As a minimum, use a layer between sharding and gtest-parallel that handles the environment variables. We don't accept any other environment variables into gtest-parallel, and this is a potential error source.

Something like (but preferably in python):

shard-gtest-parallel.sh (some other repo):

gtest-parallel.py --shard_count=${GTEST_SHARD_COUNT} --shard_index=${GTEST_SHARD_INDEX} "$@"

As background, we have started using gtest-parallel [1] to run our tests in swarming bots, so that the command that we ask swarming to run looks like:

../../testing/test_env.py ../../third_party/gtest-parallel/gtest-parallel ./test_binary -- --asan=0 --msan=1 --tsan=0 

We could add another script between test_env.py and gtest-parallel also, but it might be worth to use flags instead of environment variables.

[1] https://github.com/google/gtest-parallel/blob/master/gtest-parallel

test_env.py might be a good place to make that conversion.

I think you should add an in-between script until this bug is fixed so that gtest-parallel can be updated to support shards upstream. It's kind of clunky, but I doubt gtest flags will change in a day.

I'm not sure if gtest binaries barf on unrecognized command-line flags, so gtest-parallel might have to be a special case.

What about adding:
$ test_env.py --pass-shard-flags .../gtest-parallel ...

And that'd add --shard_count and --shard_index to the call to gtest-parallel. 

There's two potential solutions:
- Completely drop --shard flag in "swarming.py trigger" and do the sharding at the recipe level.
- Augment --shard flag to specify argument or environment variable to use.

The end goal is to remove the hardcoded strings GTEST_SHARD_INDEX and GTEST_TOTAL_SHARDS from swarming.py.

The first is logically much simpler. The second is a bit more work but not too much.

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