From 3b88a8a65da17a49c6aad2d1bcb3eb2163577c06 Mon Sep 17 00:00:00 2001
From: Barrett Ruth <62671086+barrettruth@users.noreply.github.com>
Date: Fri, 10 Jul 2026 11:40:13 -0500
Subject: [PATCH] fix(filetype): ensure directory bufname ends w/ slash sep
 #40552

Problem:
`vim.filetype.match()` needs a cheap way to recognize directory buffers
without doing filesystem stat work.

Solution:
Ensure full buffer names for directories end in a trailing slash. Now
directory buffers can proceed through the normal 'filetype' path.

Note side-effects: session and ShaDa buffer-list restore behavior must
be compatible, so those + corresponding tests must be updated.
---
 runtime/doc/api.txt                        |  3 ++
 runtime/doc/vimfn.txt                      |  4 ++-
 runtime/lua/nvim/dir.lua                   |  3 +-
 runtime/lua/vim/_meta/api.gen.lua          |  3 ++
 runtime/lua/vim/_meta/vimfn.gen.lua        |  4 ++-
 runtime/lua/vim/filetype.lua               |  3 ++
 src/nvim/api/buffer.c                      |  3 ++
 src/nvim/buffer.c                          | 42 +++++++++++++++++++++-
 src/nvim/eval.lua                          |  4 ++-
 src/nvim/path.c                            |  2 +-
 test/functional/api/buffer_spec.lua        | 26 ++++++++++++++
 test/functional/ex_cmds/mksession_spec.lua | 20 +++++++++++
 test/functional/lua/filetype_spec.lua      |  9 +++++
 test/functional/plugin/dir_spec.lua        |  5 +--
 14 files changed, 122 insertions(+), 9 deletions(-)

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index cfb30a7d30..f643bab7a0 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -2609,6 +2609,9 @@ nvim_buf_get_name({buf})                                 *nvim_buf_get_name()*
     Gets the full/absolute filepath of the buffer, or the buffer name for
     non-file buffers.
 
+    If the buffer represents a directory, the name ends with a path separator,
+    unless it was changed by |:file| or |nvim_buf_set_name()|.
+
     Attributes: ~
         Since: 0.1.0
 
diff --git a/runtime/doc/vimfn.txt b/runtime/doc/vimfn.txt
index 239bbe89d7..371779adfa 100644
--- a/runtime/doc/vimfn.txt
+++ b/runtime/doc/vimfn.txt
@@ -692,7 +692,9 @@ bufname([{buf}])                                                     *bufname()*
 
 		The result is the name of a buffer.  Mostly as it is displayed
 		by the `:ls` command, but not using special names such as
-		"[No Name]".
+		"[No Name]".  If the buffer represents a directory, the name
+		ends with a path separator, unless it was changed by |:file| or
+		|nvim_buf_set_name()|.
 		If {buf} is omitted the current buffer is used.
 		If {buf} is a Number, that buffer number's name is given.
 		Number zero is the alternate buffer for the current window.
diff --git a/runtime/lua/nvim/dir.lua b/runtime/lua/nvim/dir.lua
index f1d326bf54..23d4d8c09d 100644
--- a/runtime/lua/nvim/dir.lua
+++ b/runtime/lua/nvim/dir.lua
@@ -158,8 +158,7 @@ end
 
 ---@param buf integer
 local function open_parent(buf)
-  local path = api.nvim_buf_get_name(buf)
-  -- TODO(barrettruth): simplify after #40552
+  local path = fs.normalize(api.nvim_buf_get_name(buf))
   local name = encode_name(fs.basename(path)) .. (vim.fn.isdirectory(path) == 1 and '/' or '')
   navigate(fs.dirname(path))
   vim.fn.search([[\C\m^\V]] .. vim.fn.escape(name, [[\]]) .. [[\m$]], 'cw')
diff --git a/runtime/lua/vim/_meta/api.gen.lua b/runtime/lua/vim/_meta/api.gen.lua
index 78b3a28c18..5b207281d4 100644
--- a/runtime/lua/vim/_meta/api.gen.lua
+++ b/runtime/lua/vim/_meta/api.gen.lua
@@ -511,6 +511,9 @@ function vim.api.nvim_buf_get_mark(buf, name) end
 
