Heartbeat: Improve xmlrpc request argument handling

[oskosk]

Fixes #10070

Props to @kraftbj for finding the source of the issue.

The XML-RPC server sets a default param of array() via https://github.com/WordPress/WordPress/blob/4.9.8/wp-includes/IXR/class-IXR-message.php#L17
and it passes that to the method via https://github.com/WordPress/WordPress/blob/4.9.8/wp-includes/IXR/class-IXR-server.php#L65

The proposal here is based on a personal opinion that using generate_stats_array was being overloaded in usage as both a direct xmlrpc method handler and a statically callable method for other purposes.

Changes proposed in this Pull Request:

• Adds a new static method Jetpack_Heartbeat:: xmlrpc_data_response method as handler for the XML RPC request jetpack.getHeartbeatData.

Testing instructions:

• Start with a connected Jetpack site, and WP_DEBUG WP_DEBUG_LOG constants set to true.
• Pass your site through the Jetpack Debugger.
• Expect not to see PHP notices logged. Before this changeset, you should have expected to see PHP notices logged as described in Heartbeat: Array to string conversion PHP notice #10070 .

Proposed changelog entry for your changes:

[jetpackbot]

That's a great PR description, thank you so much for your effort!

Generated by 🚫 dangerJS

[oskosk]

not quite sure if introducing the handler method is necessary or even tidier than just checking in generate_stats_array() 🤷‍♂️

[kraftbj]

I haven't tested it, but I agree with your approach as it future-proofs us better IMO than keeping it within generate_stats_array. As we develop more via REST, etc, we avoid having to setup special checks to account for each variant within that function. We can have handler functions with each special use all referring to a clean and canonical data source.

[dereksmart]

LGTM - tested via the debugger and ran a test using the wp jetpack get-heartbeat command in wpcom.

[mdawaffe]

I wonder what all this was breaking on the WP.com side.

Looks good to me.

[oskosk]

Ported to branch-6.5 in 7ec33a9