This is because the default project substitutions are made first.

Can you please explain this statement? Other parameter substitutions work fine, and use the exact same process for performing replacements.

So this as of commit 2da2bf91851f3593a56c7d4a9408ec15e5d8aa79 as far as I can tell

During SCM checkout, when getting the perforce workspace, the first step is to get the effective client name.

1) Try to get the effective client name using the current project and build node (see call stack below)

hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1702)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1581)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1569)
hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:867)
hudson.model.AbstractProject.checkout(AbstractProject.java:1024)
hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
hudson.model.Run.run(Run.java:1198)
hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
hudson.model.ResourceController.execute(ResourceController.java:88)
hudson.model.Executor.run(Executor.java:122)

Digging deeper, if on a remote build node, then substitute tokens in the slave client name format. The problem is that the parameter maps being used for substitution don't include the current build's parameters. The substitution map is gathering entries from a number of default sources, including a set of default substitutions for the project:

hudson.plugins.perforce.utils.MacroStringHelper.getDefaultSubstitutions(MacroStringHelper.java:369)
hudson.plugins.perforce.utils.MacroStringHelper.substituteParametersNoCheck(MacroStringHelper.java:244)
hudson.plugins.perforce.utils.MacroStringHelper.substituteParameters(MacroStringHelper.java:110)
hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1735)
hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1702)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1581)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1569)
hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:867)
hudson.model.AbstractProject.checkout(AbstractProject.java:1024)
hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
hudson.model.Run.run(Run.java:1198)
hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
hudson.model.ResourceController.execute(ResourceController.java:88)
hudson.model.Executor.run(Executor.java:122)

At this point, if the slave client name format contains a token referencing a build parameter, it will be substituted by the default value of that parameter. (e.g. my_node_name_${paramA} --> my_node_name_defaultValue, as I described above in the issue). The token is now replaced and can't be substituted correctly if the build provides a different value for that parameter.

2) Finally attempt to substitute any remaining tokens using the build and current environment map. For the example I described, there's no tokens left to replace and the substitution returns:

hudson.plugins.perforce.utils.MacroStringHelper.substituteParametersNoCheck(MacroStringHelper.java:266)
hudson.plugins.perforce.utils.MacroStringHelper.substituteParameters(MacroStringHelper.java:148)
hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1707)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1581)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1569)
hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:867)
hudson.model.AbstractProject.checkout(AbstractProject.java:1024)
hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
hudson.model.Run.run(Run.java:1198)
hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
hudson.model.ResourceController.execute(ResourceController.java:88)
hudson.model.Executor.run(Executor.java:122)

Looks like the order just needs to be changed, then? Move line 1707 above 1702. Unfortunately, I do not have a development environment available to make this change and test it.

That was my first guess too -

hudson.plugins.perforce.utils.MacroStringHelper.getDefaultSubstitutions(MacroStringHelper.java:369)
hudson.plugins.perforce.utils.MacroStringHelper.substituteParametersNoCheck(MacroStringHelper.java:244)
hudson.plugins.perforce.utils.MacroStringHelper.substituteParameters(MacroStringHelper.java:110)
hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1735) <-- but don't get the the slave name format until here
hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1702) <-- order would need to change here
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1581)
hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:1569)
hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:867)
hudson.model.AbstractProject.checkout(AbstractProject.java:1024)
hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
hudson.model.Run.run(Run.java:1198)
hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
hudson.model.ResourceController.execute(ResourceController.java:88)
hudson.model.Executor.run(Executor.java:122)

I think the current build variables need to be included in the substitutions map AFTER the default values are added when substituting parameters on the client name. I can look into a potential fix tomorrow.

This issue has been caused by a new variables substitution mechanism.
https://github.com/jenkinsci/perforce-plugin/pull/54

I'll handle it

Caused by JENKINS-23467 in 1.3.28.
The issue seems to be a blocker, a noting on Wiki is required

https://github.com/jenkinsci/perforce-plugin/pull/59

Code changed in jenkins
User: Oleg Nenashev
Path:
src/test/java/hudson/plugins/perforce/PerforceSCMTest.java
http://jenkins-ci.org/commit/perforce-plugin/7fdf8d0d580661cf12c15f054462f15d30cd61b2
Log:
JENKINS-25226] - A direct unit test for the issue

The test checks that the variables substitution works properly for build parameters.
The change also refactors the provisioning of PerforceSCM stubs.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>

Code changed in jenkins
User: Oleg Nenashev
Path:
src/main/java/hudson/plugins/perforce/utils/MacroStringHelper.java
http://jenkins-ci.org/commit/perforce-plugin/7b22e0b18e27941d07269d53b63e3720841944e5
Log:
[FIXED JENKINS-25226] - Resolve environment variables with a highest priority

Other variables (projects, default parameters, etc.) will be used in the case of missing macros.
It should not happen in general, but may appear in buildEnvironment()

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>

Code changed in jenkins
User: Rob Petti
Path:
src/main/java/hudson/plugins/perforce/utils/MacroStringHelper.java
src/test/java/com/synopsys/arc/jenkinsci/plugins/perforce/MacroStringHelperTest.java
src/test/java/hudson/plugins/perforce/PerforceSCMTest.java
http://jenkins-ci.org/commit/perforce-plugin/fe2717138270aa9a927730dfc7b91e1cc9a97495
Log:
Merge pull request #59 from synopsys-arc-oss/Variables_handling_issues

[FIXED JENKINS-25635,JENKINS-25226] - Variables substitution issues

Compare: https://github.com/jenkinsci/perforce-plugin/compare/44c10b2c639c...fe2717138270

Stuart reported that the issue has not been solved for client names.
Reopening the issue

Created JENKINS-25559 for Stuart's case

I have seen this issue in 1.3.29 of the Perforce plugin where the workspace field was set to ${CLIENTSPEC} and the default value for ${CLIENTSPEC} was empty and is populated by incoming parameters from triggering jobs. The client name format for slaves was just ${basename}. 1.3.29 was selecting the default value instead of the incoming value. Downgrading to 1.3.27 addressed my issue.