Write verity first, then do fs verification

Old behavior:
   Read partition, for each block:
    Update hasher
    Update verity writer
   before reading hashtree/verity:
    write hashtree/verity to disk
   Read the last verity blocks.
   Finalize hasher, verity hashes.

The old bahvior tries to minimize fs read by only read once and feed
data to hasher and verity writer. However, in VABC, reading/writing are
handled very differently. Read can be done via regular fd, but writes
must go through special COW API. As we have seen in b/186196758, using
COW API in filesystem hashing can lead to inconsistent read and boot
failure. Therefore, we've decided to write verity first using COW API,
then read/hash partition using regular fd. This does mean that we need
to read everything twice, but we think this is a worth while tradeoff.
As verity writes can take 5 minutes, but reading the entire partition
again only takes <10 seconds.

New behavior:
    Read partition, for each block:
      Update verity writer
    Finalize verity writer, write verity to disk
    launch snapuserd, open a regular fd.
    Read partition, for each block:
      Update hasher
    Finaliaze hasher, verity hashes.

Test: th
Test: Manual testing on pixel of the following scenario:
        1. Verity enabled, VABC enabled, pause/resume multiple times
        2. Verity disabled, VABC enabled, pause/resume multiple times
        3. Verity Enabled, VABC enabled, pause/resume multiple times

Bug: 186196758
Change-Id: I2477c2dc4da5b921e84b48a54d0d8a877c1a52ef

0 files changed, 0 insertions, 0 deletions