It is possible for the same pid to be assigned to processes spawned at different points in time during a test. The way serverExitCodeMap is implemented makes it so that checking the exit code of the process in MongoRunner.stopMongod() may see the wrong exit code for the process which was just spawned.

Original description

MongoRunner.stopMongod should explicitly check for exit code 100 on instances where it is expected to fail with non-zero exit code.