After upgrading p4-plugin from 1.0.14 to 1.4.8, all our matrix build configurations started triggering continuously. It turns out, the plugin changed the way it resolves variables in the workspace name for the matrix configuration. The way the SCM polling does it and the actual build is different, which leads to the polling trying to use a non-existent workspace name, and always detecting new changes.

Build name: test-build-config
Workspace name in configuration: jenkins-${NODE_NAME}-${JOB_NAME}

Polling log, observe the workspace name has been calculated using the JOB_NAME of the job of the first matrix axis combination. In previous versions of the plugin, this would be the build name, so the workspace would be 'jenkins-master-test-build-config'.

Perforce Software Polling Log

Started on Oct 11, 2016 12:15:00 AM
Polling SCM changes on master
P4: Polling on: master with:jenkins-master-test-build-config-AXIS_1-value_1-AXIS_2-Value_2-AXIS_3-Value_3
... p4 client -o jenkins-master-test-build-config-AXIS_1-value_1-AXIS_2-Va___ +
... p4 info +

P4 Task: establishing connection.
... server: perforce-ci.tomtomgroup.com:1666
... node: michi-jenkins-00
P4: Polling with cstat: //jenkins-master-test-build-config-AXIS_1-value_1-AXIS_2-Value_2-AXIS_3-Value_3/...
... p4 cstat //jenkins-master-test-build-config-AXIS_1-value_1-AXIS_2-Valu___ +
... p4 change -o 2418201 +
... found change: 2418201
... p4 change -o 2364282 +
... found change: 2364282
... p4 change -o 2357042 +
... found change: 2357042
Done. Took 1.7 sec
Changes found

Build log (the kick off of the master build, before spawning the builds for each matrix axis combination), however, checks out 'jenkins-master-test-build-config'. So the polling workspace and the build workspace are different. And since the former does not exist, each poll triggers a build.