Resolve "`RuntimeError: Unable to release an unacquired lock` error"

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add a CHANGELOG.md entry in the Unreleased section

Once your merge request is complete, please assign it a Meltano maintainer for review cycle. Once the review cycle finished, the reviewer shall approve the merge request so it can be merged by any Meltano maintainer.

Closes #1120 (closed)

This MR fixes a bug where trying to read the meltano.yml inside an meltano_update would cause an error. This approach was bad, because it forced the read to use the same lock as the write process, and that lock is not re-entry safe.

Instead, the solution here is to make sure to always read and update inside the meltano_update critical section.

Before

plugin = config_service.find_plugin(…)
… (update the plugin)

with project.meltano_update() as meltano:
    meltano[…] = plugin

After

with project.meltano_update() as meltano:
    # we are sure nobody can update the `meltano.yml` at this point
    plugin = config_service.find_plugin(…)
    … (update the plugin)

    meltano[…] = plugin

This also fixes an issue with the json serializer that caused me not to be able to load the UI at all.