Change default value for the branch parameter to 'main'

[fniessink]

Tasks:

• Add migration script to change branches without a value to 'master'. Old sources have branch parameters without value. Apparently, the default value of parameters has not always been inserted into reports.
• Also investigate whether the Branch and Branches parameter can be combined. In that case no default value would be needed.

Combining the Branch and Branches parameters

The Branch parameter is only used by GitLab and AzureDevOps, both for the metric: source_up_to_dateness.

The Branches parameters is only used by GitLab for the metric: pipeline_duration.

Note that SonarQube also has a branch parameter, but it's not derived from Branch, but rather a separate StringParameter. The analysis below only covers the use of Branch by GitLab and AzureDevOps.

To decide whether Branch and Branches can be combined it's important to look at the default values of the parameters. Branches has as default value "all branches". Branch has master (after this issue is completed: main) as default value.

When configuring GitLab or Azure DevOps as source for the source_up_to_dateness metric, users can measure the up-to-dateness of files and of pipelines. The Branch parameter is only used in case of the files option: users can specify for which branch the up-to-dateness of the file should be measured.

If we would replace the Branch parameter with the Branches parameter, Quality-time is basically saying that by default users will want to track the up-to-dateness of files across all branches. We haven't had that request from users.

Alternatively, we could change the default value of the Branches parameter to main. That would impact only the pipeline_duration metric when used with GitLab as source. Users that want to measure the duration of all pipelines regardless of branch would have to delete main from the parameter value. It seems more likely that users would want to measure the duration of all pipelines instead of just the main branch.

Getting rid of the Branch parameter would save five lines of code in the data model.

Conclusion: merging the two parameters would worsen user experience in return for a very small reduction of code size. It's not worth it.