autocmd: add WinClosed event

- only fire once, just before freeing mem
- trigger when on a different buffer
- avoid recursive calls in another tab

runtime/doc/autocmd.txt

@@ -317,6 +317,7 @@ Name			triggered by ~
 |CursorMoved|		the cursor was moved in Normal mode
 |CursorMovedI|		the cursor was moved in Insert mode
 
+|WinClosed|		after closing a window
 |WinNew|		after creating a new window
 |WinEnter|		after entering another window
 |WinLeave|		before leaving a window
@@ -1131,6 +1132,8 @@ VimResized			After the Vim window was resized, thus 'lines'
 VimResume			After Nvim resumes from |suspend| state.
 							*VimSuspend*
 VimSuspend			Before Nvim enters |suspend| state.
+							*WinClosed*
+WinClosed			After closing a window.
 							*WinEnter*
 WinEnter			After entering another window.  Not done for
 				the first window, when Vim has just started.
@@ -1148,7 +1151,6 @@ WinLeave			Before leaving a window.  If the window to be
 				executes the BufLeave autocommands before the
 				WinLeave autocommands (but not for ":new").
 				Not used for ":qa" or ":q" when exiting Vim.
-
 							*WinNew*
 WinNew				When a new window was created.  Not done for
 				the first window, when Vim has just started.

runtime/doc/vim_diff.txt

@@ -159,6 +159,7 @@ Events:
   |UILeave|
   |VimResume|
   |VimSuspend|
+  |WinClosed|
 
 Functions:
   |dictwatcheradd()| notifies a callback whenever a |Dict| is modified

src/nvim/auevents.lua

@@ -110,6 +110,7 @@ return {
     'WinEnter',               -- after entering a window
     'WinLeave',               -- before leaving a window
     'WinNew',                 -- when entering a new window
+    'WinClosed',              -- after closing a window
   },
   aliases = {
     BufCreate = 'BufAdd',
@@ -129,5 +130,6 @@ return {
     TermOpen=true,
     UIEnter=true,
     UILeave=true,
+    WinClosed=true,
   },
 }

src/nvim/window.c

@@ -2502,9 +2502,10 @@ int win_close(win_T *win, bool free_buf)
         return FAIL;
     }
     win->w_closing = true;
-    apply_autocmds(EVENT_WINLEAVE, NULL, NULL, FALSE, curbuf);
+    apply_autocmds(EVENT_WINLEAVE, NULL, NULL, false, curbuf);
-    if (!win_valid(win))
+    if (!win_valid(win)) {
       return FAIL;
+    }
     win->w_closing = false;
     if (last_window())
       return FAIL;
@@ -2534,6 +2535,12 @@ int win_close(win_T *win, bool free_buf)
     }
   }
 
+  // Fire WinClosed just before starting to free window-related resources.
+  do_autocmd_winclosed(win);
+  // autocmd may have freed the window already.
+  if (!win_valid_any_tab(win)) {
+    return OK;
+  }
 
   /* Free independent synblock before the buffer is freed. */
   if (win->w_buffer != NULL)
@@ -2576,6 +2583,7 @@ int win_close(win_T *win, bool free_buf)
     win_close_othertab(win, false, prev_curtab);
     return FAIL;
   }
+
   // Autocommands may have closed the window already, or closed the only
   // other window or moved to another tab page.
   if (!win_valid(win) || (!win->w_floating && last_window())
@@ -2585,8 +2593,9 @@ int win_close(win_T *win, bool free_buf)
 
   // let terminal buffers know that this window dimensions may be ignored
   win->w_closing = true;
-  /* Free the memory used for the window and get the window that received
+
-   * the screen space. */
+  // Free the memory used for the window and get the window that received
+  // the screen space.
   wp = win_free_mem(win, &dir, NULL);
 
   if (help_window) {
@@ -2678,6 +2687,19 @@ int win_close(win_T *win, bool free_buf)
   return OK;
 }
 
