vim-patch:8.2.1588: cannot read back the prompt of a prompt buffer

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes vim/vim#6851)
https://github.com/vim/vim/commit/077cc7aa0e0c431e97795612374fe17fe7c88803

Updated prompt_getprompt() doc to https://github.com/vim/vim/commit/cb80aa2d53e56d3aba3b3c439fb467f29a750c5e
and removed mention of method syntax usage (not supported by Nvim).

6 files changed, 78 insertions, 5 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index cff87b2fed..b8dcfd0ff4 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2309,6 +2309,7 @@ perleval({expr})		any	evaluate |perl| expression
 pow({x}, {y})			Float	{x} to the power of {y}
 prevnonblank({lnum})		Number	line nr of non-blank line <= {lnum}
 printf({fmt}, {expr1}...)	String	format text
+prompt_getprompt({buf})		String	get prompt text
 prompt_setcallback({buf}, {expr}) none	set prompt callback function
 prompt_setinterrupt({buf}, {text}) none	set prompt interrupt function
 prompt_setprompt({buf}, {text}) none	set prompt text
@@ -6746,6 +6747,13 @@ printf({fmt}, {expr1} ...)				*printf()*
 		of "%" items.  If there are not sufficient or too many
 		arguments an error is given.  Up to 18 arguments can be used.
 
+prompt_getprompt({buf})					*prompt_getprompt()*
+		Returns the effective prompt text for buffer {buf}.  {buf} can
+		be a buffer name or number.  See |prompt-buffer|.
+
+		If the buffer doesn't exist or isn't a prompt buffer, an empty
+		string is returned.
+
 prompt_setcallback({buf}, {expr})			*prompt_setcallback()*
 		Set prompt callback for buffer {buf} to {expr}.  When {expr}
 		is an empty string the callback is removed.  This has only
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 21f5dcc815..41948f577e 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -978,6 +978,7 @@ Tags:						*tag-functions*
 	settagstack()		modify the tag stack of a window
 
 Prompt Buffer:					*promptbuffer-functions*
+	prompt_getprompt()	get the effective prompt text for a buffer
 	prompt_setcallback()	set prompt callback for a buffer
 	prompt_setinterrupt()	set interrupt callback for a buffer
 	prompt_setprompt()	set the prompt text for a buffer
diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index f2fddc89fe..999cc74185 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -1604,13 +1604,20 @@ void edit_putchar(int c, bool highlight)
   }
 }
 
+/// Return the effective prompt for the specified buffer.
+char_u *buf_prompt_text(const buf_T *const buf)
+    FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  if (buf->b_prompt_text == NULL) {
+    return (char_u *)"% ";
+  }
+  return buf->b_prompt_text;
+}
+
 // Return the effective prompt for the current buffer.
-char_u *prompt_text(void)
+char_u *prompt_text(void) FUNC_ATTR_WARN_UNUSED_RESULT
 {
-    if (curbuf->b_prompt_text == NULL) {
-      return (char_u *)"% ";
-    }
-    return curbuf->b_prompt_text;
+  return buf_prompt_text(curbuf);
 }
 
 // Prepare for prompt mode: Make sure the last line has the prompt text.
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index b10e99fc08..77e7c7b3a9 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -251,6 +251,7 @@ return {
     pow={args=2},
     prevnonblank={args=1},
     printf={args=varargs(1)},
+    prompt_getprompt={args=1},
     prompt_setcallback={args={2, 2}},
     prompt_setinterrupt={args={2, 2}},
     prompt_setprompt={args={2, 2}},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 206f76ba8e..6d328953f6 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -6491,6 +6491,26 @@ static void f_prompt_setinterrupt(typval_T *argvars,
     buf->b_prompt_interrupt= interrupt_callback;
 }
 
+/// "prompt_getprompt({buffer})" function
+void f_prompt_getprompt(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+  FUNC_ATTR_NONNULL_ALL
+{
+  // return an empty string by default, e.g. it's not a prompt buffer
+  rettv->v_type = VAR_STRING;
+  rettv->vval.v_string = NULL;
+
+  buf_T *const buf = tv_get_buf_from_arg(&argvars[0]);
+  if (buf == NULL) {
+    return;
+  }
+
+  if (!bt_prompt(buf)) {
+    return;
+  }
+
+  rettv->vval.v_string = vim_strsave(buf_prompt_text(buf));
+}
+
 // "prompt_setprompt({buffer}, {text})" function
 static void f_prompt_setprompt(typval_T *argvars,
                                typval_T *rettv, FunPtr fptr)
diff --git a/src/nvim/testdir/test_prompt_buffer.vim b/src/nvim/testdir/test_prompt_buffer.vim
index 66647018b5..6fc5850be3 100644
--- a/src/nvim/testdir/test_prompt_buffer.vim
+++ b/src/nvim/testdir/test_prompt_buffer.vim
@@ -156,4 +156,40 @@ func Test_prompt_buffer_edit()
   call assert_equal(0, prompt_setprompt([], ''))
 endfunc
 
+func Test_prompt_buffer_getbufinfo()
+  new
+  call assert_equal('', prompt_getprompt('%'))
+  call assert_equal('', prompt_getprompt(bufnr('%')))
+  let another_buffer = bufnr('%')
+
+  set buftype=prompt
+  call assert_equal('% ', prompt_getprompt('%'))
+  call prompt_setprompt( bufnr( '%' ), 'This is a test: ' )
+  call assert_equal('This is a test: ', prompt_getprompt('%'))
+
+  call prompt_setprompt( bufnr( '%' ), '' )
+  " Nvim doesn't support method call syntax yet.
+  " call assert_equal('', '%'->prompt_getprompt())
+  call assert_equal('', prompt_getprompt('%'))
+
+  call prompt_setprompt( bufnr( '%' ), 'Another: ' )
+  call assert_equal('Another: ', prompt_getprompt('%'))
+  let another = bufnr('%')
+
+  new
+
+  call assert_equal('', prompt_getprompt('%'))
+  call assert_equal('Another: ', prompt_getprompt(another))
+
+  " Doesn't exist
+  let buffers_before = len( getbufinfo() )
+  call assert_equal('', prompt_getprompt( bufnr('$') + 1))
+  call assert_equal(buffers_before, len( getbufinfo()))
+
+  " invalid type
+  call assert_fails('call prompt_getprompt({})', 'E728:')
+
+  %bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab