fix: avoid creating merge fields if we can't get merge fields

[dkoo]

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Changes proposed in this Pull Request:

When preparing a contact's data sync, we compare the contact's meta fields with the existing merge fields in the audience so that we can use the fields that exist and create the ones that don't yet exist. However, if we can't get info about the existing merge fields via the Mailchimp API for any reason, we should stop the sync to avoid potentially creating duplicate merge fields.

This PR also uses the validate method to catch any potential error-like responses from the Mailchimp API.

Coupled with Automattic/newspack-plugin#3639, this PR will also schedule a retry sync after five minutes if we fail to fetch merge fields, up to a maximum of five times.

Addresses: 1200550061930446-as-1208959720314117

How to test the changes in this Pull Request:

1. Smoke test syncing new and existing contacts with Mailchimp and confirm that contact metadata gets synced correctly to existing merge fields, and that merge fields that don't yet exist get created.

NOTE: leaving the below in place for the record, but it's no longer part of this PR.

Click to see additional testing steps (no longer part of this PR)

2. Temporarily add a line $response = []; here to simulate a non-error response from the Mailchimp API that nonetheless fails to return merge field data.
3. Trigger a contact sync to Mailchimp via any means.
4. Confirm that the sync now fails with an error, without creating any new merge fields.
5. Using WP CLI, run wp cron event list and confirm there's a non-recurring newspack_scheduled_esp_sync job scheduled for 5 minutes in the future.
6. Wait 5 minutes or run wp cron event run newspack_scheduled_esp_sync to trigger the job immediately. Repeat this 5 times.
7. After 5 failed syncs, confirm that a Failed to sync contact <email address> after 5 retries is logged, and that there's no more newspack_scheduled_esp_sync job scheduled.
8. Remove the temporary error line added in step 2 and resync the contact.
9. Using WP CLI, run Newspack\Reader_Activation\Sync::schedule_sync( $user_id, 'Testing scheduled sync', 30 ); and confirm that the scheduled sync happens successfully after 30 seconds.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 23.27%. Comparing base (ad092ca) to head (91c4834).
Report is 15 commits behind head on release.

Files with missing lines	Patch %	Lines
...mailchimp/class-newspack-newsletters-mailchimp.php	92.30%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             release    #1729      +/-   ##
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+ Coverage      23.07%   23.27%   +0.20%     
+ Complexity      2713     2709       -4     
=============================================
  Files             49       49              
  Lines          10761    10748      -13     
=============================================
+ Hits            2483     2502      +19     
+ Misses          8278     8246      -32     

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[leogermani]

This PR also uses the validate method to catch any potential error-like responses from the Mailchimp API.

Good catch! That's why this error was never being logged!

I couldn't get it all to work, here are a few things I saw:

• When the error is triggered, it doesn't stop the sync from happening, which results in the contact missing the merge fields
• When I run wp cron event run newspack_scheduled_esp_sync, it succeeds, but I don't see the actual sync going through
• Another sync is not scheduled (I'm running the two branches as expected)

[dkoo]

@leogermani I think 345229b should stop the sync by throwing an exception if we encounter a merge fields error. Note that since we're using the newspack_log action, the errors won't be logged directly to debug.log, but to the Mailchimp log file in your server's /tmp directory.

[dkoo]

When I run wp cron event run newspack_scheduled_esp_sync, it succeeds, but I don't see the actual sync going through

This might be the case if you still have the temp error being triggered by the $response = []; line. If it's still there then the scheduled syncs will continue failing with an error (logged in the local /tmp logfile, not debug.log). On my site once I remove that line the next scheduled sync will succeed.

[leogermani]

When I run wp cron event run newspack_scheduled_esp_sync, it succeeds, but I don't see the actual sync going through

This might be the case if you still have the temp error being triggered by the $response = []; line. If it's still there then the scheduled syncs will continue failing with an error (logged in the local /tmp logfile, not debug.log). On my site once I remove that line the next scheduled sync will succeed.

Right, it's expected to fail, but also to schedule a new retry until it reaches 5 attempts. It didn't schedule the second retry

[dkoo]

@leogermani I think this change is still safe because it should result in the same behavior and error code—the removed code was mainly duplicating some of the checks in the validate() method. Happy to revert this too if you prefer.

[dkoo]

@leogermani I've scaled back the changes in this PR to include only the additional validation and error logging, so this should be ready for another review.