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TracingContextFactory allocates when no tracer is configured

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Dal 16/8/2026.

Valutazione

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Descrizione

Azure.Core Client needs-team-attention

Summary

Azure::Core::Tracing::_internal::TracingContextFactory::CreateTracingContext allocates on the no-tracer path. Every call that receives a caller context without the factory performs two heap allocations, even when the application configured no tracing provider. A service that does not use the HTTP pipeline pays that cost for every top-level operation and gets nothing back.

Motivation

CreateTracingContext calls context.WithValue(TracingFactoryContextKey, this) at sdk/core/azure-core/src/tracing/tracing.cpp:66-68 before it tests HasTracer(), and Context::WithValue performs two make_shared calls at sdk/core/azure-core/inc/azure/core/context.hpp:289-293. The comment at tracing.cpp:61-64 justifies the unconditional injection by saying the factory also builds the User-Agent header. That justification is stale. At commit 99f45a5f1 RequestActivityPolicy read GetUserAgent() from the injected factory, but GetUserAgent no longer exists anywhere in sdk/core, and TelemetryPolicy now produces the header on its own at sdk/core/azure-core/src/http/telemetry_policy.cpp:16-21. The only remaining consumer of the injected factory is RequestActivityPolicy, which acts only when tracingFactory && tracingFactory->HasTracer() at sdk/core/azure-core/src/http/request_activity_policy.cpp:28, so a factory without a tracer has no observable effect. WithValue returns a child context and never changes the caller context, so two calls that share one caller context both miss the guard and both allocate. An AMQP service such as azure-messaging-eventhubs builds a fresh caller context for each operation, which measures at two allocations for every send and every receive.

Proposal

  • Return TracingContext{context, ServiceSpan{}} at the top of CreateTracingContext in sdk/core/azure-core/src/tracing/tracing.cpp when HasTracer() is false, and delete the dead else branch at lines 98-101. Copying the context copies one shared_ptr and does not reach the heap.
  • Keep the guarded injection and the span creation unchanged for the tracer-configured path, so HTTP clients behave exactly as they do today.
  • Replace the stale comment at tracing.cpp:61-64 with one that names RequestActivityPolicy as the consumer and records that TelemetryPolicy owns the User-Agent header.
  • Correct the TelemetryPolicy doc comment at sdk/core/azure-core/inc/azure/core/http/policies/policy.hpp:546-550, which claims User-Agent generation moved into RequestActivityPolicy. That merger was reverted, and the stale claim invites a future reader to restore the injection.
  • Add a test near BasicServiceSpanTests in sdk/core/azure-core/test/ut/service_tracing_test.cpp that makes sure CreateFromContext returns null for a context built with no provider, and returns a factory with HasTracer() true when a provider is set.
  • Add a CHANGELOG entry under "Other Changes" recording that CreateTracingContext no longer allocates when no tracer is configured.

Rejected alternatives. An opt-out constructor flag works, but it adds a data member to a class whose inline definition ships in a public header, and it forces every non-HTTP service to opt in by hand. Passing the factory to RequestActivityPolicy outside the context would let the injection be deleted for everyone, but the pipeline constructor carries no service name, which az.namespace needs, so it requires plumbing through every service that builds a pipeline. That route remains the right long-term direction if User-Agent duty ever returns to RequestActivityPolicy.

Validation

  • Both symbols live in Azure::Core::Tracing::_internal, and a repository-wide search for TracingFactoryContextKey and CreateFromContext finds one non-test consumer, so no public header changes and no API or ABI break.
  • Every existing RequestActivityPolicy test configures a tracing provider, at sdk/core/azure-core/test/ut/request_activity_policy_test.cpp:161-246, so no current test pins the no-tracer injection.
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