tracing: route in-process SDK writers through the v2 ring - #7326
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Add the temporary SDK-to-v2 path used to exercise the producer-local ring in a real tracing session. The consumer still receives ordinary TracePackets through the existing v1 service connection.
The temporary hop is:
SDK -> TraceWriterV2 -> v2 ring -> relay reader/rewriter
-> v1 writer -> v1 SMB -> traced
When traced reads the v2 ring directly, the relay, v1 writer and second SMB go away; the reader and rewriter move into the service ahead of the trace buffer.
- Select v2 through the C/C++ SDK initialization arguments. Keep the producer-wide option default-off, require encoding-aware data sources and leave Rust without an opt-in.
- Keep the optional implementation behind the established linker indirection so SDK binaries that do not enable it can discard it.
- Decorate ProducerEndpoint::CreateTraceWriter(). Startup writers and writers without an id remain on v1.
- Drain rings on one relay sequence. Reassemble fragments, rewrite proto groups and forward through one v1 writer per WriterID.
- Serialize endpoint controls, per-writer flushes and retirement through ring-position watermarks and downstream v1 acknowledgements.
- Preserve no_flush data-source semantics while still draining the ring before the producer acknowledges a service flush.
- Retire each downstream writer only after its last ring position has been drained and flushed.
- Complete Sync callbacks after endpoint loss instead of stranding their callers.
- Report malformed and unsupported ring chunks as data loss on the next packet instead of silently discarding them.
- Keep v1 writers and their arbiters alive through disconnect and shutdown, and release them on the relay sequence.
- Test SDK selection, routing, reassembly, barrier ordering, no_flush, data loss and teardown.
Bug: 536851377
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Add the temporary SDK-to-v2 path used to exercise the producer-local ring in a real tracing session. The consumer still receives ordinary TracePackets through the existing v1 service connection.
The temporary hop is:
When traced reads the v2 ring directly, the relay, v1 writer and second SMB go away; the reader and rewriter move into the service ahead of the trace buffer.