vim-patch:8.1.0039: cannot easily delete lines in another buffer

Problem:    Cannot easily delete lines in another buffer.
Solution:   Add deletebufline().
d79a26219d

runtime/doc/eval.txt

@@ -2028,6 +2028,8 @@ cursor({lnum}, {col} [, {off}])
 cursor({list})			Number	move cursor to position in {list}
 deepcopy({expr} [, {noref}])	any	make a full copy of {expr}
 delete({fname} [, {flags}])	Number	delete the file or directory {fname}
+deletebufline({expr}, {first}[, {last}])
+				Number	delete lines from buffer {expr}
 dictwatcheradd({dict}, {pattern}, {callback})
 				Start watching a dictionary
 dictwatcherdel({dict}, {pattern}, {callback})
@@ -3209,6 +3211,17 @@ delete({fname} [, {flags}])					*delete()*
 		The result is a Number, which is 0 if the delete operation was
 		successful and -1 when the deletion failed or partly failed.
 
+deletebufline({expr}, {first}[, {last}])		*deletebufline()*
+		Delete lines {first} to {last} (inclusive) from buffer {expr}.
+		If {last} is omitted then delete line {first} only.
+		On success 0 is returned, on failure 1 is returned.
+
+		For the use of {expr}, see |bufname()| above.
+
+		{first} and {last} are used like with |setline()|. Note that
+		when using |line()| this refers to the current buffer. Use "$"
+		to refer to the last line in buffer {expr}.
+
 dictwatcheradd({dict}, {pattern}, {callback})		      *dictwatcheradd()*
 		Adds a watcher to a dictionary. A dictionary watcher is
 		identified by three components:

src/nvim/eval.c

@@ -1114,6 +1114,24 @@ static void restore_vimvar(int idx, typval_T *save_tv)
   }
 }
 
+/*
+ * If there is a window for "curbuf", make it the current window.
+ */
+    static void
+find_win_for_curbuf(void)
+{
+    wininfo_T *wip;
+
+    for (wip = curbuf->b_wininfo; wip != NULL; wip = wip->wi_next)
+    {
+	if (wip->wi_win != NULL)
+	{
+	    curwin = wip->wi_win;
+	    break;
+	}
+    }
+}
+
 /*
  * Evaluate an expression to a list with suggestions.
  * For the "expr:" part of 'spellsuggest'.
@@ -8017,6 +8035,89 @@ static void f_dictwatcherdel(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   callback_free(&callback);
 }
 
+/*
+ * "deletebufline()" function
+ */
+    static void
+f_deletebufline(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+    buf_T	*buf;
+    linenr_T	first, last;
+    linenr_T	lnum;
+    long	count;
+    int		is_curbuf;
+    buf_T	*curbuf_save = NULL;
+    win_T	*curwin_save = NULL;
+
+    buf = tv_get_buf(&argvars[0], FALSE);
+    if (buf == NULL)
+    {
+	rettv->vval.v_number = 1; /* FAIL */
+	return;
+    }
+    is_curbuf = buf == curbuf;
+
+    first = tv_get_lnum_buf(&argvars[1], buf);
+    if (argvars[2].v_type != VAR_UNKNOWN)
+	last = tv_get_lnum_buf(&argvars[2], buf);
+    else
+	last = first;
+
+    if (buf->b_ml.ml_mfp == NULL || first < 1
+			   || first > buf->b_ml.ml_line_count || last < first)
+    {
+	rettv->vval.v_number = 1;	/* FAIL */
+	return;
+    }
+
+    if (!is_curbuf)
+    {
+	curbuf_save = curbuf;
+	curwin_save = curwin;
+	curbuf = buf;
+	find_win_for_curbuf();
+    }
+    if (last > curbuf->b_ml.ml_line_count)
+	last = curbuf->b_ml.ml_line_count;
+    count = last - first + 1;
+
+    // When coming here from Insert mode, sync undo, so that this can be
+    // undone separately from what was previously inserted.
+    if (u_sync_once == 2)
+    {
+	u_sync_once = 1; // notify that u_sync() was called
+	u_sync(TRUE);
+    }
+
+    if (u_save(first - 1, last + 1) == FAIL)
+    {
+	rettv->vval.v_number = 1;	/* FAIL */
+	return;
+    }
+
+    for (lnum = first; lnum <= last; ++lnum)
+	ml_delete(first, TRUE);
+
+    FOR_ALL_TAB_WINDOWS(tp, wp)
+	if (wp->w_buffer == buf)
+	{
+	    if (wp->w_cursor.lnum > last)
+		wp->w_cursor.lnum -= count;
+	    else if (wp->w_cursor.lnum> first)
+		wp->w_cursor.lnum = first;
+	    if (wp->w_cursor.lnum > wp->w_buffer->b_ml.ml_line_count)
+		wp->w_cursor.lnum = wp->w_buffer->b_ml.ml_line_count;
+	}
+    check_cursor_col();
+    deleted_lines_mark(first, count);
+
+    if (!is_curbuf)
+    {
+	curbuf = curbuf_save;
+	curwin = curwin_save;
+    }
+}
+
 /*
  * "did_filetype()" function
  */

src/nvim/eval.lua

@@ -75,6 +75,7 @@ return {
     cursor={args={1, 3}},
     deepcopy={args={1, 2}},
     delete={args={1,2}},
+    deletebufline={args={2,3}},
     dictwatcheradd={args=3},
     dictwatcherdel={args=3},
     did_filetype={},