[6to7] Restore public DDTraceId class API#5021
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DD64bTraceId is a subclass of DDTraceId, so the JVM must initialize DDTraceId before DD64bTraceId. DDTraceId.<clinit> in turn initialized DD64bTraceId by building its ZERO/ONE constants via DD64bTraceId.from(), a circular initialization dependency. When the two classes were first touched concurrently from opposite ends -- the service-discovery task (muteTracing() -> blackholeSpan() -> DDTraceId.ZERO) racing the application's first span (IdGenerationStrategy.generateTraceId() -> DD64bTraceId.from()) -- each thread held one class-init lock and waited for the other, hanging trace creation. This surfaced as recurring 30s LogInjectionSmokeTest timeouts in CI (latent since #9705 added the scheduled muteTracing task). Break the cycle at its source while keeping DDTraceId.ZERO/ONE as public fields (preserving the API restored in #5021): ZERO/ONE are now instances of a private DDTraceId subtype (a sibling of DD64bTraceId), so DDTraceId.<clinit> no longer references the subclass. Replace the fragile "== DDTraceId.ZERO" identity checks with a value-based DDTraceId.isZero(). Those identity checks relied on every zero id being the single ZERO instance; isZero() recognizes a zero id of any concrete type, so the factories need not route 0 to the singleton and the propagation codecs no longer mishandle a zero parsed via the direct 64-bit factories. Add a forked regression test that initializes the two classes concurrently from opposite ends (deadlocks without the fix), plus isZero() coverage across the DDTraceId subtypes. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What Does This Do
Fix broken public API due to changing
DDTraceIdfrom class into an interface.Motivation
Additional Notes
Fixes #4918
Related to #4938