From ce9dbd398b4ad31d8febde004cb7799964ecb009 Mon Sep 17 00:00:00 2001
From: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Date: Tue, 17 Feb 2026 00:06:03 +0000
Subject: [PATCH] fix(undo): u_savecommon uses wrong buffer

Problem: u_savecommon with reload = true wrongly uses curbuf.

Solution: use buf. Fix comments.
---
 src/nvim/undo.c                      |  6 +++---
 test/functional/editor/undo_spec.lua | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index e1e2238a36..67c2921a8f 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -382,7 +382,7 @@ int u_savecommon(buf_T *buf, linenr_T top, linenr_T bot, linenr_T newbot, bool r
   u_entry_T *prev_uep;
   linenr_T size = bot - top - 1;
 
-  // If curbuf->b_u_synced == true make a new header.
+  // If buf->b_u_synced == true make a new header.
   if (buf->b_u_synced) {
     // Need to create new entry in b_changelist.
     buf->b_new_change = true;
@@ -401,7 +401,7 @@ int u_savecommon(buf_T *buf, linenr_T top, linenr_T bot, linenr_T newbot, bool r
     }
 
     // If we undid more than we redid, move the entry lists before and
-    // including curbuf->b_u_curhead to an alternate branch.
+    // including buf->b_u_curhead to an alternate branch.
     u_header_T *old_curhead = buf->b_u_curhead;
     if (old_curhead != NULL) {
       buf->b_u_newhead = old_curhead->uh_next.ptr;
@@ -608,7 +608,7 @@ int u_savecommon(buf_T *buf, linenr_T top, linenr_T bot, linenr_T newbot, bool r
   buf->b_u_newhead->uh_entry = uep;
   if (reload) {
     // buffer was reloaded, notify text change subscribers
-    curbuf->b_u_newhead->uh_flags |= UH_RELOAD;
+    buf->b_u_newhead->uh_flags |= UH_RELOAD;
   }
   buf->b_u_synced = false;
   undo_undoes = false;
diff --git a/test/functional/editor/undo_spec.lua b/test/functional/editor/undo_spec.lua
index 2d4b79cc8c..0d75e072c3 100644
--- a/test/functional/editor/undo_spec.lua
+++ b/test/functional/editor/undo_spec.lua
@@ -225,3 +225,22 @@ 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)