tree be13fdc3730fa12136d52820bc01fd38ca23a593
parent a4cba6578f09d606d32fe32bb0a0c6283ea6d198
author Primiano Tucci <primiano@google.com> 1597836472 +0200
committer Primiano Tucci <primiano@google.com> 1597836472 +0200

Protozero: fall back on the heap when nesting too deep

This CL lifts the historic restriction on the max nesting
of protozero messages. It does so by removing the
stack-based |nested_message_arena_| hack and instead using
an explicit MessgeArena object that is responsible for
memory ownership of protozero Message objects.
The difference between root and nested messages is made
explicit by introducing RootMessage<T> (as opposite to
just Message<T>), which owns the storage for all the
submessages.
This CL still maintains the property of "(mostly) zero
allocations": the MessageArena 0th block is allocated
when the TraceWriterImpl is constructed an in most cases
never expanded. It gets expanded allocating new blocks
only when writing a message that is nested too deep.
Turns out the change is quite self-contained as most
tracing obtains Message objects via
TraceWriter::NewTracePacket(). The places that require
code changes are very limited.

Bug: 163125063
Bug: 125322557
Bug: crbug.com/960303
Change-Id: I8d7a313d81d0c407ef2c549ff5bc9febb30a54a0