tree 57e7d527572f2d23da5edd07e13f6d92980d82b1
parent cea3eb082a93b32c03c2d528ea1d0cb9654629a1
author Tom Cherry <tomcherry@google.com> 1624429208 -0700
committer Tom Cherry <tomcherry@google.com> 1624636255 -0700

logd: replay the entire log when UID clearing

SerializedLogChunk::ClearUidLogs() has long been a source of pain, as
it attempts to modify log chunks in-place, leading to various edge
cases.  Worse, there are pathological cases that can cause many small
chunks, which will not compress cleanly, defeating the purpose of
SerializedLogBuffer.

Recently a bug was discovered that when the last-most chunk in a log
buffer is deleted during UID clearing, the new last-most chunk isn't
set up correctly for writing and triggers a CHECK.

This CL attempts to rectify both problems by rethinking UID clearing.
Instead of modifying chunks in place, this CL iterates through all log
messages and logs the messages that won't be cleared into a parallel
log buffer.

This prevents edge cases, replaces the error prone in-place
modification with the logging mechanism as logging naturally.

This solves the pathological cases, since the log buffers will be
re-packed during the clear.

This contains an optimization that chunks are copied to the new log
buffer as-is until a message from the UID to be cleared is found, such
that back-to-back UID clears of the same UID are no less efficient
than the previous algorithm.

Lastly, old chunks are either spliced into the new list or erased as
soon as they're handled to keep memory usage low during clearing.

Bug: 188858988
Test: unit tests
Merged-In: I2bd51aa4309f3ac48e4f523fb18d67de5e7234c7
Change-Id: I2bd51aa4309f3ac48e4f523fb18d67de5e7234c7
(cherry picked from commit da4752eb9e7c96b53da54105fd355a5173561a0b)