @@ describe(':undo! command', function()
     )
   end)
 end)
-
-describe('undo', function()
-  before_each(clear)
-
-  it('u_savecommon uses correct buffer with reload = true', function()
-    -- Easiest to repro in a prompt buffer. prompt_setprompt's buffer must not yet have an undo
-    -- header to trigger this. Will crash if it wrongly uses the unloaded curbuf in nvim_buf_call,
-    -- as that has no undo buffer.
-    eq(0, #fn.undotree().entries)
-    exec_lua(function()
-      local buf = vim.api.nvim_get_current_buf()
-      vim.bo.buftype = 'prompt'
-      vim.api.nvim_buf_call(vim.fn.bufadd(''), function()
-        vim.fn.prompt_setprompt(buf, 'hej > ')
-      end)
-    end)
-    eq('hej > ', fn.prompt_getprompt(''))
-  end)
-end)

test/functional/legacy/prompt_buffer_spec.lua

@@ -514,6 +514,60 @@ describe('prompt buffer', function()
       {1:~                        }|*3
       1 line {MATCH:.*} |
     ]])
+
+    -- "S" does not clear undo
+    feed('ihello<Esc>S')
+    screen:expect([[
+      cmd: tests-initial       |
+      Command: "tests-initial" |
+      cmd: ^                    |
+      {1:~                        }|
+      {3:[Prompt] [+]             }|
+      other buffer             |
+      {1:~                        }|*3
+      {5:-- INSERT --}             |
+    ]])
+    feed('<Esc>u')
+    screen:expect([[
+      cmd: tests-initial       |
+      Command: "tests-initial" |
+      ^cmd: hello               |
+      {1:~                        }|
+      {3:[Prompt] [+]             }|
+      other buffer             |
+      {1:~                        }|*3
+      1 change; {MATCH:.*} |
+    ]])
+
+    -- undo cleared if prompt changes
+    -- (otherwise undoing would abort it and append a new prompt, which isn't useful)
+    fn('prompt_setprompt', '', 'cmd > ')
+    feed('u')
+    screen:expect([[
+      cmd: tests-initial       |
+      Command: "tests-initial" |
+      c^md > hello              |
+      {1:~                        }|
+      {3:[Prompt] [+]             }|
+      other buffer             |
+      {1:~                        }|*3
+      Already at oldest change |
+    ]])
+
+    -- new prompt line appended to fix missing prompt also clears undo
+    feed('A there')
+    fn('setpos', "':", { 0, fn('line', '.'), 99, 0 })
+    feed('<Esc>u')
+    screen:expect([[
+      cmd: tests-initial       |
+      Command: "tests-initial" |
+      cmd > hello there        |
+      cmd >^                    |
+      {3:[Prompt] [+]             }|
+      other buffer             |
+      {1:~                        }|*3
+      Already at oldest change |
+    ]])
   end)
 
   it('o/O can create new lines', function()