type ExecLogSink
type ExecLogIter
type file
type iterator
function compact_file
Write this build’s compact execution log, byte for byte as Bazel’s
--execution_log_compact_file would.
One path holds one log. A task that retries its Bazel invocation spawns
a build per attempt, and each writes this path afresh, so what is left
when the task ends is the last attempt’s log rather than all of them.
Name a path per attempt if every attempt’s log has to survive.
