In v2.5, we introduced an optimization to avoid rereading data when
seeking inside the file cache. Unfortunately this used a slightly
wrong condition to check if the cache was "live", which meant seeks from
end-of-blocks could end up with invalid caches and wrong data. Not
great.
The problem is the nuance of when a file's cache is "live":
1. The file is marked as LFS_F_READING or LFS_F_WRITING.
But we can't reuse the cache when writing, so we only care about
LFS_F_READING.
2. file->off != lfs->cfg->block_size (end-of-block).
This is an optimization to avoid eagerly reading blocks we may not
actually care about.
We weren't checking for the end-of-block case, which meant if you seeked
_from_ the end of a block to a seemingly valid location in the file
cache, you could end up with an invalid cache.
Note that end-of-block may not be powers-of-two due to CTZ skip-list
pointers.
---
The fix is to check for the end-of-block case in lfs_file_seek. Note
this now matches the need-new-block logic in lfs_file_flushedread.
This logic change may also make lfs_file_seek call lfs_file_flush more
often, but only in cases where lfs_file_flush is a noop.
I've also extended the test_seek tests to cover a few more boundary-read
cases and prevent a regression in the future.
Found by wjl and lrodorigo
366100b140