terminal: handle &confirm and :confirm on unloading (#8726)

Show a proper confirmation dialog when trying to unload a terminal buffer while
the confirm option is set or when :confirm is used.

Fixes https://github.com/neovim/neovim/issues/4651

src/nvim/buffer.c

@@ -1172,16 +1172,23 @@ do_buffer (
         }
       } else {
         if (buf->terminal) {
+          if (p_confirm || cmdmod.confirm) {
+            if (!dialog_close_terminal(buf)) {
+              return FAIL;
+            }
+          } else {
             EMSG2(_("E89: %s will be killed(add ! to override)"),
                   (char *)buf->b_fname);
+            return FAIL;
+          }
         } else {
           EMSGN(_("E89: No write since last change for buffer %" PRId64
                   " (add ! to override)"),
                 buf->b_fnum);
-        }
           return FAIL;
         }
       }
+    }
 
     // When closing the current buffer stop Visual mode.
     if (buf == curbuf && VIsual_active) {

src/nvim/ex_cmds2.c

@@ -1339,6 +1339,22 @@ void dialog_changed(buf_T *buf, int checkall)
   }
 }
 
+/// Ask the user whether to close the terminal buffer or not.
+///
+/// @param buf The terminal buffer.
+/// @return bool Whether to close the buffer or not.
+bool dialog_close_terminal(buf_T *buf)
+{
+  char_u buff[DIALOG_MSG_SIZE];
+
+  dialog_msg(buff, _("Close \"%s\"?"),
+             (buf->b_fname != NULL) ? buf->b_fname : (char_u *)"?");
+
+  int ret = vim_dialog_yesnocancel(VIM_QUESTION, NULL, buff, 1);
+
+  return (ret == VIM_YES) ? true : false;
+}
+
 /// Return true if the buffer "buf" can be abandoned, either by making it
 /// hidden, autowriting it or unloading it.
 bool can_abandon(buf_T *buf, int forceit)

test/functional/terminal/buffer_spec.lua

@@ -201,6 +201,28 @@ describe('terminal buffer', function()
     feed([[<C-\><C-n>]])
     feed_command('bdelete!')
   end)
+
+  describe('handles confirmations', function()
+    it('with :confirm', function()
+      feed_command('terminal')
+      feed('<c-\\><c-n>')
+      feed_command('confirm bdelete')
+      screen:expect('Close "term://', nil, true, nil, true)
+    end)
+
+    it('with &confirm', function()
+      feed_command('terminal')
+      feed('<c-\\><c-n>')
+      feed_command('bdelete')
+      screen:expect('E89', nil, true, nil, true)
+      feed('<cr>')
+      eq('terminal', eval('&buftype'))
+      feed_command('set confirm | bdelete')
+      screen:expect('Close "term://', nil, true, nil, true)
+      feed('y')
+      neq('terminal', eval('&buftype'))
+    end)
+  end)
 end)
 
 describe('No heap-buffer-overflow when using', function()