[BEAM-10311] Sharing restriction trackers between SDF and DataSource.

[youngoli]

Adding a channel that can be used to retrieve the current restriction
tracker from the DataSource.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark
Go		---		---
Java
Python				---
XLang	---	---		---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[youngoli]

R: @lostluck

Ended up being a really small PR. I tried to think of a way to test it (like testing that the channel gets set in the DataSource maybe), but it seems too specific of an interaction to test, and any test for it would be a bit complicated.

For context of what I'm intending when it comes to the actual splitting: The idea is the DataSource will receive the restriction tracker from the channel when it needs to check progress or perform a split, and then return the restriction tracker to the channel when done with it. The goal is to make sure there's only one copy of that tracker being sent around, so that nothing else is using the tracker when processing is done and the tracker is getting deleted.

[lostluck]

Agreed that this isn't something that can be tested independently without just testing that the language works. It's not currently doing anything beyond configuration and set up and what it needs to do is best finished up in the other with the tests.

[lostluck]

Typo" Rt

Suggested change

	// rt allows this unit to send out restriction trackers being processed.
	// Rt allows this unit to send out restriction trackers being processed.

[youngoli]

Fixed. (Also switched the channel from *RTracker to RTracker)

[lostluck]

Ah yes. Good catch. Don't use pointers to interface types :)