fix(events): don't expand `args.file` for Lua callback (#31473)

Problem:  In an autocommand Lua callback whether `args.file` is expanded
          depends on whether `expand('<afile>')` has been called.
Solution: Always use the unexpanded file name for `args.file`.

Related to #31306 and vim/vim#16106. This doesn't provide `sfname`, but
at least makes `args.file` have a consistent value.

6 files changed, 49 insertions, 26 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 9cb8f72348..cb3b2a3f77 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3494,9 +3494,9 @@ nvim_create_autocmd({event}, {opts})                   *nvim_create_autocmd()*
                    • event: (string) name of the triggered event
                      |autocmd-events|
                    • group: (number|nil) autocommand group id, if any
-                   • match: (string) expanded value of <amatch>
-                   • buf: (number) expanded value of <abuf>
-                   • file: (string) expanded value of <afile>
+                   • file: (string) <afile> (not expanded to a full path)
+                   • match: (string) <amatch> (expanded to a full path)
+                   • buf: (number) <abuf>
                    • data: (any) arbitrary data passed from
                      |nvim_exec_autocmds()|                       *event-data*
                  • command (string) optional: Vim command to execute on event.
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index d74ee11b46..b2385197bd 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -963,9 +963,9 @@ function vim.api.nvim_create_augroup(name, opts) end
 ---     - id: (number) autocommand id
 ---     - event: (string) name of the triggered event `autocmd-events`
 ---     - group: (number|nil) autocommand group id, if any
----     - match: (string) expanded value of [<amatch>]
----     - buf: (number) expanded value of [<abuf>]
----     - file: (string) expanded value of [<afile>]
+---     - file: (string) [<afile>] (not expanded to a full path)
+---     - match: (string) [<amatch>] (expanded to a full path)
+---     - buf: (number) [<abuf>]
 ---     - data: (any) arbitrary data passed from [nvim_exec_autocmds()] [event-data]()
 --- - command (string) optional: Vim command to execute on event. Cannot be used with
 --- {callback}
diff --git a/src/nvim/api/autocmd.c b/src/nvim/api/autocmd.c
index db87500d08..45e2de69e0 100644
--- a/src/nvim/api/autocmd.c
+++ b/src/nvim/api/autocmd.c
@@ -386,9 +386,9 @@ cleanup:
 ///                 - id: (number) autocommand id
 ///                 - event: (string) name of the triggered event |autocmd-events|
 ///                 - group: (number|nil) autocommand group id, if any
-///                 - match: (string) expanded value of [<amatch>]
-///                 - buf: (number) expanded value of [<abuf>]
-///                 - file: (string) expanded value of [<afile>]
+///                 - file: (string) [<afile>] (not expanded to a full path)
+///                 - match: (string) [<amatch>] (expanded to a full path)
+///                 - buf: (number) [<abuf>]
 ///                 - data: (any) arbitrary data passed from [nvim_exec_autocmds()] [event-data]()
 ///             - command (string) optional: Vim command to execute on event. Cannot be used with
 ///             {callback}
diff --git a/src/nvim/autocmd.c b/src/nvim/autocmd.c
index c08ef7a4c1..118a50e15d 100644
--- a/src/nvim/autocmd.c
+++ b/src/nvim/autocmd.c
@@ -1666,7 +1666,9 @@ bool apply_autocmds_group(event_T event, char *fname, char *fname_io, bool force
   } else {
     autocmd_fname = fname_io;
   }
+  char *afile_orig = NULL;  ///< Unexpanded <afile>
   if (autocmd_fname != NULL) {
+    afile_orig = xstrdup(autocmd_fname);
     // Allocate MAXPATHL for when eval_vars() resolves the fullpath.
     autocmd_fname = xstrnsave(autocmd_fname, MAXPATHL);
   }
@@ -1798,6 +1800,7 @@ bool apply_autocmds_group(event_T event, char *fname, char *fname_io, bool force
     // save vector size, to avoid an endless loop when more patterns
     // are added when executing autocommands
     .ausize = kv_size(autocmds[(int)event]),
+    .afile_orig = afile_orig,
     .fname = fname,
     .sfname = sfname,
     .tail = tail,
@@ -1865,6 +1868,7 @@ bool apply_autocmds_group(event_T event, char *fname, char *fname_io, bool force
   autocmd_nested = save_autocmd_nested;
   xfree(SOURCING_NAME);
   estack_pop();
+  xfree(afile_orig);
   xfree(autocmd_fname);
   autocmd_fname = save_autocmd_fname;
   autocmd_fname_full = save_autocmd_fname_full;
@@ -2029,8 +2033,8 @@ static bool call_autocmd_callback(const AutoCmd *ac, const AutoPatCmd *apc)
     MAXSIZE_TEMP_DICT(data, 7);
     PUT_C(data, "id", INTEGER_OBJ(ac->id));
     PUT_C(data, "event", CSTR_AS_OBJ(event_nr2name(apc->event)));
+    PUT_C(data, "file", CSTR_AS_OBJ(apc->afile_orig));
     PUT_C(data, "match", CSTR_AS_OBJ(autocmd_match));
-    PUT_C(data, "file", CSTR_AS_OBJ(autocmd_fname));
     PUT_C(data, "buf", INTEGER_OBJ(autocmd_bufnr));
 
