tree b7d8c4bd7411b8056ab6a218ffec9beb13994558
parent eb9de160773d1277a6c3447ebedcfe27d2a45a77
author Kelvin Zhang <zhangkelvin@google.com> 1601313820 -0400
committer Kelvin Zhang <zhangkelvin@google.com> 1605635279 -0500

Use FileDescriptorPtr to implement async reads in verify stage

During FileSystemVerify stage, update_engine needs to read from source
or target partition to verify hashes && write verity. Previously we use
brillow's file stream to implement async reads. WIth Virtual AB
Compression, reading from target partition must go through libsnapshot's
interface(FileDescriptorPtr). So we replace brillo::FileStream with
FileDescriptorPtr for ease of integrating with VABC.

Test: serve an OTA update, verify: slot switch resume, regular resume

Change-Id: Id8531757468f60e3e21667b7761b83f7c2af2dbf