Fix SCM parsing to use explicit descriptor selection process

[AhmedFatthy1040]

See JENKINS-75352.

Testing done

To reproduce the issue, I created a custom SCM plugin that implements isApplicable() to only support certain job types (like WorkflowJob):

@Extension
public static class DescriptorImpl extends SCMDescriptor<MySCM> {
    @Override
    public boolean isApplicable(Job project) {
        return project instanceof WorkflowJob;
    }
}

I installed the plugin in my Jenkins instance and did the following steps:

1. Create a new Free Style Job:

   • Go to Jenkins Dashboard
   • Click "New Item"
   • Enter a name
   • Select "Freestyle project"
   • Click "OK"

2. In the configuration page:

   • Scroll down to the "Source Code Management" section
   • Select an SCM option that would appear after your custom SCM in the list if your custom SCM were applicable (the other SCM option was git plugin, it is alphabetically after my custom plugin as I named my custom plugin "A Custom Plugin")
   • Configure the selected SCM with any required parameters
   • Click "Save"

I encountered the error (same error as in the issue) after completing these steps. After fixing the issue, I repeated the same steps and did not encounter any errors.

Proposed changelog entries

• Improve SCM parsing logic to handle null objects and index out of range scenarios.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@daniel-beck
@MarkEWaite
@mawinter69

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[welcome]

Yay, your first pull request towards Jenkins core was created successfully! Thank you so much!

A contributor will provide feedback soon. Meanwhile, you can join the chats and community forums to connect with other Jenkins users, developers, and maintainers.

[MarkEWaite]

@AhmedFatthy1040 I'm unable to duplicate the original problem. I copied the source code of the Mercurial plugin, renamed the packages in the source from hudson.plugins.mercurial to hudson.plugins.aamercurial and started Jenkins 2.479.3 with it. I confirmed that my modified Mercurial plugin was able to clone a mercurial repository in a Pipeline job. I confirmed that the freestyle job configuration does not offer my modified Mercurial plugin as an SCM choice at all (since isApplicable is returning false for a freestyle job in the modified Mercurial plugin).

Can you share the repository that contains your custom SCM plugin?

[AhmedFatthy1040]

Here's the repository link for my custom SCM plugin: https://github.com/AhmedFatthy1040/my-scm-plugin

Regarding the issue, I think the problem might be related to the display name ordering. The plugin's display name should start with an alphabetic character that comes before the other plugin’s name. I used this approach in my implementation:

@Override
@Nonnull
public String getDisplayName() {
    return "A My Custom SCM";
}

This ensures that the plugin appears before others in the selection list.

@MarkEWaite