Unlike lfsr_setattr/getattr/etc, file-attached custom attributes are
RAM-backed snapshots attached to, well, files, that can be committed
atomically along with the file's contents. Great for power-loss
resilience, but boy does it make a mess of an API.
This API was really where custom attributes needed some TLC.
The biggest change is how file-attached custom attributes interact with
file sync broadcasting.
A common complaint from users is that setting custom attributes did not
update attributes in open file handles. This behavior is _very_
inconsistent with other filesystems and created a lot of confusion.
Since we're nailing down littlefs's snapshot/broadcasting model as a
part of larger changes, it makes sense to also nail down how custom
attributes interact.
In the new model:
- Custom attributes are still in-RAM snapshots. Updates do not
immediately take effect, even across write calls.
- On lfsr_file_sync or lfsr_file_close, custom attributes are written
atomically to disk and broadcasted to all open file handles.
- lfsr_setattr/removeattr also take part in attribute broadcasting. When
called, lfsr_setattr/removeattr updates the attribute on disk and
broadcasts the attribute changes to all open file handles.
- Desynced files do _not_ recieve any attribute broadcasts in the same
way they do not recieve any data broadcasts.
This should hopefully make littlefs behave much more consistently with
other filesystems, while still maintaining a well-defined snapshot and
power-loss properties.
---
The lfs_attr struct also gained several new fields:
// Custom attribute structure, used to describe custom attributes
// committed atomically during file writes.
struct lfs_attr {
// Type of attribute
//
// Note some of this range is reserved:
// 0x00-0x7f - Free for custom attributes
// 0x80-0xff - May be assigned a standard attribute
uint8_t type;
// Flags that control how attr is read/written/removed
uint8_t flags;
// Pointer the buffer where the attr will be read/written
void *buffer;
// Size of the attr buffer in bytes, this can be set to
// LFS_ERR_NOATTR to remove the attr
lfs_ssize_t buffer_size;
// Optional pointer to a mutable attr size, updated on read/write,
// set to LFS_ERR_NOATTR if attr does not exist
//
// Defaults to buffer_size if NULL
lfs_ssize_t *size;
};
Which are useful for several new features:
- lfs_attr now supports LFS_A_RDONLY/WRONLY/RDWR modes.
One of the blockers for attribute broadcasting was in-ROM attributes,
where broadcast updates would hard-fault. But now if you mark in-ROM
attributes as WRONLY, and in-RAM attributes as RDWR, this problem goes
away.
- When opened, lfs_attr now optionally writes the attribute size to the
indirect size field.
No more hacky zero padding and not knowing an attribute's size.
Note this follows the same rules as lfsr_getattr, so it does truncate
if the buffer is too small.
The size field can also be set to NULL, in which case lfs_attr
defaults to the buffer_size. This can be quite useful for pure
ROM-backed attributes.
- Missing attributes are now represented with size=LFS_ERR_NOATTR.
No more zero-sized vs missing attribute ambiguity.
This also makes it possible to remove attributes via lfs_attr, by
setting the size to LFS_ERR_NOATTR manually.
This does lead to a bit of a quirk where buffer_size can be
LFS_ERR_NOATTR, which is a bit weird but at least consistent.
- Changes to lfs_attrs will now always trigger file syncs by default.
Previously, if you changed an attribute, you had to also change the
file's contents for it to get written to disk. As pointed out by users
this is both surprising and difficult to work around.
Solving this is quite tricky since there's no real signalling
mechanism between attribute buffers and littlefs. The best I could
come up with is to read attributes from disk during lfsr_file_sync to
see if anything changed.
At the very least, the new flag LFS_A_LAZY restores the old behavior
in case the extra reads in lfsr_file_sync are problematic.
Though I suspect _most_ calls to lfsr_file_sync immediately follow
intentional changes to a file. It would be interesting to know of
examples where this is not the case...
These new fields do increase the size of lfs_attr, which is a downside,
but thanks to flags fitting in type's padding, this is only an increase
from 3 words (12 bytes) -> 4 words (16 bytes).
---
Other implementation notes:
- I did try to implement LFS_A_CREAT/EXCL in lfs_attr but this proved
to be too messy and inconsistent, so I dropped the idea for now.
The idea was to error with NOATTR/EXIST if the lfs_attr flag in
incompatible with what's on disk, but this led to a lot of complexity
for what is a pretty niche use case.
It's also inconsistent with rdonly attrs, which do _not_ error with
NOATTR during lfsr_file_opencfg, because that would be kind of
annoying.
- Having both `struct lfs_attr` and `lfsr_attr_t` to represent different
things in the codebase is both fragile and confusing. One of these
needs to change, probably `lfsr_attr_t`.
If only I could think of a good name...
One of the nice side-effects of the now-dropped uattr/sattr split was
avoiding this conflict.
- We still need more tests related to how custom attributes interact
with other filesystem operations, but I wanted to get what is
currently working committed, see the TODOs in test_attrs.toml.
All of the new bells and whistles unfortunately do add up.
lfsr_file_sync is also the root of our current stack hot-path, so the
additional attr also adds a bit of stack:
code stack
before: 37116 2608
after: 38104 (+2.7%) 2624 (+0.6%)
Still, having a consistent and flexible API is well worth it.
Though I do think at some point we should add a compile-time option to
opt-out of custom attributes (LFS_NO_ATTR?).
Christopher Haster
f80db15c7e