[#23043] Re-enable single iteration for the Go SDK.

[lostluck]

Re-enable single iteration "decode on-demand" stream for the Go SDK.

Originally disabled due to #22933, in #23042, the principle remains sound, and it's long past time to look at why it failed in the first place.

The issue was that we didn't drain the iterable properly if there were no reads at all, so we go with the slightly more ham fisted approach of iterating until io.EOF, or stream != nil. If the stream isn't nil, it means the reader got handed off, and it's up to that stream to close.

This avoids unnecessarily draining something off of the main reader which could be very large and costly.

Fixes #23043

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[lostluck]

Run Go Spark ValidatesRunner

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Run Go Flink ValidatesRunner

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Run Go PostCommit

[codecov]

Codecov Report

Merging #27744 (7073e7c) into master (b668799) will increase coverage by 0.03%.
Report is 8 commits behind head on master.
The diff coverage is 85.71%.

❗ Current head 7073e7c differs from pull request most recent head 3ddbae0. Consider uploading reports for the commit 3ddbae0 to get more accurate results

@@            Coverage Diff             @@
##           master   #27744      +/-   ##
==========================================
+ Coverage   70.92%   70.95%   +0.03%     
==========================================
  Files         860      860              
  Lines      104860   104866       +6     
==========================================
+ Hits        74368    74406      +38     
+ Misses      28934    28901      -33     
- Partials     1558     1559       +1     

Flag	Coverage Δ
go	53.66% <85.71%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
sdks/go/pkg/beam/core/runtime/exec/fullvalue.go	84.10% <81.81%> (+6.85%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/datasource.go	70.62% <100.00%> (+3.82%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/plan.go	68.07% <100.00%> (+0.19%)	⬆️

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[lostluck]

Whoops. Forgot to mark this as draft Jack. Just wanted to see if we could replicate the failure still (as listed in the description).

It doesn't occur on Prism (validated on laptop).

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Run Go Spark ValidatesRunner

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Run Go Spark ValidatesRunner

[lostluck]

It's very strange, since that element doesn't exist, it's just a windowed value header with no element, since there's only one element at all. Current idea is that element "always" exists, and it's being skipped somehow in the non-singleIterate path.

[lostluck]

Run Go Spark ValidatesRunner

[lostluck]

Ready for a look. Ready to submit if the Spark Validates runner passes.

[lostluck]

Run Go Spark ValidatesRunner

[jrmccluskey]

Looks like everything is good apart from the go fmt check

[lostluck]

Thanks for the review!