One of the unexpected side-effects of lazy file creation is that
suddenly LFS_O_EXCL doesn't make sense.
The standard definition: "Fail if the file exists", is easy enough to
implement, but doesn't really match what the user expects.
The user expects one of these calls to fail:
lfsr_file_open(&lfs, &file_a, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
lfsr_file_open(&lfs, &file_b, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
But because we create files lazily (to prevent zero-length files after
powerloss), these both succeed.
---
I considered deferring the "file exists" check until we actually would
create the file, but while this _technically_ satisfies the
exclusitivity requirement, I decided against it as I think it just makes
the API way too confusing:
lfsr_file_open(&lfs, &file_a, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
lfsr_file_open(&lfs, &file_b, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
lfsr_file_close(&lfs, &file_a) => 0;
lfsr_file_close(&lfs, &file_b) => LFS_ERR_EXIST;
---
Instead, a simpler, more pragmatic approach: Fail if the file exists
_or_ if the file is open in a mode that will create the file:
lfsr_file_open(&lfs, &file_a, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
lfsr_file_open(&lfs, &file_b, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => LFS_ERR_EXIST;
This explicitly does _not_ error on zombie/desync files:
lfsr_file_open(&lfs, &file_a, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
lfsr_file_desync(&lfs, &file_a) => 0;
lfsr_file_open(&lfs, &file_b, "file.txt",
LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
And it does mean we aren't necessarily guaranteeing the file will be
created, but I think this does more-or-less what the user expects:
- open(a) -> desync(a) -> open(b) -> resync(a) is roughly equivalent to
opening a after creating b, which is perfectly fine with LFS_O_EXCL.
- open(a) -> open(b) (errors) -> desync(a) is one way to not actually
create the file, but is somewhat similar to removing the file after
creation.
If you're using desync files you should probably have a good
understanding of littlefs's sync model anyways.
And of course the user can always sync immediately after open to
guarantee file creation, while opting into the possibility of
zero-length files after powerloss.
Code changes:
code stack ctx
before: 38084 2624 752
after: 38128 (+0.1%) 2624 (+0.0%) 752 (+0.0%)
Christopher Haster
5055a40d8b