     if (apc->data) {
diff --git a/src/nvim/autocmd_defs.h b/src/nvim/autocmd_defs.h
index 490782b209..cba947e85f 100644
--- a/src/nvim/autocmd_defs.h
+++ b/src/nvim/autocmd_defs.h
@@ -52,6 +52,7 @@ struct AutoPatCmd_S {
   AutoPat *lastpat;         ///< Last matched AutoPat
   size_t auidx;             ///< Current autocmd index to execute
   size_t ausize;            ///< Saved AutoCmd vector size
+  char *afile_orig;         ///< Unexpanded <afile>
   char *fname;              ///< Fname to match with
   char *sfname;             ///< Sfname to match with
   char *tail;               ///< Tail of fname
diff --git a/test/functional/api/autocmd_spec.lua b/test/functional/api/autocmd_spec.lua
index 3f9883f43f..bb6456f45b 100644
--- a/test/functional/api/autocmd_spec.lua
+++ b/test/functional/api/autocmd_spec.lua
@@ -273,54 +273,72 @@ describe('autocmd api', function()
       eq({}, api.nvim_get_autocmds({ event = 'User', pattern = 'Test' }))
     end)
 
-    it('receives an args table', function()
+    local function test_autocmd_args(event)
+      local function get_amatch(pat)
+        return event == 'User' and pat or vim.fs.normalize(n.fn.fnamemodify(pat, ':p'))
+      end
+
       local group_id = api.nvim_create_augroup('TestGroup', {})
       -- Having an existing autocmd calling expand("<afile>") shouldn't change args #18964
-      api.nvim_create_autocmd('User', {
+      api.nvim_create_autocmd(event, {
         group = 'TestGroup',
         pattern = 'Te*',
         command = 'call expand("<afile>")',
       })
 
-      local autocmd_id = exec_lua [[
-        return vim.api.nvim_create_autocmd("User", {
+      local autocmd_id = exec_lua(([[
+        return vim.api.nvim_create_autocmd(%q, {
           group = "TestGroup",
           pattern = "Te*",
           callback = function(args)
             vim.g.autocmd_args = args
           end,
         })
-      ]]
+      ]]):format(event))
 
-      api.nvim_exec_autocmds('User', { pattern = 'Test pattern' })
+      local exec_pat = 'Test pattern'
+      local amatch = get_amatch(exec_pat)
+      api.nvim_exec_autocmds(event, { pattern = exec_pat })
       eq({
         id = autocmd_id,
         group = group_id,
-        event = 'User',
-        match = 'Test pattern',
-        file = 'Test pattern',
+        event = event,
+        match = amatch,
+        file = exec_pat,
         buf = 1,
       }, api.nvim_get_var('autocmd_args'))
 
       -- Test without a group
-      autocmd_id = exec_lua [[
-        return vim.api.nvim_create_autocmd("User", {
+      autocmd_id = exec_lua(([[
+        return vim.api.nvim_create_autocmd(%q, {
           pattern = "*",
           callback = function(args)
             vim.g.autocmd_args = args
           end,
         })
-      ]]
+      ]]):format(event))
 
-      api.nvim_exec_autocmds('User', { pattern = 'some_pat' })
+      exec_pat = 'some_pat'
+      amatch = get_amatch(exec_pat)
+      api.nvim_exec_autocmds(event, { pattern = exec_pat })
       eq({
         id = autocmd_id,
         group = nil,
-        event = 'User',
-        match = 'some_pat',
-        file = 'some_pat',
+        event = event,
+        match = amatch,
+        file = exec_pat,
         buf = 1,
       }, api.nvim_get_var('autocmd_args'))
+    end
+
+    describe('receives correct args table', function()
+      it('for event that takes non-file pattern', function()
+        test_autocmd_args('User')
+      end)
+
+      it('for event that takes file pattern', function()
+        test_autocmd_args('BufEnter')
+      end)
     end)
 
     it('can receive arbitrary data', function()