 --- Gets the full/absolute filepath of the buffer, or the buffer name for non-file buffers.
 ---
+--- If the buffer represents a directory, the name ends with a path separator,
+--- unless it was changed by `:file` or `nvim_buf_set_name()`.
+---
 --- @param buf integer Buffer id, or 0 for current buffer
 --- @return string # Buffer name
 function vim.api.nvim_buf_get_name(buf) end
diff --git a/runtime/lua/vim/_meta/vimfn.gen.lua b/runtime/lua/vim/_meta/vimfn.gen.lua
index ede1efae4e..b7ba4d138d 100644
--- a/runtime/lua/vim/_meta/vimfn.gen.lua
+++ b/runtime/lua/vim/_meta/vimfn.gen.lua
@@ -605,7 +605,9 @@ function vim.fn.bufloaded(buf) end
 ---
 --- The result is the name of a buffer.  Mostly as it is displayed
 --- by the `:ls` command, but not using special names such as
---- "[No Name]".
+--- "[No Name]".  If the buffer represents a directory, the name
+--- ends with a path separator, unless it was changed by |:file| or
+--- |nvim_buf_set_name()|.
 --- If {buf} is omitted the current buffer is used.
 --- If {buf} is a Number, that buffer number's name is given.
 --- Number zero is the alternate buffer for the current window.
diff --git a/runtime/lua/vim/filetype.lua b/runtime/lua/vim/filetype.lua
index d04a355cf5..1c355fe2e8 100644
--- a/runtime/lua/vim/filetype.lua
+++ b/runtime/lua/vim/filetype.lua
@@ -3254,6 +3254,9 @@ function M.match(args)
   end
 
   if name then
+    if name:sub(-1) == '/' then
+      return 'directory'
+    end
     name = normalize_path(name)
 
     local ok_abspath, path = pcall(vim.fs.abspath, name)
diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 2995bd64d8..8b73c5bc1b 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -942,6 +942,9 @@ void nvim_buf_del_var(Buffer buf, String name, Error *err)
 
 /// Gets the full/absolute filepath of the buffer, or the buffer name for non-file buffers.
 ///
+/// If the buffer represents a directory, the name ends with a path separator,
+/// unless it was changed by |:file| or |nvim_buf_set_name()|.
+///
 /// @param buf     Buffer id, or 0 for current buffer
 /// @param[out] err   Error details, if any
 /// @return Buffer name
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 0903d729ce..f914e09591 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -404,6 +404,19 @@ int open_buffer(bool read_stdin, exarg_T *eap, int flags_arg)
     curbuf->b_flags |= BF_READERR;
   }
 
+  // Directory buf:
+  // readfile() returns NOTDONE without firing BufReadPost when it did not read a
+  // file (e.g. a directory). Since BufReadPost is what normally runs filetype
+  // detection, do it here so FileType fires before the BufEnter below.
+  if (retval == NOTDONE && *curbuf->b_p_ft == NUL
+      && curbuf->b_ffname != NULL
+      && after_pathsep(curbuf->b_ffname,
+                       curbuf->b_ffname + strlen(curbuf->b_ffname))) {
+    if (augroup_exists("filetypedetect")) {
+      do_doautocmd("filetypedetect BufRead", false, NULL);
+    }
+  }
+
   // Need to update automatic folding.  Do this before the autocommands,
   // they may use the fold info.
   foldUpdateAll(curwin);
@@ -2022,6 +2035,7 @@ buf_T *buflist_new(char *ffname_arg, char *sfname_arg, linenr_T lnum, int flags)
   }
 
   if (ffname != NULL) {
+    assert(sfname != NULL);
     buf->b_ffname = ffname;
     buf->b_sfname = xstrdup(sfname);
   }
@@ -3597,6 +3611,9 @@ int append_arg_number(win_T *wp, char *buf, size_t buflen)
 }
 
