blockdev: open API design questions

[lorenz]

There are a number of warts/open questions related to the blockdev design:

See also code at https://review.monogon.dev/c/monogon/+/1873

1. Blocks vs bytes and alignment

   • Most arguments are bytes, which must be aligned to blocks
   • WriteAt/ReadAt might only take block-aligned IO depending on platform and O_DIRECT
   • BlockCount returns the number of blocks, not bytes (maybe we should have Size())?

2. ReadWriteSeeker

   • BlockDev does not require Read/Write/Seek to be present, but these are commonly required to for example copy data onto block devices
   • Underlying block device can require block alignment, which would require buffering

3. GPT/partitions

   • I migrated GPT onto blockdev as it makes extensive use of the block device geometry.
   • Problem is that now GPT has a weird API surface where magic happens when AddPartition magically creates a section of the block device it's added to (and not updated when you change first/last block manually)
   • Reading/writing partitions requires creating a RWS manually.
   • Support for dynamically-sized block devices when backed by files.

/cc @q3k