Fix race condition

[damccorm]

It seems that not all Datasources are thread safe when creating connections and can hang when many threads try to create connections at once. Since datasources can be shared across threads, this creates a problem. This pr attempts to resolve the problem, though right now I'm mostly using it to test/validate the fix.

Fixes #34120

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[damccorm]

Launching some performance tests to see if this change has any impact: https://github.com/apache/beam/actions/runs/13592236722

I'm hopeful it won't since:

1. It should only be paid once per bundle
2. Actually reading/writing should be the dominant part of this workload, not getting a connection. If getting a connection is slow, that probably means that there just aren't enough connections to go around

[damccorm]

Yeah, seems like little to no perf impact.

My run with this change:

Previous 5 runs:

run 1:

run 2:

run 3:

run 4:

run 5:

My updated version actually did the best here on both metrics, so it seems like it should be pretty safe.

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @Abacn for label java.
R: @Abacn for label io.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[damccorm]

R: @Abacn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[Abacn]

Thank you!

[liferoad]

Shall we call this out in CHANGES.md?

[damccorm]

Shall we call this out in CHANGES.md?

Sure, #34147