Add BUILD_TARGET environment variable to all build actions

[Qinusty]

Description of the feature request:

Currently bazel adds TEST_TARGET as an environment variable to all test actions which is great for tracking down targets causing failures within actions executed remotely. Tracking down the related target for a failed build action is much harder.

Feature requests: what underlying problem are you trying to solve with this feature?

It is currently quite awkward to trace a failed build action back to the target which created it.

Have you found anything relevant by searching the web?

The test-encyclopedia details TEST_TARGET but I haven't found anything for other actions.

[janakdr]

Could you expand a bit on the use case here? Within a Bazel invocation, Bazel should print out enough information to identify the failing rule. This is for the case of debugging failed actions on the remote execution server?

I don't think that currently build actions have any environment variables set, so this would be a completely new thing. I think that a two-line hacky change to AbstractAction to set env to

    this.env = env.addFixedVariables(ImmutableMap.of("BUILD_TARGET", getPrimaryOutput().getExecPathString()));

would work. This seems to work for genrules and Starlark, but not for tests, which is maybe fine. I'm open to a PR implementing this, but we will have to think about how to limit this: I think we'd only want a very few such variables. Note that we could easily bork remote caching if we weren't careful.

As a current workaround, if the relevant rules are Starlark, you could manually add the action label to the command line, perhaps?

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