Don't export via deps, only via exports

[illicitonion]

#557 replaced using
JavaInfo.transitive_exports by instead using
HasMavenDeps.transitive_exports but it changed how this was computed.

The JavaInfo traversal code:
https://github.com/bazelbuild/bazel/blob/5a647bb234ba9817f51fdb2b4b5973a0602f51a4/src/main/java/com/google/devtools/build/lib/rules/java/JavaCommon.java#L341-L357
only follows the exports attribute, and merges together each of the
transitive exports it finds that way.

The replacement starlark code was traversing each of deps, exports,
and runtime_deps, looking for anything which may have been exported
to those targets, which exports significantly more targets.

This adds a unit test to prevent future regressions.

[illicitonion]

/cc @comius in case the change in behaviour was intended

[comius]

/cc @comius in case the change in behaviour was intended

No, it was unintentional; I assume you have a case that is broken/different now. Thanks for the fix and the test!

Didn't the old code here https://github.com/bazelbuild/rules_jvm_external/pull/557/files#diff-4c35294998684c53cec071ff25a96119a3545b55579a7a21233c83870ed37c6cL56-L58 also collect transitive_exports from deps and runtime_deps? Because artifact_infos are gathered over _ASPECT_ATTRS?

[illicitonion]

Sorry for the delay - I've been out on PTO :)

/cc @comius in case the change in behaviour was intended

I assume you have a case that is broken/different now.

I do indeed!

Didn't the old code here https://github.com/bazelbuild/rules_jvm_external/pull/557/files#diff-4c35294998684c53cec071ff25a96119a3545b55579a7a21233c83870ed37c6cL56-L58 also collect transitive_exports from deps and runtime_deps? Because artifact_infos are gathered over _ASPECT_ATTRS?

I have spent quite a while staring at this code, and I think I now understand the key difference. JavaInfo itself does filtering for us to only follow exports attributes when computing transitive_exports - so transitive_exports is arguably a misleading name - it's really transitive_exports_via_my_exports - target X only has something appear in its transitive_exports if the route from X to that target is in the exports attribute of X.

Your change had the effect of removing this "only via the exports of X" filtering for maven_project_jar targets - before we consulted its target's target[JavaInfo].transitive_exports which performed the "only via my exports" filtering, and now we don't because we're reading its target[MavenInfo].transitive_exports which includes transitive_exports from its target's deps attribute.

I've just pushed a commit which I think makes this more clear - when traversing the tree to gather, we only accumulate the transitive_exports if the attribute we went via to get to the target we're accumulating from was itself exports. I also added a test on the transitive_runtime_deps the maven_project_jar sees, as this is the root cause of my failure in a private repo.

Note that this is still a slightly change in behaviour, but based on non-public examples I think it's closer to the original behaviour.

[shs96c]

Sorry it took so long to review. Thank you @illicitonion!