Protobuf repo recompilation sensitivity

[pcj]

As discussed on the mailing list https://groups.google.com/forum/#!topic/bazel-discuss/UKLK7Y-jsW4 the protobuf repo is notoriously sensitive to recompilation. Presumably something in the environment used in the action key for protobuf C++ compilation leads to cache invalidation and hence recompiling protobuf from source.

Given protobuf libs are often base dependencies for many others, leads to degraded user experience.

Repro steps include taking any workspace that has a proto dependency and source ~/.bashrc, watch protobuf recompile.

Observed up to and including bazel 0.20.0.

/assign @hlopko

[hlopko]

Can you double check that:

git clone https://github.com/bazelbuild/rules_cc
cd rules_cc
bazel test //tools/migration/...
source ~/.bashrc
bazel test //tools/migration/...

results in recompilation? What is the minimal content of the ~/.bashrc file that triggers the issue? What platform are you on?

[pcj]

Looks like it's the PATH. Any changes to PATH will trigger recompilation.

Perhaps this is working as intended, or perhaps we can split it and use a set to dedup entries to normalize the path to the minimal necessary set.

Is it possible to exclude PATH here and rely solely on the toolchain?

[pcj]

$ uname -a
Linux bina 4.13.0-46-generic #51-Ubuntu SMP Tue Jun 12 12:36:29 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

[hlopko]

Hmmm then #6648 should actually help you (afaik). Can you try with bazel 0.21?

[hlopko]

or with 0.20 and with --incompatible_strict_action_env

[pcj]

Yes! Tested on bazel 0.21.0 and indeed no longer recompiles with PATH change. I guess I will go ahead and close this. The migration path from 0.19.2 to 0.20.0 and 0.21.0 is a little challenging (for this same reason), but clearly beneficial. Thanks for your help @hlopko.

[hlopko]

Awesome, thanks Paul. Can you pls share this solution on the mailing list?

[hlopko]

@pcj What toolchain do you use? Was it the Bazel's autoconfigured C++ toolchain?