patch 8.2.2036: buffer messed up if creating the quickfix window fails

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

src/quickfix.c

@@ -4151,13 +4151,15 @@ 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,
-		ECMD_HIDE + ECMD_OLDBUF, oldwin);
+	if (do_ecmd(qf_buf->b_fnum, NULL, NULL, NULL, ECMD_ONE,
+				      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;
 
 	// save the number of the new buffer
 	qi->qf_bufnr = curbuf->b_fnum;

src/testdir/dumps/Test_quickfix_window_fails.dump

@@ -0,0 +1,13 @@
+|a+0&#ffffff0|n|y|t|h|i|n|g| @66
+> @74
+|t|r|y| @71
+|X+3&&|q|u|i|c|k|f|i|x|F|a|i|l|s| |[|+|]| @38|2|,|0|-|1| @9|T|o|p
+| +0&&@74
+|t|r|y| @71
+@2|a|n|y|t|h|i|n|g| @64
+|X+1&&|q|u|i|c|k|f|i|x|F|a|i|l|s| |[|+|]| @38|2|,|0|-|1| @9|5|0|%
+| +0&&@74
+|~+0#4040ff13&| @73
+|~| @73
+|[+1#0000000&|N|o| |N|a|m|e|]| @47|0|,|0|-|1| @9|A|l@1
+| +0&&@74

src/testdir/test_quickfix.vim

@@ -5218,4 +5218,39 @@ 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

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2036,
 /**/
     2035,
 /**/