[0/2] reftable/stack: fix race in up-to-date check

Message

Patrick Steinhardt Jan. 18, 2024, 1:41 p.m. UTC

Hi,

In 6fdfaf15 (reftable/stack: use stat info to avoid re-reading stack
list, 2024-01-11) we have introduced a new mechanism to avoid re-reading
the table list in case stat(3P) figures out that the stack didn't change
since the last time we read it.

Unfortunately, I've been able to hit a bug there that can happen under
very specific circumstances when a reading and writing process race with
each other. When the writer appends to the stack and then compacts it,
it may happen that we recycle the previously-used inode of "tables.list"
so that the resulting `struct stat` exactly matches the cached value of
the reading process.

The first patch is merely cosmetic, whereas the second patch fixes the
described race. The topic depends on ps/reftable-optimize-io, which has
introduced the issue.

Patrick

Patrick Steinhardt (2):
  reftable/stack: unconditionally reload stack after commit
  reftable/stack: fix race in up-to-date check

 reftable/stack.c  | 101 ++++++++++++++++++++++++++++++++++++++++++----
 reftable/stack.h  |   4 +-
 reftable/system.h |   1 -
 3 files changed, 96 insertions(+), 10 deletions(-)


base-commit: 718a93ecc06ed59dda4e6a5d91b1c2169275694f