The Jenkins build here didn't hit the failures because the precommit builds only run the tests in the maven projects that were touched by the patch. This patch touched MapReduce but only in the history server project. If it had touched the jobclient project then you would have seen the failures. In short, you cannot know for certain that a patch doesn't break something when the precommit build is clean because the precommit build doesn't run all of the tests due to how long that would take for each precommit build.
See https://builds.apache.org/job/hadoop-qbt-trunk-java8-linux-x86/157/#showFailuresLink for a nightly build sample. Any precommit build that needs to run tests in jobclient is going to report a lot of failures due to this still being in the build.
The changing of the default RPC engine is unexpected and deeply sorry for that.
The summary implies this is just about tests, but it is not. It changes the default RPC engine. If that change is unexpected then this patch should be reverted, that portion removed from the patch, and then reviewed and recommitted.