Add fields property

Fixes #1104

Author: ocelotl

Diff for: opentelemetry-api/src/opentelemetry/baggage/propagation/__init__.py

@@ -87,6 +87,15 @@ def inject(
         baggage_string = _format_baggage(baggage_entries)
         set_in_carrier(carrier, self._BAGGAGE_HEADER_NAME, baggage_string)
 
+    @property
+    def fields(self) -> typing.Set[str]:
+        """Returns a set with the fields set in `inject`.
+
+        See
+        `opentelemetry.trace.propagation.textmap.TextMapPropagator.fields`
+        """
+        return {self._BAGGAGE_HEADER_NAME}
+
 
 def _format_baggage(baggage_entries: typing.Mapping[str, object]) -> str:
     return ",".join(

Diff for: opentelemetry-api/src/opentelemetry/propagators/composite.py

@@ -65,3 +65,18 @@ def inject(
         """
         for propagator in self._propagators:
             propagator.inject(set_in_carrier, carrier, context)
+
+    @property
+    def fields(self) -> typing.Set[str]:
+        """Returns a set with the fields set in `inject`.
+
+        See
+        `opentelemetry.trace.propagation.textmap.TextMapPropagator.fields`
+        """
+        composite_fields = set()
+
+        for propagator in self._propagators:
+            for field in propagator.fields:
+                composite_fields.add(field)
+
+        return composite_fields

Diff for: opentelemetry-api/src/opentelemetry/trace/propagation/textmap.py

@@ -129,3 +129,15 @@ def inject(
                 context if not set.
 
         """
+
+    @abc.abstractmethod
+    def fields(self) -> typing.Set[str]:
+        """
+        Gets the fields set in the carrier by the `inject` method.
+
+        If the carrier is reused, its fields that correspond with the ones
+        present in this attribute should be deleted before calling `inject`.
+
+        Returns:
+            A set with the fields set in `inject`.
+        """

Diff for: opentelemetry-api/src/opentelemetry/trace/propagation/tracecontext.py

@@ -132,6 +132,17 @@ def inject(
                 carrier, self._TRACESTATE_HEADER_NAME, tracestate_string
             )
 
+    @property
+    def fields(self) -> typing.Set[str]:
+        """Returns a set with the fields set in `inject`.
+
+        See
+        `opentelemetry.trace.propagation.textmap.TextMapPropagator.fields`
+        """
+        return {
+            self._TRACEPARENT_HEADER_NAME, self._TRACESTATE_HEADER_NAME
+        }
+
 
 def _parse_tracestate(header_list: typing.List[str]) -> trace.TraceState:
     """Parse one or more w3c tracestate header into a TraceState.

Diff for: opentelemetry-api/tests/baggage/test_baggage_propagation.py

@@ -12,8 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-import typing
 import unittest
+from unittest.mock import patch, Mock
 
 from opentelemetry import baggage
 from opentelemetry.baggage.propagation import BaggagePropagator
@@ -142,3 +142,18 @@ def test_inject_non_string_values(self):
         self.assertIn("key1=True", output)
         self.assertIn("key2=123", output)
         self.assertIn("key3=123.567", output)
+
+    @patch("opentelemetry.baggage.propagation.baggage")
+    @patch("opentelemetry.baggage.propagation._format_baggage")
+    def test_fields(self, mock_format_baggage, mock_baggage):
+
+        mock_set_in_carrier = Mock()
+
+        self.propagator.inject(mock_set_in_carrier, {})
+
+        inject_fields = set()
+
+        for mock_call in mock_set_in_carrier.mock_calls:
+            inject_fields.add(mock_call.args[1])
+
+        self.assertEqual(inject_fields, self.propagator.fields)

