fix(tracing): ensure trace info is correctly set on subprocess (#12741)

Resolves an issue where trace information, such as span links, baggage,
and trace-level tags (ex: sampling decision maker), could be lost when a
new process was created while a trace was active.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: mabdinur

ddtrace/_trace/tracer.py

@@ -701,13 +701,7 @@ def _start_span(
             # strong span reference (which will never be finished) is replaced
             # with a context representing the span.
             if isinstance(child_of, Span):
-                new_ctx = Context(
-                    sampling_priority=child_of.context.sampling_priority,
-                    span_id=child_of.span_id,
-                    trace_id=child_of.trace_id,
-                    is_remote=False,
-                )
-
+                new_ctx = child_of.context
                 # If the child_of span was active then activate the new context
                 # containing it so that the strong span referenced is removed
                 # from the execution.

releasenotes/notes/fix-new-process-creation-7f997661cf5ace22.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    tracing: Resolves an issue where trace information, such as span links, baggage, and trace-level tags (ex: sampling decision maker), could be lost when a new process was created while a trace was active.

tests/integration/test_integration_snapshots.py

@@ -118,9 +118,7 @@ def test_synchronous_writer():
 def test_tracer_trace_across_popen():
     """
     When a trace is started in a parent process and a child process is spawned
-        The trace should be continued in the child process. The fact that
-        the child span has does not have '_dd.p.dm' shows that sampling was run
-        before fork automatically.
+        The trace should be continued in the child process.
     """
 
     def task(tracer):
@@ -140,9 +138,7 @@ def task(tracer):
 def test_tracer_trace_across_multiple_popens():
     """
     When a trace is started and crosses multiple process boundaries
-        The trace should be continued in the child processes. The fact that
-        the child span has does not have '_dd.p.dm' shows that sampling was run
-        before fork automatically.
+        The trace should be continued in the child processes.
     """
 
     def task(tracer):

tests/snapshots/tests.integration.test_integration_snapshots.test_tracer_trace_across_multiple_popens.json

@@ -32,6 +32,7 @@
        "type": "",
        "error": 0,
        "meta": {
+         "_dd.p.dm": "-0",
          "language": "python",
          "runtime-id": "60f937fe2fa5494c8aeceab4b18a5174"
        },
@@ -54,6 +55,7 @@
           "type": "",
           "error": 0,
           "meta": {
+            "_dd.p.dm": "-0",
             "language": "python",
             "runtime-id": "0a98b167e74c408abbb58db3fd26a2e7"
           },

tests/snapshots/tests.integration.test_integration_snapshots.test_tracer_trace_across_popen.json

@@ -32,6 +32,7 @@
        "type": "",
        "error": 0,
        "meta": {
+         "_dd.p.dm": "-0",
          "language": "python",
          "runtime-id": "36f1a5c9396e469291422d40f4296160"
        },

tests/tracer/test_tracer.py

@@ -1696,12 +1696,16 @@ def test_fork_manual_span_same_context():
     from ddtrace.trace import tracer
 
     span = tracer.trace("test")
+    span.context.set_baggage_item("key", "value")
+    span.context._meta["_dd.p.dm"] = "-1"
     pid = os.fork()
 
     if pid == 0:
         child = tracer.start_span("child", child_of=span)
         assert child.parent_id == span.span_id
         assert child._parent is None
+        # Ensure the child context is the same as the parent context
+        assert child.context == span.context
         # No more current span strong reference to avoid memory leaks.
         assert tracer.current_span() is None
         child.finish()