fix(cwd): stale buffer names after temp context-switch #41433

Problem:
1. `ctx_dirs_restore()` is the only chdir site that doesn't re-shorten
   buffer names; `post_chdir()`, `update_cwd()` and `do_autochdir()` all
   call `shorten_fnames(true)`.  So after a temp window-context switch
   that moved the CWD, every buffer's `b_fname` is still relative to the
   other directory.  ui2 renders messages in a float, and entering it is
   such a switch, so with a `:bcd` in a `BufReadPost` handler 'statusline'
   "%f" shows ".config/nvim/init.lua" while the CWD is already
   `~/.config/nvim`, and :write resolves the name against it:
   ```
   E212: Can't open file for writing: no such file or directory
   ```
2. `msg_multihl()` leaves `msg_ext_id` pointing at the caller's storage
   when nothing was emitted (e.g. 'msg_silent'): the reset only ran on
   a flush that produced chunks.  The next message then ships a dead
   stack frame as its msg_show id.  After ":silent write" the id is
   buf_write()'s `msg_id[MAXPATHL + 32]`:
   ```
   id = "\0\0\0\0\0\0\0\0\29\0(<C6>k\24R\17p<C7><C7><C7>\1\0\0\0..."
   ```

Solution:
1. `shorten_fnames(true)` after restoring the CWD.  Drop `cs_save_sfname`,
   which was a partial workaround for the same bug.
2. Release the id in `msg_multihl()`, where it is set and the caller's
   frame is still alive. Stop reading `.data.integer` out of a String
   union member.

src/nvim/context.c

@@ -33,6 +33,7 @@
 #include "nvim/eval/userfunc.h"
 #include "nvim/eval/vars.h"
 #include "nvim/ex_docmd.h"
+#include "nvim/fileio.h"
 #include "nvim/globals.h"
 #include "nvim/hashtab.h"
 #include "nvim/keycodes.h"
@@ -348,9 +349,6 @@ static void ctx_dirs_save(CtxSwitch *cs, win_T *wp, tabpage_T *tp, buf_T *buf)
   // If 'acd' is set, check we are using that directory.  If yes, then
   // apply 'acd' afterwards, otherwise restore the current directory.
   if (cs->cs_cwd != NULL && p_acd) {
-    if (curbuf->b_sfname != NULL && curbuf->b_fname == curbuf->b_sfname) {
-      cs->cs_save_sfname = xstrdup(curbuf->b_sfname);
-    }
     do_autochdir();
     char autocwd[MAXPATHL];
     if (os_dirname(autocwd, MAXPATHL) == OK) {
@@ -387,17 +385,11 @@ static void ctx_dirs_restore(CtxSwitch *cs)
 
   // Restore the CWD itself. After an explicit chdir, ctx_restore() re-derives it instead.
   if (cs->cs_apply_acd) {
-    xfree(cs->cs_save_sfname);
     do_autochdir();
   } else if (cs->cs_cwd != NULL && ((cs->cs_flags & kCtxKeepDirs) || !_ctx_did_chdir)) {
     os_chdir(cs->cs_cwd);
-    if (cs->cs_save_sfname != NULL) {
-      xfree(curbuf->b_sfname);
-      curbuf->b_sfname = cs->cs_save_sfname;
-      curbuf->b_fname = curbuf->b_sfname;
-    }
-  } else {
-    xfree(cs->cs_save_sfname);
+    // Buffer names are relative to the CWD, so they must follow it back. #41424
+    shorten_fnames(true);
   }
   XFREE_CLEAR(cs->cs_cwd);
 }

src/nvim/context_defs.h

@@ -116,5 +116,4 @@ typedef struct {
   char *cs_globaldir;             ///< Saved globaldir
   char *cs_cwd;                   ///< Saved CWD (kCtxKeepCwd/kCtxKeepDirs).
   bool cs_apply_acd;              ///< Re-apply 'autochdir' on ctx_restore().
-  char *cs_save_sfname;           ///< Saved b_sfname (kCtxKeepCwd/kCtxKeepDirs).
 } CtxSwitch;

src/nvim/message.c

@@ -415,6 +415,9 @@ MsgID msg_multihl(MsgID id, HlMessage hl_msg, const char *kind, bool history, bo
   if (hl_msg_updated && !(history && kv_size(hl_msg))) {
     hl_msg_free(hl_msg);
   }
+  // Release the id: it belongs to this message, and a String id only borrows the caller's storage
+  // (often a stack buffer). Redundant unless nothing was emitted (e.g. 'msg_silent').
+  msg_ext_id = INTEGER_OBJ(msg_id_next);
   return id;
 }
 
