Bug 1848200 - add migration flag to init

[rosahbruno]

Conditionally migrate IDB data only if it already exists. If the project has never used an older version of Glean, then we want to avoid migration because of a potential race condition when submitting pings on first page load.

Pull Request checklist

• Quality: Make sure this PR builds and runs cleanly.
  • Inside the glean/ folder, run:
    • npm run test Runs all tests
    • npm run lint Runs all linters
• Tests: This PR includes thorough tests or an explanation of why it does not
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• Documentation: This PR includes documentation changes, an explanation of why it does not need that or a follow-up bug has been filed to do that work
  • Documentation should be added to The Glean Book when making changes to Glean's user facing API
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  • Documentation should be added to the Glean.js developer documentation when making internal code changes

[travis79]

What happens if Local Storage is cleared? will this continue to try and migrate metrics? How long does the migration code need to be in tree, have you considered this and planned (and filed a bug) to remove it at some time in the future?

[rosahbruno]

@travis79

What happens if Local Storage is cleared? will this continue to try and migrate metrics?

If LocalStorage is cleared, it will try and run just once. Currently, we aren't migrating most metrics - we are just migrating client_id and first_run_date. So on a second migration the flag will be set and the values will be set to the same thing as before.

How long does the migration code need to be in tree, have you considered this and planned (and filed a bug) to remove it at some time in the future?

That is something I've been thinking about. I'd like to maybe chat with you during our next 1:1 to get some thoughts here. In the mean time, I will file a bug so that this isn't lost in the shuffle.

[Dexterp37]

How long does the migration code need to be in tree, have you considered this and planned (and filed a bug) to remove it at some time in the future?

That is something I've been thinking about. I'd like to maybe chat with you during our next 1:1 to get some thoughts here. In the mean time, I will file a bug so that this isn't lost in the shuffle.

Only as long as MDN hasn't migrated (or any other project requiring migration).

[travis79]

Okay, let's discuss in our next 1:1 how to track when everyone using Glean.js has migrated to the new version and we will file a bug about cleaning up the migration code then! Other than that, this looks good to me.