Return an empty synthetic span context even if there was no trace context found

Author: Andrey Slotin

instana/http_propagator.py

@@ -108,6 +108,9 @@ def extract(self, carrier):  # noqa
                                   baggage={},
                                   sampled=True,
                                   synthetic=synthetic)
+            elif synthetic:
+                ctx = SpanContext(synthetic=synthetic)
+
             return ctx
 
         except Exception:

instana/tracer.py

@@ -84,7 +84,7 @@ def start_span(self,
         # Assemble the child ctx
         gid = generate_id()
         ctx = SpanContext(span_id=gid)
-        if parent_ctx is not None:
+        if parent_ctx is not None and parent_ctx.trace_id is not None:
             if parent_ctx._baggage is not None:
                 ctx._baggage = parent_ctx._baggage.copy()
             ctx.trace_id = parent_ctx.trace_id

tests/clients/test_urllib3.py

@@ -509,6 +509,8 @@ def test_client_error(self):
         self.assertEqual(1, urllib3_span.ec)
 
     def test_requestspkg_get(self):
+        self.recorder.clear_spans()
+        
         with tracer.start_active_span('test'):
             r = requests.get(testenv["wsgi_server"] + '/', timeout=2)
 

tests/opentracing/test_ot_propagators.py

@@ -71,6 +71,18 @@ def test_http_mixed_case_extract():
     assert not ctx.synthetic
 
 
+def test_http_extract_synthetic_only():
+    ot.tracer = InstanaTracer()
+
+    carrier = {'X-Instana-Synthetic': '1'}
+    ctx = ot.tracer.extract(ot.Format.HTTP_HEADERS, carrier)
+
+    assert isinstance(ctx, span.SpanContext)
+    assert ctx.trace_id is None
+    assert ctx.span_id is None
+    assert ctx.synthetic
+
+
 def test_http_no_context_extract():
     ot.tracer = InstanaTracer()