Reworked gstate/commit interactions
The main change is moving away from applying gstate changes via special
attrs. Instead, gstate changes are applied implicitly, whenever the
relevant field in lfs_t differs from the gstate on-disk.
How do we recover from errors then? Well, we already need to track the
exact on-disk encoding of any gstate (grm_p) to avoid issues with minor
encoding differences, so if we encounter an error, we can revert any
changes to gstate by re-decoding the on-disk gstate. This is more
fragile: 1. all error paths in lfsr_mdir_commit need to revert gstate,
2. logic must not error between gstate updates and lfsr_mdir_commit, but
it gets the job done.
The benefit of this approach is that it's much easier to manipulate
gstate inside of lfsr_mdir_commit. No more hacky attr-list scanning to
patch grms mid-commit! It also in theory saves stack usage by dropping
an attr, but none of these attrs were on our stack hot-path.
Other gstate changes:
- Moved all grm adjustments into lfsr_mdir_commit.
This should deduplicate the messy grm adjust logic and make grms
easier to work with.
One hiccup though is the temporarily self-removing bookmark created in
lfsr_mkdir, which needs to create a grm referencing an mid that
doesn't exist yet. To work around this, lfsr_mdir_commit now
automatically creates grms for new bookmarks.
This might be a problem if we ever elide same-mdir mkdirs, but if so
we can solve that problem then.
- Dropped lfsr_data_t xoring, the added complexity wasn't really worth
it since all gstate should be small enough to buffer on the stack.
- Renamed several things:
- lfsr_grm_push/poprm -> lfsr_grm_push/pop
- lfsr_grm_isrm -> lfsr_grm_ispending
- grm_g -> grm_p
- grm.rms -> grm.mids
- Moved things around so grm/gstate logic is grouped together.
Unfortunately none of these attrs were on our stack hot-path, so no
stack savings. But thanks to the simpler logic, this does save quite a
bit of code:
code stack
before: 33514 2632
after: 33338 (+0.5%) 2640 (+0.3%)
This commit is contained in:
Christopher Haster