This doesn't resolve the issue for me. I see the master computer, but it is not used for the main matrix job. Instead, a cloud slave is provisioned. Steps to repro:

1. Create a matrix job with a single configuration, no labels
2. Ensure that Jenkins.numExecutors = 0
3. Ensure that a cloud plugin can provision slaves
4. Run the matrix job

The matrix job won't run on the master, but will result in a slave being provisioned.

@recampbell: you backported https://github.com/jenkinsci/jenkins/commit/a114c693bc61c564232e78c9fffb5de1ef946ea8, or are testing a 1.510+ build with the original fix, and still see the issue?

And did you verify that it is the matrix run which waits for, and runs on, the cloud slave instead of master, as opposed to the matrix configuration run which naturally has to run on the cloud slave? (Does testFlyweightTasksWithoutMasterExecutors pass?)

Sorry, my mistake in backporting.

Code changed in jenkins
User: Kohsuke Kawaguchi
Path:
core/src/main/java/hudson/model/AbstractCIBase.java
core/src/main/java/hudson/model/Computer.java
core/src/main/java/hudson/model/Queue.java
core/src/main/java/hudson/slaves/SlaveComputer.java
core/src/main/java/jenkins/model/Jenkins.java
core/src/main/resources/lib/hudson/executors.jelly
test/src/test/java/hudson/model/QueueTest.java
http://jenkins-ci.org/commit/jenkins/c5f2d2df58c7781c8003e11b6d389352f4e39adc
Log:
[FIXED JENKINS-7291] Permit flyweight tasks to run on master even when it has zero configured executors.

Always adding Computer for master as a fallback

The original proposed fix for JENKINS-7291 creates a Computer object
transitively. This seems unwise as it violates the design of Computer
as stated in the javadoc, and for example we can end up creating two
Computers for the master.

I think a better fix is to create a Computer for the master all the
time, even if there's no executors configured. The discrimination in
Queue.makeBuildable would ensure that such phantom Computer is only used
as a last resort (statistically speaking).

I've also tweaked executors.jelly a bit. I simplified it somewhat
based on the idea that "if there's only one computer to show, the
context is likely making it obvious".

(I must be missing the intricacy in the current code.)

Originally developed in a branch at
2c5b57fcc1f39ed39057254e802f4183db5aa0dc then squashed for clarity.

(cherry picked from commit a114c693bc61c564232e78c9fffb5de1ef946ea8)

Conflicts:
changelog.html