Jetpack: optimize the Sharing module hook

[sergeymitr]

Proposed changes:

• Make the Sharing module run the template_redirect action callback only when necessary.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

https://github.com/Automattic/vulcan/issues/529

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

1. Go to "Jetpack -> Settings -> Sharing" and activate the "Sharing buttons".
2. Visit a post, confirm the sharing buttons show up under the content.
3. Click on a social service and confirm you get redirected to its sharing page.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/optimize-sharing-hook branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/optimize-sharing-hook
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen semi-continuously (PCYsg-Jjm-p2).
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[jeherve]

If we check for this before the hook, should we remove that check from the actual sharing_process_requests function?

[sergeymitr]

I think we could keep it there as well
The hook uses the data from $_GET['share'], and it's always helpful to check the array element for existence before using it.