java_grpc_library: Add support for protobuf lite

[ejona86]

gRPC's protobuf-lite auto-selects between full and lite protobuf based on the
value of crosstool_top. If the user is specifying their own
--android_crosstool_top, then it will not auto-detect correctly. One day,
platforms will fix problems like this, but for the moment it seems we get to
live with it.

This includes #4288 as the first commit.

Note: I would have added a BUILD for the helloworld Android app, but ran
into unrelated problems ("cannot access TaskStackBuilder").

[ejona86]

CC @codesuki

[codesuki]

Thanks! That looks good. You didn't even need to add a new rule. I will give it a spin when I am back from vacation.
Sorry I couldn't help after all.

[codesuki]

I am on mobile and didn't dig any deeper, let me ask a stupid question. Why does the 'protobuf-lite' rule even need the full protobuf in the default case.

"//conditions:default": ["@com_google_protobuf//:protobuf_java"],

I am not saying it's not correct just thinking it's unintuitive :)

[ejona86]

@codesuki, so... Lite is a subset of full proto. Or rather, it was a subset before the fork. The fork should be temporary, but it will be with us a while longer.

grcp-protobuf-lite uses only the subset of full proto supported by lite. But it can work with both full and lite protobufs. grpc-protobuf actually depends on grpc-protobuf-lite. However, since Lite is a fork now, it's dangerous to depend on both full and lite protobuf as you'll get conflicts in the classpath. Thus we have to select which version we want.

[ejona86]

Oh, and since it is dangerous to mix lite and full protobuf, I am sort of imposing the solution we have done internally: use lite if and only if on Android.

[ericgribkoff]

s/closstool/crosstool

[ejona86]

Done.

[ericgribkoff]

LGTM.

Coverage for this on Kokoro is blocked by getting the Android examples building with bazel, right? I manually built protobuf-lite with --crosstool_top=//external:android/crosstool which I think demonstrates that this works - is this or another similar command appropriate to add to the bazel build script as a measure?

[ejona86]

Coverage is blocked by getting a android_library in the build, yes.

I manually built protobuf-lite with --crosstool_top=//external:android/crosstool which I think demonstrates that this works

It shows that grpc-protobuf-lite can be built against protobuf-lite (which we already do with gradle). It doesn't show the rest of the stuff works. And it doesn't show that the select() is right. And such an argument command could break other targets. So such a test seems pretty limited use at the moment. I'd much rather introduce an android_library of some sort.