 /// Make "*ffname" a full file name, set "*sfname" to "*ffname" if not NULL.
+/// For a directory the resulting "*ffname" gets a trailing path separator. When
+/// "*sfname" already ends with a separator the final component is not resolved,
+/// so the symbolic link path is preserved.
 /// "*ffname" becomes a pointer to allocated memory (or NULL).
 /// When resolving a link both "*sfname" and "*ffname" will point to the same
 /// allocated memory.
@@ -3610,7 +3627,30 @@ void fname_expand(buf_T *buf, char **ffname, char **sfname)
   if (*sfname == NULL) {  // no short file name given, use ffname
     *sfname = *ffname;
   }
-  *ffname = fix_fname((*ffname));     // expand to full path
+  *ffname = fix_fname(*ffname);     // expand to full path
+  if (*ffname == NULL) {
+    *sfname = NULL;
+    return;
+  }
+  // Preserve a symbolic link path for directory buffers. fix_fname("link/")
+  // resolves to the target, so re-expand without the trailing separator: if
+  // "link" is a symbolic link to directory "target", ":edit link/" produces
+  // ".../link/", not ".../target/".
+  if (os_isdir(*ffname)) {
+    char *name = xstrdup(*sfname);
+    size_t name_len = strlen(name);
+    if (name_len > 0 && after_pathsep(name, name + name_len)
+        && name + name_len > get_past_head(name)) {
+      *path_tail_with_sep(name) = NUL;
+    }
+    char *full = fix_fname(name);
+    xfree(name);
+    if (full != NULL) {
+      xfree(*ffname);
+      *ffname = full;
+    }
+    *ffname = concat_fnames_realloc(*ffname, "", true);
+  }
 
 #ifdef MSWIN
   if (!buf->b_p_bin) {
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 980044fac3..a14d498d28 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -823,7 +823,9 @@ M.funcs = {
     desc = [=[
       The result is the name of a buffer.  Mostly as it is displayed
       by the `:ls` command, but not using special names such as
-      "[No Name]".
+      "[No Name]".  If the buffer represents a directory, the name
+      ends with a path separator, unless it was changed by |:file| or
+      |nvim_buf_set_name()|.
       If {buf} is omitted the current buffer is used.
       If {buf} is a Number, that buffer number's name is given.
       Number zero is the alternate buffer for the current window.
diff --git a/src/nvim/path.c b/src/nvim/path.c
index 184783d25c..79391674c7 100644
--- a/src/nvim/path.c
+++ b/src/nvim/path.c
@@ -389,7 +389,7 @@ int path_fnamecmp(const char *fname1, const char *fname2)
   const size_t len2 = strlen(fname2);
   return path_fnamencmp(fname1, fname2, MAX(len1, len2));
 #else
-  return mb_strcmp_ic((bool)p_fic, fname1, fname2);
+  return pathcmp(fname1, fname2, -1);
 #endif
 }
 
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index a44af2b4eb..ec8f8248e5 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -2392,6 +2392,32 @@ describe('api/buf', function()
       os.remove(new_name)
     end)
 
