doc.scaladoc is failing silently when command-line options list is too long.

[thoward]

Sometimes the list of command-line options passed to Scaladoc is very large (especially w/ generating combined Scaladocs, with many dependencies). This is due to the need to pass many classpath locations and source file locations in large build graphs. There is a practical limit to the size of a commandline string, and it can sometimes surpass that. When it's too long, it will fail to run the task, but not produce an error.

For example, I have a target which I'd like to generate Scaladocs for, which generated a 1MB string for the commandline. This is too large, even though it's a valid target/Scaladoc task.

There are two ways to fix:

1. Output the options into a text file, then pass this as an "options file" to Scaladoc. I tried this by modifying Pant's code locally to test, and it worked well. Generated a 1MB text file which Scaldoc had no problem loading as options, and it successfully built the Scaladoc for this large graph.

2. Per recommendation from @stuhood, run via Nailgun (e.g. change the base class to NailgunTask instead of JvmTask). This resolves by passing the command directly to Nailgun instead of going through a subprocess, which should allow larger option sets. Converting to this base class requires broader changes, and touches doc.javadoc as well. This is a little beyond me in terms of my familiarity w/ Pants code.

I will file a PR for the first option, as a less-than-ideal "broken->working" fix for this issue, but another ticket should probably be made for migrating to a Nailgun task to go from "ok->good" in terms of implementation quality.

[stuhood]

Thank you @thoward ! That sounds great.

[Eric-Arellano]

Closing because the JVM backend was removed in Pants 2.0 and will be rebuilt from the ground up.