@@ -3462,7 +3465,10 @@ void msg_ext_ui_flush(void)
     msg_ext_append = false;
     msg_ext_fast = true;
     msg_ext_kind = NULL;
-    msg_id_next += (msg_ext_id.data.integer == msg_id_next);
+    // Consume the pre-allocated id, if the message did not get one from msg_multihl().
+    if (msg_ext_id.type == kObjectTypeInteger && msg_ext_id.data.integer == msg_id_next) {
+      msg_id_next++;
+    }
     msg_ext_id = INTEGER_OBJ(msg_id_next);
   }
 }

test/functional/ex_cmds/cd_spec.lua

@@ -572,6 +572,23 @@ describe('cd during temp context-switch', function()
     eq({ 1, tabdir, tabdir, startdir }, { tlwd(), tcwd(), cwd(), cwd(-1, -1) })
   end)
 
+  it('does not linger in buffer names after switch back #41424', function()
+    local bufdir = join(startdir, directories.buffer)
+    command('edit ' .. join(bufdir, tmpfile))
+    command('bcd ' .. bufdir)
+    command('split Xtest-cd-other') -- Buffer with no local dir.
+    local otherwin = call('win_getid')
+    command('wincmd p')
+    eq({ bufdir, tmpfile }, { cwd(), call('bufname', '%') })
+
+    -- Entering a window leaves `bufdir`, since the buffer there has no local dir.
+    call('win_execute', otherwin, 'split | close')
+
+    -- Names relative to the old CWD would cause ":write" to target nonsense.
+    eq({ bufdir, tmpfile }, { cwd(), call('bufname', '%') })
+    command('write')
+  end)
+
   it("nvim_open_win / nvim_win_set_buf keep the caller's cwd", function()
     local bufdir = join(startdir, directories.buffer)
     command('bcd ' .. bufdir)

test/functional/options/autochdir_spec.lua

@@ -6,6 +6,7 @@ local clear = n.clear
 local eq = t.eq
 local fn = n.fn
 local command = n.command
+local api = n.api
 local mkdir = t.mkdir
 
 describe("'autochdir'", function()
@@ -44,4 +45,26 @@ describe("'autochdir'", function()
     n.rmdir(dir_a)
     n.rmdir(dir_b)
   end)
+
+  it('win_execute() keeps buffer names #41417', function()
+    local root = vim.fs.normalize(t.tmpname(false))
+    mkdir(root)
+    mkdir(root .. '/a')
+    mkdir(root .. '/b')
+    clear()
+    command('set shellslash')
+    command('edit ' .. root .. '/a/file_a')
+    command('vsplit ' .. root .. '/b/file_b')
+    local win_b, buf_b = api.nvim_get_current_win(), api.nvim_get_current_buf()
+    command('wincmd p')
+    command('set autochdir')
+    -- Sit in an ancestor of both files: 'autochdir' moves away from it during win_execute(), and
+    -- both buffer names stay shortened, i.e. relative to the CWD.
+    command('cd ' .. root)
+    eq({ 'a/file_a', 'b/file_b' }, { fn.bufname('%'), fn.bufname(buf_b) })
+
+    fn.win_execute(win_b, 'echo')
+
+    eq({ 'a/file_a', 'b/file_b' }, { fn.bufname('%'), fn.bufname(buf_b) })
+  end)
 end)

test/functional/ui/messages_spec.lua

@@ -3859,6 +3859,26 @@ describe('progress-message', function()
     eq(8, id8)
   end)
 
+  it('msg-id is not inherited by the next message #41417', function()
+    local fname = 'Xtest_progress_msgid'
+    finally(function()
+      os.remove(fname)
+    end)
+    -- The write emits nothing ('msg_silent'), but must still release its msg-id.
+    command('silent write ' .. fname)
+    feed(':echoerr "boom"<CR>')
+    screen:expect({
+      messages = {
+        {
+          content = { { 'boom', 9, 'ErrorMsg' } },
+          history = true,
+          id = 1,
+          kind = 'echoerr',
+        },
+      },
+    })
+  end)
+
   it('accepts caller-defined id (string)', function()
     -- string id works
     local id = api.nvim_echo({ { 'supports str-id' } }, true, {

test/functional/ui/screen.lua

@@ -1597,17 +1597,23 @@ function Screen:_extstate_repr(attr_state, exp)
 
   local messages = {}
   for i, entry in ipairs(self.messages) do
+    local exp_msg = exp and exp.messages and exp.messages[i]
+    -- Late addition, only include when expected state includes it.
     local trigger = nil
-    if exp and exp.messages and exp.messages[i] and exp.messages[i].trigger ~= nil then
-      -- Late addition, only include when expected state includes it.
+    if exp_msg and exp_msg.trigger ~= nil then
       trigger = entry.trigger
     end
+    -- Progress messages are identified by their id, so always show it for them.
+    local id = nil
+    if entry.kind == 'progress' or (exp_msg and exp_msg.id ~= nil) then
+      id = entry.id
+    end
     messages[i] = {
       kind = entry.kind,
       content = self:_chunks_repr(entry.content, attr_state),
       history = entry.history or nil,
       append = entry.append or nil,
-      id = entry.kind == 'progress' and entry.id or nil,
+      id = id,
       trigger = trigger,
     }
   end