JdtToBeBuiltComputer may skip indexing relevant jars

The following has been observed in Xtext 2.28, but it seems the code in question hasn't changed since:

• Assume a jar file Lib.jar containing DSL files X1.mydsl ...
• Assume a chain of dependent DSL projects, P3 -> P2 -> P1, each with an additional dependency on Lib.jar
• By some cause the index for Lib.jar has gone missing or is badly outdated
  • Replacing the file on disk seems to trigger this. This might be the result of m2e updating dependencies.
• User sees resolve errors in P3 "Couldn't resolve reference to MyThing 'X1'" and triggers a clean build on P3
• Errors remain - bummer.

This happens because

• the full build on P3 will ask JdtToBeBuiltComputer whether or not Lib.jar should be reindexed
  • the answer is "no"
  • this happens because the code (via isBuiltByUpstream()) assumes that Lib.jar will have been indexed already via P1 (and P2).
  • this assumption is wrong in the case of building individual projects, rather than the entire workspace

User is left with these (non-obvious) workarounds:

• clean build the entire workspace - which can be a real performance problem, if it's needed for each update of maven dependencies, e.g.
• figure out which is the most-upstream project that depends on Lib.jar to perform the clean build there. In a large workspace this may not be an obvious thing to figure out.

I believe this could best be addressed by replacing the following expression in updateProject()

• workspace.computeProjectOrder(workspace.getRoot().getProjects())
• with some invocation buildContext.getAllReferencedBuildConfigs()
  • this would require exposing the builder instance to JdtToBeBuiltComputer so it can call getContext().

This change would ensure that the assumption "isBuiltByUpstream()` considers only those projects that participate in the current builder invocation.

Injecting such a change from a DSL implementation is not straight forward, as there doesn't seem to be a way to replace JdtToBeBuiltComputer with your own variant, one can only add additional contributions. Moreover, the issue may potentially affect all DSLs, so a fix in Xtext itself seems preferable.

Start at JdtToBeBuiltComputer.updateProject() and trace how isBuiltByUpstream() uses workspace.computeProjectOrder(). Inspect the builder context and the proposed getAllReferencedBuildConfigs() entry point, then reproduce the P3 → P2 → P1 scenario with Lib.jar. Done means a clean build of an individual project reindexes Lib.jar when it is part of the current builder invocation.