Assertion ‘n & PENDING’ failed in fil_space_t::set_needs_flush()

This debug assertion failure was observed while testing MDEV-14795, but it could affect older releases as well.

The scenario is as follows:

1. A write of a page of a tablespace has been initiated.
2. Something is suspending further I/O on the tablespace (fil_space_t::set_stopping()). This could be as part of executing DROP TABLE or anything that rebuilds a table (TRUNCATE TABLE, OPTIMIZE TABLE, ALTER TABLE), or possibly as part of executing innodb_undo_log_truncate=ON.
3. The buf_flush_page_cleaner thread encounters some more dirty pages for the table in buf_pool.flush_list, but notices that all further writes to the tablespace need to be discarded. It will release a tablespace reference that it did not hold in the first place.
4. write_io_callback calls IORequest::write_complete() on the previously submitted write, which will hit the debug assertion failure, catching the corruption of the reference count and flags.

The following patch fixes this so that the tablespace reference will be incremented even though the flags are set.

diff --git a/storage/innobase/include/fil0fil.h b/storage/innobase/include/fil0fil.h

index 10365d167b7..10c084bbbf1 100644

--- a/storage/innobase/include/fil0fil.h

+++ b/storage/innobase/include/fil0fil.h

@@ -1526,9 +1526,11 @@ template<bool have_reference> inline void fil_space_t::flush()

     flush_low();

   else

   {

-    if (!(acquire_low() & (STOPPING | CLOSING)))

+    if (!(acquire_low(STOPPING | CLOSING) & (STOPPING | CLOSING)))

+    {

       flush_low();

-    release();

+      release();

+    }

   }

 }

The impact of this for non-debug builds should be that the reference count wraps around (from 0 to 536,870,911) and the 3 flags are "decremented".

I believe that this should lead to InnoDB hanging at some point, and possibly a missed fdatasync() or fsync() call (which should not matter much, because the file is typically going to be deleted anyway).