fix(terminal): crash if TermClose deletes own buffer #19222

- Partially fixes #10386 except for the case where the alternate buffer
  is the default, empty, first buffer created on startup. #vimlife
- TODO: port patches related to `can_unload_buffer`, maybe that fully
  fixes #10386?

vim-patch:8.0.1732: crash when terminal API call deletes the buffer

src/nvim/buffer.c

@@ -89,6 +89,7 @@
 static char *msg_loclist = N_("[Location List]");
 static char *msg_qflist = N_("[Quickfix List]");
 static char *e_auabort = N_("E855: Autocommands caused command to abort");
+static char *e_buflocked = N_("E937: Attempt to delete a buffer that is in use");
 
 // Number of times free_buffer() was called.
 static int buf_free_count = 0;
@@ -438,7 +439,7 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
   // Disallow deleting the buffer when it is locked (already being closed or
   // halfway a command that relies on it). Unloading is allowed.
   if (buf->b_locked > 0 && (del_buf || wipe_buf)) {
-    emsg(_("E937: Attempt to delete a buffer that is in use"));
+    emsg(_(e_buflocked));
     return false;
   }
 
@@ -529,7 +530,9 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
   }
 
   if (buf->terminal) {
+    buf->b_locked++;
     terminal_close(&buf->terminal, -1);
+    buf->b_locked--;
   }
 
   // Always remove the buffer when there is no file name.
@@ -1182,6 +1185,10 @@ int do_buffer(int action, int start, int dir, int count, int forceit)
   if (unload) {
     int forward;
     bufref_T bufref;
+    if (buf->b_locked) {
+      emsg(_(e_buflocked));
+      return FAIL;
+    }
     set_bufref(&bufref, buf);
 
     // When unloading or deleting a buffer that's already unloaded and

src/nvim/testdir/test_autocmd.vim

@@ -169,7 +169,9 @@ func Test_autocmd_bufunload_avoiding_SEGV_01()
     exe 'autocmd BufUnload <buffer> ' . (lastbuf + 1) . 'bwipeout!'
   augroup END
 
-  call assert_fails('edit bb.txt', 'E937:')
+  " Todo: check for E937 generated first
+  " call assert_fails('edit bb.txt', 'E937:')
+  call assert_fails('edit bb.txt', 'E517:')
 
   autocmd! test_autocmd_bufunload
   augroup! test_autocmd_bufunload
@@ -540,7 +542,7 @@ func Test_three_windows()
   e Xtestje2
   sp Xtestje1
   call assert_fails('e', 'E937:')
-  call assert_equal('Xtestje2', expand('%'))
+  call assert_equal('Xtestje1', expand('%'))
 
   " Test changing buffers in a BufWipeout autocommand.  If this goes wrong
   " there are ml_line errors and/or a Crash.
@@ -563,7 +565,6 @@ func Test_three_windows()
 
   au!
   enew
-  bwipe! Xtestje1
   call delete('Xtestje1')
   call delete('Xtestje2')
   call delete('Xtestje3')

test/functional/autocmd/termxx_spec.lua

@@ -7,6 +7,8 @@ local eval, eq, neq, retry =
   helpers.eval, helpers.eq, helpers.neq, helpers.retry
 local ok = helpers.ok
 local feed = helpers.feed
+local pcall_err = helpers.pcall_err
+local assert_alive = helpers.assert_alive
 local iswin = helpers.iswin
 
 describe('autocmd TermClose', function()
@@ -16,6 +18,24 @@ describe('autocmd TermClose', function()
     command('set shellcmdflag=EXE shellredir= shellpipe= shellquote= shellxquote=')
   end)
 
+
+  local function test_termclose_delete_own_buf()
+    command('autocmd TermClose * bdelete!')
+    command('terminal')
+    eq('Vim(bdelete):E937: Attempt to delete a buffer that is in use', pcall_err(command, 'bdelete!'))
+    assert_alive()
+  end
+
+  -- TODO: fixed after merging patches for `can_unload_buffer`?
+  pending('TermClose deleting its own buffer, altbuf = buffer 1 #10386', function()
+    test_termclose_delete_own_buf()
+  end)
+
+  it('TermClose deleting its own buffer, altbuf NOT buffer 1 #10386', function()
+    command('edit foo1')
+    test_termclose_delete_own_buf()
+  end)
+
   it('triggers when fast-exiting terminal job stops', function()
     command('autocmd TermClose * let g:test_termclose = 23')
     command('terminal')