Fixed B3 Propagator

B3 propagator was returning `None` instead of invalid context after last
change. This resulted in many apps crashing when B3 failed to extract a
valid context from carrier. This PR patches it to not make apps crash
and return an invalid context when one cannot be extracted from the
carrier.

CHANGELOG.md

@@ -19,6 +19,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#1721](https://github.com/open-telemetry/opentelemetry-python/pull/1721))
 - Update bootstrap cmd to use exact version when installing instrumentation packages. 
   ([#1722](https://github.com/open-telemetry/opentelemetry-python/pull/1722))
+- Fix B3 propagator to never return None.
+  ([#1750](https://github.com/open-telemetry/opentelemetry-python/pull/1750))
 
 
 ## [1.0.0](https://github.com/open-telemetry/opentelemetry-python/releases/tag/v1.0.0) - 2021-03-26

propagator/opentelemetry-propagator-b3/src/opentelemetry/propagators/b3/__init__.py

@@ -97,6 +97,8 @@ def extract(
             or self._trace_id_regex.fullmatch(trace_id) is None
             or self._span_id_regex.fullmatch(span_id) is None
         ):
+            if context is None:
+                return trace.set_span_in_context(trace.INVALID_SPAN, context)
             return context
 
         trace_id = int(trace_id, 16)

propagator/opentelemetry-propagator-b3/tests/test_b3_format.py

@@ -334,3 +334,12 @@ def test_fields(self):
             inject_fields.add(call[1][1])
 
         self.assertEqual(FORMAT.fields, inject_fields)
+
+    def test_extract_none_context(self):
+        """Given no trace ID, do not modify context"""
+        old_ctx = None
+
+        carrier = {}
+        new_ctx = FORMAT.extract(carrier, old_ctx)
+        self.assertIsNotNone(new_ctx)
+        self.assertEqual(new_ctx["current-span"], trace_api.INVALID_SPAN)