commit 38e23cc44e46bd8f96f1250c3be9c8460cf51ed6
author Lalit Maganti <lalitm@google.com> Wed Oct 30 21:04:12 2024 +0000
committer Lalit Maganti <lalitm@google.com> Wed Oct 30 21:04:12 2024 +0000
tree 4780460cd6830fbb2d1eb6225f2c346abdafc3c5
parent dfc1541486d2567ecf5b0814fe0f0f3c615307d3

perfetto: rework proto descriptor gen to inform gn about deps

The state before today:
1) All the non descriptor rules (lite, zero, gen, ipc) all need
   to know about all the recursive proto deps to ensure rebuilds
   when files are changeds (even though techincally the protoc
   compiler does not actually look at this info)
2) For descriptor rules, we actually rely on protoc to do all
   the lookups on its side.
3) For Bazel/Android.bp, we hacked soemthing custom to propogate
   the depenendencies instead of using gn properly (at the time
   I did not understand gn enough to write the correct templates)

The problem:
If we are using RBE, we need GN to know about deps so that scheduling
of compile actions can correctly figure out which sources are necessary
to build which others. We cannot just rely on protoc for this

The solution:
Unhack a bunch of custom stufff around how we handle descriptors to
teach GN properly about the deps. This involes the following:
1) Make source_sets groups instead of actions which allows proper
   propogation of deps (which was something badly missing with action)
2) Remove source_set and descriptor from being proto_generators:
   proto_generators now strictly corresponds to C++ codegen.
3) Always generate a source_set target (behind the scenes) to allow
   depending on it from any other proto target (this is very similar
   to how Bazel/Blaze work)
4) Make descriptors just rely on generate_descriptor option and
   specify the top level proto with descriptor_root_source (which is
   what will be passed to protoc in the end)

This should have a minimal diff for bazel/gn which fixing the RBE
problem. It also reduces the complexity of a bunch of random proto
build rules throughout the codebase.

Really as hard as it is to believe, this is noop change...

Change-Id: Ieab7ea58efe75ad919d45763965529b3cda02751