Diff for: opentelemetry-api/tests/propagators/test_composite.py

@@ -26,6 +26,8 @@ def get_as_list(dict_object, key):
 def mock_inject(name, value="data"):
     def wrapped(setter, carrier=None, context=None):
         carrier[name] = value
+        setter(None, "inject_field_{}_0".format(name), None)
+        setter(None, "inject_field_{}_1".format(name), None)
 
     return wrapped
 
@@ -39,18 +41,29 @@ def wrapped(getter, carrier=None, context=None):
     return wrapped
 
 
+def mock_fields(name):
+    return {
+        "inject_field_{}_0".format(name), "inject_field_{}_1".format(name)
+    }
+
+
 class TestCompositePropagator(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         cls.mock_propagator_0 = Mock(
-            inject=mock_inject("mock-0"), extract=mock_extract("mock-0")
+            inject=mock_inject("mock-0"),
+            extract=mock_extract("mock-0"),
+            fields=mock_fields("mock-0"),
         )
         cls.mock_propagator_1 = Mock(
-            inject=mock_inject("mock-1"), extract=mock_extract("mock-1")
+            inject=mock_inject("mock-1"),
+            extract=mock_extract("mock-1"),
+            fields=mock_fields("mock-1"),
         )
         cls.mock_propagator_2 = Mock(
             inject=mock_inject("mock-0", value="data2"),
             extract=mock_extract("mock-0", value="context2"),
+            fields=mock_fields("mock-0"),
         )
 
     def test_no_propagators(self):
@@ -105,3 +118,23 @@ def test_multiple_propagators_same_key(self):
             get_as_list, carrier=new_carrier, context={}
         )
         self.assertEqual(context, {"mock-0": "context2"})
+
+    def test_fields(self):
+        propagator = CompositeHTTPPropagator(
+            [
+                self.mock_propagator_0,
+                self.mock_propagator_1,
+                self.mock_propagator_2
+            ]
+        )
+
+        mock_set_in_carrier = Mock()
+
+        propagator.inject(mock_set_in_carrier, {})
+
+        inject_fields = set()
+
+        for mock_call in mock_set_in_carrier.mock_calls:
+            inject_fields.add(mock_call.args[1])
+
+        self.assertEqual(inject_fields, propagator.fields)

Diff for: opentelemetry-sdk/src/opentelemetry/sdk/trace/propagation/b3_format.py

@@ -152,6 +152,15 @@ def inject(
             )
         set_in_carrier(carrier, self.SAMPLED_KEY, "1" if sampled else "0")
 
+    @property
+    def fields(self) -> typing.Set[str]:
+        return {
+            self.TRACE_ID_KEY,
+            self.SPAN_ID_KEY,
+            self.PARENT_SPAN_ID_KEY,
+            self.SAMPLED_KEY,
+        }
+
 
 def format_trace_id(trace_id: int) -> str:
     """Format the trace id according to b3 specification."""

Diff for: opentelemetry-sdk/tests/trace/propagation/test_b3_format.py

@@ -14,6 +14,7 @@
 
 import unittest
 from unittest.mock import patch
+from unittest.mock import Mock
 
 import opentelemetry.sdk.trace as trace
 import opentelemetry.sdk.trace.propagation.b3_format as b3_format
@@ -329,3 +330,21 @@ def setter(carrier, key, value):
 
         ctx = FORMAT.extract(CarrierGetter(), {})
         FORMAT.inject(setter, {}, ctx)
+
+    def test_fields(self):
+        """Make sure the fields attribute returns the fields used in inject"""
+
+        tracer = trace.TracerProvider().get_tracer("sdk_tracer_provider")
+
+        mock_set_in_carrier = Mock()
+
+        with tracer.start_as_current_span("parent"):
+            with tracer.start_as_current_span("child"):
+                FORMAT.inject(mock_set_in_carrier, {})
+
+        inject_fields = set()
+
+        for call in mock_set_in_carrier.mock_calls:
+            inject_fields.add(call.args[1])
+
+        self.assertEqual(FORMAT.fields, inject_fields)