site-kit-wp: Retry failed requests: implement exponential backoff

Feature Description

Currently when the plugin makes an API request that results in an error we display the error to the user and stop processing. We can improve this behavior for specific errors by implementing exponential backoff. We will retry the request in (roughly) 1, 2 and 4 seconds.

Let’s start by turning on the client’s default retry behavior (which handles 50x errors and curl/connection errors) and set maximum retries to 3. We can then follow up for other API specific codes that should be retried. Exact conditions/codes/reasons for sending a request will vary by API.

For example, for the Analytics Reporting API, the following error codes should use exponential retry:

429 - Quota Error: Number of recent failed reporting API requests is too high, please implement exponential back off.

For the Analytics Management API, when the error message is “Quota Error: Rate limit for writes exceeded.” we should use exponential backoff to retry.

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Acceptance criteria

• When the PHP client encounters a retry-able errors for API requests, it should use exponential backoff to re-request that data, trying up to 3 times.

Implementation Brief

• When Google API request are made, the client should handle error conditions with up to 3 retries:

• In includes/Core/Authentication/Clients/OAuth_Client.php:setup_client configure the $client to add support for retries:

// Enable exponential retries, try up to three times.
$client->setConfig( 'retry', array( 'retries' => 3 ) );

Test Coverage

We can test that the default client uses exponential backoff. We shouldn’t need tests for actual retries - the client itself includes these tests.

• In tests/phpunit/integration/Core/Authentication/Clients/OAuth_ClientTest.php expand the test_get_client test which calls Google_Site_Kit_Client:get_client. Add assertions that verify retries is set to 3:

$retry = $client->getConfig( 'retry' );
assertEquals( $retry[ 'retries' ], 3 );

Visual Regression Changes

• No changes in VRT.

QA Brief

• To test that this is working as expected we need to be able to mock API error responses. I will work separately to add this capability in the tester plugin.

Changelog entry

• Implement exponential backoff to retry Google service API requests a limited amount of time if they fail with temporary errors.