vim-patch:8.2.2036: buffer messed up if creating the quickfix window fails (#13365)

Problem:    Current buffer is messed up if creating a new buffer for the
            quickfix window fails.
Solution:   Check that creating the buffer succeeds. (closes vim/vim#7352)
9e40c4b15e

src/nvim/quickfix.c

@@ -3608,11 +3608,15 @@ static int qf_open_new_cwindow(qf_info_T *qi, int height)
   }
   if (qf_buf != NULL) {
     // Use the existing quickfix buffer
-    (void)do_ecmd(qf_buf->b_fnum, NULL, NULL, NULL, ECMD_ONE,
+    if (do_ecmd(qf_buf->b_fnum, NULL, NULL, NULL, ECMD_ONE,
-                  ECMD_HIDE + ECMD_OLDBUF, oldwin);
+                ECMD_HIDE + ECMD_OLDBUF, oldwin) == FAIL) {
+      return FAIL;
+    }
   } else {
     // Create a new quickfix buffer
-    (void)do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, oldwin);
+    if (do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, oldwin) == FAIL) {
+      return FAIL;
+    }
   }
 
   // Set the options for the quickfix buffer/window (if not already done)

src/nvim/testdir/test_quickfix.vim

@@ -4423,4 +4423,39 @@ func Test_add_invalid_entry_with_qf_window()
   cclose
 endfunc
 
+" Test for very weird problem: autocommand causes a failure, resulting opening
+" the quickfix window to fail. This still splits the window, but otherwise
+" should not mess up buffers.
+func Test_quickfix_window_fails_to_open()
+  CheckScreendump
+
+  let lines =<< trim END
+      anything
+      try
+        anything
+      endtry
+  END
+  call writefile(lines, 'XquickfixFails')
+
+  let lines =<< trim END
+      split XquickfixFails
+      silent vimgrep anything %
+      normal o
+      au BufLeave * ++once source XquickfixFails
+      " This will trigger the autocommand, which causes an error, what follows
+      " is aborted but the window was already split.
+      silent! cwindow
+  END
+  call writefile(lines, 'XtestWinFails')
+  let buf = RunVimInTerminal('-S XtestWinFails', #{rows: 13})
+  call VerifyScreenDump(buf, 'Test_quickfix_window_fails', {})
+
+  " clean up
+  call term_sendkeys(buf, ":bwipe!\<CR>")
+  call term_wait(buf)
+  call StopVimInTerminal(buf)
+  call delete('XtestWinFails')
+  call delete('XquickfixFails')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab