[BEAM-9799] Automated validation for RTrackers.

[youngoli]

Adds automated validation for RTrackers to the DoFn invokers. I ran the
ParDo benchmark with and without my change and got similar results, so
this doesn't seem to have an obvious performance impact.

Note that the currently documented benchmark number doesn't seem accurate before (or it's just my desktop that's different).

Benchmark before change:
BenchmarkParDo_EmitSumFn-6 948087 1918 ns/op 484 B/op 4 allocs/op

Benchmark after change:
BenchmarkParDo_EmitSumFn-6 681343 1919 ns/op 521 B/op 4 allocs/op

Note that exact numbers wavered a bunch. It seemed to average around 1900 ns/op, going as low as 1850 and up to 1950. I chose two examples that had similar runtimes.

The error message emitted looks like this to the end-user:

...
Caused by: java.lang.RuntimeException: Error received from SDK harness for instruction 2: process bundle failed for instruction 2 using plan 5-6 : <emitted error>
...

I was toying with making it a bit more descriptive, but it matches the structure you get when the user emits an error from ProcessElement, so I kept it as is for consistency.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	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]

Completely forgot to actually send this for review.

R: @lostluck

[lostluck]

Just the one question really.

[lostluck]

Suggested change

	} else {
	return errors.New("RTracker IsDone failed for unspecified reason")
	}
	}
	return errors.New("RTracker.IsDone() failed for unspecified reason")

[lostluck]

Also, in both places this is called, error or no, we don't know IsDone failed(), we know that the DoFn terminated without fully processing the restriction (If I understand IsDone's semantics correctly). Perhaps the message should indicate that?

[youngoli]

Yeah good point. I'll rephrase it.