Add MapCarrier (#2334)

* Add MapCarrier

* Update CHANGELOG.md

* Lint propagation_test.go

* Remove trailing space in changelog entry

* Revert change to internal/tools/go.sum

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - Add the `"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc".WithGRPCConn` option so the exporter can reuse an existing gRPC connection. (#2002)
 - Added a new `schema` module to help parse Schema Files in OTEP 0152 format. (#2267)
+- Added a new `MapCarrier` to the `go.opentelemetry.io/otel/propagation` package to hold propagated coss-cutting concerns as a `map[string]string` held in memory. (#2334)
 
 ## [1.1.0] - 2021-10-27
 

propagation/propagation.go

@@ -40,6 +40,32 @@ type TextMapCarrier interface {
 	// must never be done outside of a new major release.
 }
 
+// MapCarrier is a TextMapCarrier that uses a map held in memory as a storage
+// medium for propagated key-value pairs.
+type MapCarrier map[string]string
+
+// Compile time check that MapCarrier implements the TextMapCarrier.
+var _ TextMapCarrier = MapCarrier{}
+
+// Get returns the value associated with the passed key.
+func (c MapCarrier) Get(key string) string {
+	return c[key]
+}
+
+// Set stores the key-value pair.
+func (c MapCarrier) Set(key, value string) {
+	c[key] = value
+}
+
+// Keys lists the keys stored in this carrier.
+func (c MapCarrier) Keys() []string {
+	keys := make([]string, 0, len(c))
+	for k := range c {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
 // HeaderCarrier adapts http.Header to satisfy the TextMapCarrier interface.
 type HeaderCarrier http.Header
 

propagation/propagation_test.go

@@ -16,9 +16,12 @@ package propagation_test
 
 import (
 	"context"
+	"sort"
 	"strings"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+
 	"go.opentelemetry.io/otel/propagation"
 )
 
@@ -102,3 +105,33 @@ func TestCompositeTextMapPropagatorExtract(t *testing.T) {
 		t.Errorf("invalid extract order: %s", got)
 	}
 }
+
+func TestMapCarrierGet(t *testing.T) {
+	carrier := propagation.MapCarrier{
+		"foo": "bar",
+		"baz": "qux",
+	}
+
+	assert.Equal(t, carrier.Get("foo"), "bar")
+	assert.Equal(t, carrier.Get("baz"), "qux")
+}
+
+func TestMapCarrierSet(t *testing.T) {
+	carrier := make(propagation.MapCarrier)
+	carrier.Set("foo", "bar")
+	carrier.Set("baz", "qux")
+
+	assert.Equal(t, carrier["foo"], "bar")
+	assert.Equal(t, carrier["baz"], "qux")
+}
+
+func TestMapCarrierKeys(t *testing.T) {
+	carrier := propagation.MapCarrier{
+		"foo": "bar",
+		"baz": "qux",
+	}
+
+	keys := carrier.Keys()
+	sort.Strings(keys)
+	assert.Equal(t, []string{"baz", "foo"}, keys)
+}