(I feel like this was already filed, but I can't find the dupe. Feel free to merge this into the previous task if you've got better search foo than I do.)
We already provide a "lookside" to composer in order to run parsoid on selenium against a specific commit of Parsoid, instead of whatever the train build includes. But sometimes the latest parsoid depends on a new library which has not yet been train-deployed. We need a similar trick in order to ensure that the round-trip testing runs Parsoid with a specific git tag of mediawiki-vendor as well as a specific git tag of Parsoid, in order to avoid a two week release delay (upgrade library, wait a week for library to ride the train, then commit corresponding fixes to parsoid and rt-test them, wait a week for that to ride the train, and if any problems are found during rt-testing, loop back to the beginning and start over).