Fix flaky: shutdown 'executes only once'

[p-datadog]

What does this PR do?

Fixes flaky test spec/datadog/tracing/integration_spec.rb:592 — the shutdown "executes only once" integration test.

Motivation:

Flaky test reported in PR #5581. The test intermittently fails with traces_flushed: 0 under CI load.

Failure: The assertion expect(stats).to include(traces_flushed: 1) fails with traces_flushed: 0.

Root cause: The test asserts traces_flushed: 1 immediately after concurrent shutdown! calls join. shutdown! calls Writer#stop, which calls AsyncTransport#stop, which joins the worker thread with a 1-second timeout (DEFAULT_SHUTDOWN_TIMEOUT). When the HTTP round-trip to the agent takes longer than 1 second under CI load, the join times out. The worker thread is still alive completing the HTTP call, but @traces_flushed hasn't been incremented yet. The assertion fires and sees 0.

Fix: Add try_wait_until { tracer.writer.stats[:traces_flushed] >= 1 } AFTER the shutdown threads join, not before them. This preserves the test's coverage: the concurrent shutdowns still race with an in-flight flush (the buffer has data when shutdowns begin), so the shutdown guard and worker stop logic are exercised under real contention. The wait just gives the orphaned worker thread time to complete its HTTP call before the assertion checks final stats.

This is deliberately different from PR #5581's approach (which waits BEFORE shutdowns). Waiting before drains the buffer, so all shutdown flushes are no-ops — making traces_flushed: 1 tautological regardless of whether the shutdown guard works. Waiting after preserves the race between shutdowns and the flush.

Change log entry

None.

How to test the change?

Integration test validated via the three-PR reproducer/fix/validation pattern:

• Reproducer PR: Reproduce flaky: shutdown 'executes only once' #5584 (forces the race — CI should fail)
• Validation PR: Validate fix for flaky: shutdown 'executes only once' #5586 (merges reproducer + fix — CI should pass)

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 95.35% (-0.02%)

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