refactor: remove global tracer variable from core (#19083)

This change replaces `core.tracer` with a tracer instance stored on the root `ExecutionContext` object, increasing the separation of concerns between tracing and core. With this change, tracing becomes a user of the core API rather than being a part of it.

This change also increases a missed microbenchmark by a small amount to account for slightly increased execution time during span creation as a result of the `ExecutionContext.get_item` call added here.

Co-authored-by: emmett.butler <emmett.butler@datadoghq.com>

Author: emmettbutler

.gitlab/benchmarks/bp-runner.microbenchmarks.fail-on-breach.template.yml

@@ -215,7 +215,7 @@ experiments:
               - max_rss_usage < 62.50 MB
           - name: flasksqli-tracer-enabled
             thresholds:
-              - execution_time < 2.25 ms
+              - execution_time < 2.27 ms
               - max_rss_usage < 62.00 MB
 
           # httppropagationextract

ddtrace/contrib/internal/trace_utils.py

@@ -317,7 +317,7 @@ def wrapper(wrapped, instance, args, kwargs):
 
 
 def is_tracing_enabled() -> bool:
-    tracer = core.tracer
+    tracer = core.root.get_item("tracer")
     return tracer is not None and (tracer.enabled or asm_config._apm_opt_out)
 
 

ddtrace/internal/core/__init__.py

@@ -145,8 +145,6 @@ def done_callback(f):
 
 _MISSING = object()
 
-tracer = None
-
 log = logging.getLogger(__name__)
 
 
@@ -332,7 +330,8 @@ def span(self) -> "Span":
                 "Creating fallback 'default' span.",
                 self,
             )
-            self._inner_span = tracer.current_span() or tracer.trace("default")  # type: ignore
+            tracer = self.get_item("tracer")
+            self._inner_span = tracer.current_span() or tracer.trace("default")
         return self._inner_span
 
     @span.setter
@@ -439,6 +438,7 @@ def get_span() -> Optional["Span"]:
 
 def get_root_span() -> Optional["Span"]:
     span = get_span()
+    tracer = _CURRENT_CONTEXT.get().root().get_item("tracer")
     if span is None:
         return None if tracer is None else tracer.current_root_span()
     return span._local_root or span

ddtrace/propagation/http.py

@@ -1132,13 +1132,14 @@ def _get_sampled_injection_context(
         If sampling_priority is already set, returns immediately. Otherwise, finds the
         appropriate span and triggers sampling before returning the injection context.
         """
+        core_tracer = core.root.get_item("tracer")
         # Extract context for header injection (non_active_span takes precedence)
         injection_context = trace_info.context if isinstance(trace_info, Span) else trace_info
 
         # Find root span for sampling decisions
         if injection_context.sampling_priority is not None:
             return injection_context
-        elif core.tracer is None:
+        elif core_tracer is None:
             # This should never happen, tracer should be initialized before headers can be injected.
             log.error(
                 "No tracer found and injection context %s has no sampling priority, skipping sampling",
@@ -1153,13 +1154,13 @@ def _get_sampled_injection_context(
         elif isinstance(trace_info, Span):
             # Use span's root for sampling
             sampling_span = trace_info._local_root
-        elif (current_root := core.tracer.current_root_span()) and current_root.trace_id == trace_info.trace_id:
+        elif (current_root := core_tracer.current_root_span()) and current_root.trace_id == trace_info.trace_id:
             # Get the local root span for the current trace (if it is active, otherwise we can't sample)
             sampling_span = current_root
 
         # Sample the local root span before injecting headers.
         if sampling_span:
-            core.tracer.sample(sampling_span)
+            core_tracer.sample(sampling_span)
             log.debug("%s sampled before propagating trace: span_context=%s", sampling_span, injection_context)
 
         return injection_context

ddtrace/trace/__init__.py

@@ -8,7 +8,7 @@
 
 # a global tracer instance with integration settings
 tracer = Tracer()
-core.tracer = tracer  # type: ignore
+core.root.set_item("tracer", tracer)
 
 
 __all__ = [