provider/RPC: apply_autocmds_group(): fix double-free

During provider dispatch, eval_call_provider() saves global
state--including pointers, such as `autocmd_fname`--into
`provider_caller_scope` which is later restored by f_rpcrequest().

But `autocmd_fname` is special-cased in eval_vars(), for performance
(see Vim patch 7.2.021; this is also the singular purpose of the
`autocmd_fname_full` global. Yay!)

If eval_vars() frees `autocmd_fname` then its provider-RPC-scoped alias
becomes a problem.

Solution: Don't free autocmd_fname in eval_vars(), just copy into it.

closes #5245
closes #5617

Reference
------------------------------------------------------------------------

Vim patch 7.2.021
https://github.com/vim/vim/commit/f6dad43c98f47da1ff9d8c99b320fc3674f83c63
Problem:    When executing autocommands getting the full file name may be
            slow. (David Kotchan)
Solution:   Postpone calling FullName_save() until autocmd_fname is used.

vim_dev discussion (2008): "Problem with CursorMoved AutoCommand when
Editing Files on a Remote WIndows Share"
https://groups.google.com/d/msg/vim_dev/kj95weZa_eE/GTgj4aq5sIgJ

5 files changed, 48 insertions, 18 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index feccec85c6..2b4df14d78 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -13828,7 +13828,7 @@ static void f_rpcrequest(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     save_autocmd_fname_full = autocmd_fname_full;
     save_autocmd_bufnr = autocmd_bufnr;
     save_funccalp = save_funccal();
-    //
+
     current_SID = provider_caller_scope.SID;
     sourcing_name = provider_caller_scope.sourcing_name;
     sourcing_lnum = provider_caller_scope.sourcing_lnum;
@@ -22481,7 +22481,8 @@ typval_T eval_call_provider(char *provider, char *method, list_T *arguments)
   restore_funccal(provider_caller_scope.funccalp);
   provider_caller_scope = saved_provider_caller_scope;
   provider_call_nesting--;
-  
+  assert(provider_call_nesting >= 0);
+
   return rettv;
 }
 
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index e1d28b4a9c..51f11b7b56 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -8547,15 +8547,16 @@ eval_vars (
       break;
 
     case SPEC_AFILE:            /* file name for autocommand */
-      result = autocmd_fname;
-      if (result != NULL && !autocmd_fname_full) {
-        /* Still need to turn the fname into a full path.  It is
-         * postponed to avoid a delay when <afile> is not used. */
-        autocmd_fname_full = TRUE;
-        result = (char_u *)FullName_save((char *)autocmd_fname, FALSE);
-        xfree(autocmd_fname);
-        autocmd_fname = result;
+      if (autocmd_fname != NULL && !autocmd_fname_full) {
+        // Still need to turn the fname into a full path.  It was
+        // postponed to avoid a delay when <afile> is not used.
+        autocmd_fname_full = true;
+        result = (char_u *)FullName_save((char *)autocmd_fname, false);
+        // Copy into `autocmd_fname`, don't reassign it. #8165
+        xstrlcpy((char *)autocmd_fname, (char *)result, MAXPATHL);
+        xfree(result);
       }
+      result = autocmd_fname;
       if (result == NULL) {
         *errormsg = (char_u *)_(
             "E495: no autocommand file name to substitute for \"<afile>\"");
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 91b0a695f1..4e70daea90 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -6755,7 +6755,8 @@ static bool apply_autocmds_group(event_T event, char_u *fname, char_u *fname_io,
     autocmd_fname = fname_io;
   }
   if (autocmd_fname != NULL) {
-    autocmd_fname = vim_strsave(autocmd_fname);
+    // Allocate MAXPATHL for when eval_vars() resolves the fullpath.
+    autocmd_fname = vim_strnsave(autocmd_fname, MAXPATHL);
   }
   autocmd_fname_full = false;   // call FullName_save() later
 
diff --git a/test/functional/provider/python3_spec.lua b/test/functional/provider/python3_spec.lua
index f06728ec0e..93ac3ae017 100644
--- a/test/functional/provider/python3_spec.lua
+++ b/test/functional/provider/python3_spec.lua
@@ -3,6 +3,7 @@ local eval, command, feed = helpers.eval, helpers.command, helpers.feed
 local eq, clear, insert = helpers.eq, helpers.clear, helpers.insert
 local expect, write_file = helpers.expect, helpers.write_file
 local feed_command = helpers.feed_command
+local source = helpers.source
 local missing_provider = helpers.missing_provider
 
 do
@@ -13,7 +14,7 @@ do
   end
 end
 
-describe('python3 commands and functions', function()
+describe('python3 provider', function()
   before_each(function()
     clear()
     command('python3 import vim')
@@ -82,4 +83,20 @@ describe('python3 commands and functions', function()
   it('py3eval', function()
     eq({1, 2, {['key'] = 'val'}}, eval([[py3eval('[1, 2, {"key": "val"}]')]]))
   end)
+
+  it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    source([=[
+      python3 << EOF
+      import vim
+      def foo():
+        vim.eval('expand("<afile>:p")')
+        vim.eval('bufnr(expand("<afile>:p"))')
+      EOF
+      autocmd BufDelete * python3 foo()
+      autocmd BufUnload * python3 foo()]=])
+    feed_command("exe 'split' tempname()")
+    feed_command("bwipeout!")
+    feed_command('help help')
+    eq(2, eval('1+1'))  -- Still alive?
+  end)
 end)
diff --git a/test/functional/provider/ruby_spec.lua b/test/functional/provider/ruby_spec.lua
index a2c6c6a10e..e049ac7c41 100644
--- a/test/functional/provider/ruby_spec.lua
+++ b/test/functional/provider/ruby_spec.lua
@@ -1,16 +1,18 @@
 local helpers = require('test.functional.helpers')(after_each)
 
+local clear = helpers.clear
+local command = helpers.command
+local curbufmeths = helpers.curbufmeths
 local eq = helpers.eq
+local eval = helpers.eval
+local expect = helpers.expect
 local feed = helpers.feed
-local clear = helpers.clear
+local feed_command = helpers.feed_command
 local funcs = helpers.funcs
-local meths = helpers.meths
 local insert = helpers.insert
-local expect = helpers.expect
-local command = helpers.command
-local write_file = helpers.write_file
-local curbufmeths = helpers.curbufmeths
+local meths = helpers.meths
 local missing_provider = helpers.missing_provider
+local write_file = helpers.write_file
 
 do
   clear()
@@ -90,3 +92,11 @@ describe(':rubydo command', function()
     eq(false, curbufmeths.get_option('modified'))
   end)
 end)
+
+describe('ruby provider', function()
+  it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    command([=[autocmd BufDelete * ruby VIM::evaluate('expand("<afile>")')]=])
+    feed_command('help help')
+    eq(2, eval('1+1'))  -- Still alive?
+  end)
+end)