+    it('directory buffer-name always ends with slash', function()
+      local cwd = t.fix_slashes(fn.getcwd())
+      local dir = 'Xtest_dir_name'
+      t.mkdir(dir)
+      finally(function()
+        n.rmdir(dir)
+      end)
+      api.nvim_buf_set_name(0, dir)
+      eq(cwd .. '/' .. dir .. '/', t.fix_slashes(api.nvim_buf_get_name(0)))
+    end)
+
+    it('directory buffer-name preserves symbolic link path', function()
+      t.skip(t.is_os('win'), 'N/A for Windows')
+      local cwd = t.fix_slashes(fn.getcwd())
+      local target = 'Xtest_dir_target'
+      local link = 'Xtest_dir_link'
+      t.mkdir(target)
+      assert(vim.uv.fs_symlink(assert(vim.uv.fs_realpath(target)), link, { dir = true }))
+      finally(function()
+        os.remove(link)
+        n.rmdir(target)
+      end)
+      api.nvim_buf_set_name(0, link .. '/')
+      eq(cwd .. '/' .. link .. '/', t.fix_slashes(api.nvim_buf_get_name(0)))
+    end)
+
     describe("with 'autochdir'", function()
       local topdir
       local oldbuf
diff --git a/test/functional/ex_cmds/mksession_spec.lua b/test/functional/ex_cmds/mksession_spec.lua
index 3b4d17377b..f4819a1bc2 100644
--- a/test/functional/ex_cmds/mksession_spec.lua
+++ b/test/functional/ex_cmds/mksession_spec.lua
@@ -166,6 +166,26 @@ describe(':mksession', function()
     eq(cwd_dir .. '/' .. tmpfile_base .. '2', fn.expand('%:p'))
   end)
 
+  it('restores symlinked directory buffer names', function()
+    skip(is_os('win'), 'N/A for Windows')
+
+    local cwd_dir = t.fix_slashes(fn.getcwd())
+    local link_dir = file_prefix .. '-link'
+    assert(vim.uv.fs_symlink(assert(vim.uv.fs_realpath(tab_dir)), link_dir, { dir = true }))
+    finally(function()
+      os.remove(link_dir)
+    end)
+
+    command('set sessionoptions=buffers')
+    command('edit ' .. link_dir)
+    local expected = cwd_dir .. '/' .. link_dir .. '/'
+    eq(expected, api.nvim_buf_get_name(0))
+    command('mksession ' .. session_file)
+    command('%bwipeout!')
+    command('source ' .. session_file)
+    eq(expected, api.nvim_buf_get_name(0))
+  end)
+
   it('restores CWD for :terminal buffers #11288', function()
     local cwd_dir = fn.fnamemodify('.', ':p:~'):gsub([[[\/]*$]], '')
     cwd_dir = t.fix_slashes(cwd_dir) -- :mksession always uses unix slashes.
diff --git a/test/functional/lua/filetype_spec.lua b/test/functional/lua/filetype_spec.lua
index c174a8c065..2593a08d71 100644
--- a/test/functional/lua/filetype_spec.lua
+++ b/test/functional/lua/filetype_spec.lua
@@ -73,6 +73,15 @@ describe('vim.filetype', function()
     )
   end)
 
+  it('detects directory filenames', function()
+    eq(
+      'directory',
+      exec_lua(function()
+        return vim.filetype.match({ filename = '/tmp/example/' })
+      end)
+    )
+  end)
+
   it('works without defined g:ft_ignore_pat', function()
     local match_opts = { filename = 'unknown-ft', buf = api.nvim_create_buf(false, true) }
     eq(
diff --git a/test/functional/plugin/dir_spec.lua b/test/functional/plugin/dir_spec.lua
index 68ee235c0a..53839b2cd1 100644
--- a/test/functional/plugin/dir_spec.lua
+++ b/test/functional/plugin/dir_spec.lua
@@ -37,8 +37,9 @@ local function bufopt(name)
 end
 
 local function assert_directory(path)
-  eq(path, api.nvim_buf_get_name(0))
-  eq(path, fn.bufname('%'))
+  local ffname = path:sub(-1) == '/' and path or path .. '/'
+  eq(ffname, api.nvim_buf_get_name(0))
+  eq(path, vim.fs.normalize(fn.bufname('%')))
   eq('directory', bufopt('filetype'))
   eq(true, bufopt('buflisted'))
 end