vim-patch:9.1.1376: quickfix dummy buffer may remain as dummy buffer

Problem:  when failing to wipeout a quickfix dummy buffer, it will
          remain as a dummy buffer, despite being kept.
Solution: clear its dummy BF_DUMMY flag in this case (Sean Dewar).

closes: vim/vim#17283

270124f46a

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
(cherry picked from commit d95b0a5396)

src/nvim/quickfix.c

@@ -5837,7 +5837,7 @@ static void wipe_dummy_buffer(buf_T *buf, char *dirname_start)
       }
     }
     if (!did_one) {
-      return;
+      goto fail;
     }
   }
 
@@ -5859,7 +5859,13 @@ static void wipe_dummy_buffer(buf_T *buf, char *dirname_start)
       // When autocommands/'autochdir' option changed directory: go back.
       restore_start_dir(dirname_start);
     }
+
+    return;
   }
+
+fail:
+  // Keeping the buffer, remove the dummy flag.
+  buf->b_flags &= ~BF_DUMMY;
 }
 
 /// Unload the dummy buffer that load_dummy_buffer() created. Restores

test/old/testdir/test_quickfix.vim

@@ -6618,4 +6618,23 @@ func Test_vimgrep_dummy_buffer_crash()
   %bw!
 endfunc
 
+func Test_vimgrep_dummy_buffer_keep()
+  augroup DummyKeep
+    autocmd!
+    " Trigger a wipe of the dummy buffer by aborting script processing. Prevent
+    " wiping it by splitting it from the autocmd window into an only window.
+    autocmd BufReadCmd * ++once let s:dummy_buf = bufnr()
+          \| tab split | call interrupt()
+  augroup END
+
+  call assert_fails('vimgrep /./ .')
+  call assert_equal(1, bufexists(s:dummy_buf))
+  " Ensure it's no longer considered a dummy; should be able to switch to it.
+  execute s:dummy_buf 'sbuffer'
+
+  unlet! s:dummy_buf
+  autocmd! DummyKeep
+  %bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab