is it safe to open a file multiple times?

[yamt]

i have a use case to open a file multiple times. eg. keep two lfs_file_t opened for a file. is it a safe operation for littlefs?

[geky]

Ah. Yes, this is well defined, though the behavior may be unintuitive coming from POSIX filesystems.

The way this works right now is that every open file handle gets a "snapshot" of the file.

So if you open a file twice, say with lfs_file_t A and B, and then wrote to B, A would still contain the original contents.

If you open a file twice and write to both A and B, the last handle to be synced or closed will be what ends up on disk.

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The reason for this behavior is because it derives naturally from files in littlefs being copy-on-write. But users have noted it's unintuitive and initially confusing.

There are some changes in the works to make littlefs behave a bit more like POSIX here (by broadcasting file changes to other open file handles on sync/close), but these changes will need to wait for a major release with API changes, so it will take some time before they land.

[yamt]

thank you for explanation. it makes sense. is it documented in some places?

does directory work is a similar way? i vaguely remember that some traditional applications, which modify a directory (create/remove entries) while readdir'ing it, have some assumptions on what readdir contents can contain/can not contain.

[geky]

thank you for explanation. it makes sense. is it documented in some places?

It's not documented very well currently. Eventually it should be. But with changes in the works it's been hard to prioritize soon-to-be-deprecated behavior when that time could be spent getting to the better behavior sooner. Some hard decisions time-management-wise here.

does directory work is a similar way?

Ah this is a good question.

It would be quite nice if directories worked this way, since it would solve the modify-while-readdir issue you mentioned. Unfortunately because of technical limitations littlefs can't really snapshot directories. Files in littlefs are copy-on-write, but directories are not***.

The modify-while-readdir issue is a really interesting one, and I've read it has caused quite a bit of headache for other filesystems.

According to POSIX:

If a file is removed from or added to the directory after the most recent call to opendir() or rewinddir(), whether a subsequent call to readdir() returns an entry for that file is unspecified.

It doesn't really seem to comment on if modifying a directory is allowed to effect readdir in other ways. My understanding is the expectation is that readdir should at minimum return all files in the directory at the time of opendir. littlefs should be doing this, though it may not have the best test coverage.

At the very least we do have tests over some common modify-while-readdir patterns: test_dirs_recursive_remove

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***:

Ok, so technically you could snapshot a directory by marking its mdirs as "frozen", and forcing any modifications to the mdir to copy first. This may be interesting in the future for full filesystem-level snapshots, which is a feature in other copy-on-write filesystems, but not currently possible in littlefs. But this would be too heavy-handed/expensive for readdir operations.

[yamt]

thank you for explanation. it makes sense. is it documented in some places?

It's not documented very well currently. Eventually it should be. But with changes in the works it's been hard to prioritize soon-to-be-deprecated behavior when that time could be spent getting to the better behavior sooner. Some hard decisions time-management-wise here.

does directory work is a similar way?

Ah this is a good question.

It would be quite nice if directories worked this way, since it would solve the modify-while-readdir issue you mentioned. Unfortunately because of technical limitations littlefs can't really snapshot directories. Files in littlefs are copy-on-write, but directories are not***.

The modify-while-readdir issue is a really interesting one, and I've read it has caused quite a bit of headache for other filesystems.

According to POSIX:

If a file is removed from or added to the directory after the most recent call to opendir() or rewinddir(), whether a subsequent call to readdir() returns an entry for that file is unspecified.

It doesn't really seem to comment on if modifying a directory is allowed to effect readdir in other ways. My understanding is the expectation is that readdir should at minimum return all files in the directory at the time of opendir. littlefs should be doing this, though it may not have the best test coverage.

At the very least we do have tests over some common modify-while-readdir patterns: test_dirs_recursive_remove

***:

Ok, so technically you could snapshot a directory by marking its mdirs as "frozen", and forcing any modifications to the mdir to copy first. This may be interesting in the future for full filesystem-level snapshots, which is a feature in other copy-on-write filesystems, but not currently possible in littlefs. But this would be too heavy-handed/expensive for readdir operations.

when i was working on some filesystems decades ago, the access pattern of cvs caused a lot of trouble wrt readdir. i don't remember details though.

hopefully there are less applications assuming UFS-like behaviors these days. (snapshoting a directory might make the situation worse for those apps.)