+static void do_autocmd_winclosed(win_T *win)
+  FUNC_ATTR_NONNULL_ALL
+{
+  static bool recursive = false;
+  if (recursive || !has_event(EVENT_WINCLOSED)) {
+    return;
+  }
+  recursive = true;
+  apply_autocmds(EVENT_WINCLOSED, win->w_buffer->b_fname,
+                 win->w_buffer->b_fname, false, win->w_buffer);
+  recursive = false;
+}
+
 /*
  * Close window "win" in tab page "tp", which is not the current tab page.
  * This may be the last window in that tab page and result in closing the tab,
@@ -2698,6 +2720,13 @@ void win_close_othertab(win_T *win, int free_buf, tabpage_T *tp)
     return;  // window is already being closed
   }
 
+  // Fire WinClosed just before starting to free window-related resources.
+  do_autocmd_winclosed(win);
+  // autocmd may have freed the window already.
+  if (!win_valid_any_tab(win)) {
+    return;
+  }
+
   if (win->w_buffer != NULL) {
     // Close the link to the buffer.
     close_buffer(win, win->w_buffer, free_buf ? DOBUF_UNLOAD : 0, false);

test/functional/autocmd/autocmd_spec.lua

@@ -2,6 +2,7 @@ local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
 
 local dedent = helpers.dedent
+local neq = helpers.neq
 local eq = helpers.eq
 local eval = helpers.eval
 local feed = helpers.feed
@@ -40,6 +41,71 @@ describe('autocmd', function()
     assert.same(expected, eval('g:foo'))
   end)
 
+  it(':close triggers WinClosed event', function()
+    command('let g:triggered = 0')
+    command('new')
+    command('autocmd WinClosed <buffer> :let g:triggered+=1')
+    eq(0, eval('g:triggered'))
+    command('close')
+    eq(1, eval('g:triggered'))
+  end)
+
+  it(':bdelete triggers WinClosed event', function()
+    command('let g:triggered = 0')
+    command('autocmd WinClosed <buffer> :let g:triggered+=1')
+    local first_buffer = eval("bufnr('%')")
+    command('new')
+    command('bdelete ' .. first_buffer )
+    eq(1, eval('g:triggered'))
+  end)
+
+  it(':close triggers WinClosed event in another tab', function()
+    command('let g:triggered = 0')
+    local current_buffer = eval("bufnr('%')")
+    command('autocmd WinClosed <buffer> :let g:triggered+=1')
+    command('tabnew')
+    command('bdelete ' .. current_buffer)
+    eq(1, eval('g:triggered'))
+  end)
+
+  it('WinClosed events are not recursive in different window', function()
+    command('let g:triggered = 0')
+    local first_buffer = eval("bufnr('%')")
+    command('autocmd WinClosed <buffer> :let g:triggered+=1')
+    command('new')
+    local second_buffer = eval("bufnr('%')")
+    command('autocmd WinClosed <buffer> :bdelete ' .. first_buffer)
+    command('new')
+    neq(-1, funcs.bufwinnr(first_buffer))
+    command('bdelete ' .. second_buffer )
+    eq(0, eval('g:triggered'))
+
+    -- first event was triggered, second wasn't
+    eq(-1, funcs.bufwinnr(first_buffer))
+  end)
+
+  it('WinClosed events are not recursive in the same window', function()
+    command('let g:triggered = 0')
+    command('new')
+    local second_buffer = eval("bufnr('%')")
+    command('autocmd WinClosed <buffer> :let g:triggered+=1 | bdelete ' .. second_buffer)
+    neq(-1, funcs.bufwinnr(second_buffer))
+    eq(0, eval('g:triggered'))
+    command('bdelete ' .. second_buffer )
+    eq(-1, funcs.bufwinnr(second_buffer))
+    eq(1, eval('g:triggered'))
+  end)
+
+  it('WinClosed events are not recursive in different tab', function()
+    command('let g:triggered = 0')
+    command('new')
+    local second_buffer = eval("bufnr('%')")
+    command('autocmd WinClosed <buffer> :let g:triggered+=1 | bdelete ' .. second_buffer)
+    command('tabnew')
+    command('bdelete ' .. second_buffer )
+    eq(1, eval('g:triggered'))
+  end)
+
   it('v:vim_did_enter is 1 after VimEnter', function()
     eq(1, eval('v:vim_did_enter'))
   end)