Prototypes propagation with multiple values. Adds MultiTextMapCarrier…

…, extending TextMapCarrier.

Gives example extracting requests with multiple 'baggage' headers set.

propagation/baggage.go

@@ -29,6 +29,19 @@ func (b Baggage) Inject(ctx context.Context, carrier TextMapCarrier) {
 
 // Extract returns a copy of parent with the baggage from the carrier added.
 func (b Baggage) Extract(parent context.Context, carrier TextMapCarrier) context.Context {
+	multiCarrier, isMultiCarrier := carrier.(MultiTextMapCarrier)
+	if isMultiCarrier {
+		return extractMultiBaggage(parent, multiCarrier)
+	}
+	return extractSingleBaggage(parent, carrier)
+}
+
+// Fields returns the keys who's values are set with Inject.
+func (b Baggage) Fields() []string {
+	return []string{baggageHeader}
+}
+
+func extractSingleBaggage(parent context.Context, carrier TextMapCarrier) context.Context {
 	bStr := carrier.Get(baggageHeader)
 	if bStr == "" {
 		return parent
@@ -41,7 +54,20 @@ func (b Baggage) Extract(parent context.Context, carrier TextMapCarrier) context
 	return baggage.ContextWithBaggage(parent, bag)
 }
 
-// Fields returns the keys who's values are set with Inject.
-func (b Baggage) Fields() []string {
-	return []string{baggageHeader}
+func extractMultiBaggage(parent context.Context, carrier MultiTextMapCarrier) context.Context {
+	bVals := carrier.GetAll(baggageHeader)
+	members := make([]baggage.Member, 0)
+	for _, bStr := range bVals {
+		currBag, err := baggage.Parse(bStr)
+		if err != nil {
+			continue
+		}
+		members = append(members, currBag.Members()...)
+	}
+
+	b, err := baggage.New(members...)
+	if err != nil || b.Len() == 0 {
+		return parent
+	}
+	return baggage.ContextWithBaggage(parent, b)
 }

propagation/baggage_test.go

@@ -128,6 +128,55 @@ func TestExtractValidBaggageFromHTTPReq(t *testing.T) {
 	}
 }
 
+func TestExtractValidMultipleBaggageHeaders(t *testing.T) {
+	prop := propagation.TextMapPropagator(propagation.Baggage{})
+	tests := []struct {
+		name    string
+		headers []string
+		want    members
+	}{
+		{
+			name:    "non conflicting headers",
+			headers: []string{"key1=val1", "key2=val2"},
+			want: members{
+				{Key: "key1", Value: "val1"},
+				{Key: "key2", Value: "val2"},
+			},
+		},
+		{
+			name:    "conflicting keys, uses last val",
+			headers: []string{"key1=val1", "key1=val2"},
+			want: members{
+				{Key: "key1", Value: "val2"},
+			},
+		},
+		{
+			name:    "single empty",
+			headers: []string{"", "key1=val1"},
+			want: members{
+				{Key: "key1", Value: "val1"},
+			},
+		},
+		{
+			name:    "all empty",
+			headers: []string{"", ""},
+			want:    members{},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req, _ := http.NewRequest("GET", "http://example.com", nil)
+			req.Header["Baggage"] = tt.headers
+
+			ctx := context.Background()
+			ctx = prop.Extract(ctx, propagation.HeaderCarrier(req.Header))
+			expected := tt.want.Baggage(t)
+			assert.Equal(t, expected, baggage.FromContext(ctx))
+		})
+	}
+}
+
 func TestExtractInvalidDistributedContextFromHTTPReq(t *testing.T) {
 	prop := propagation.TextMapPropagator(propagation.Baggage{})
 	tests := []struct {

propagation/propagation.go

@@ -29,6 +29,15 @@ type TextMapCarrier interface {
 	// must never be done outside of a new major release.
 }
 
+// MultiTextMapCarrier is a TextMapCarrier that can return multiple values for a single key.
+type MultiTextMapCarrier interface {
+	TextMapCarrier
+	// GetAll returns all values associated with the passed key.
+	GetAll(key string) []string
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// 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
@@ -58,11 +67,16 @@ func (c MapCarrier) Keys() []string {
 // HeaderCarrier adapts http.Header to satisfy the TextMapCarrier interface.
 type HeaderCarrier http.Header
 
-// Get returns the value associated with the passed key.
+// Get returns the first value associated with the passed key.
 func (hc HeaderCarrier) Get(key string) string {
 	return http.Header(hc).Get(key)
 }
 
+// GetAll returns all values associated with the passed key.
+func (hc HeaderCarrier) GetAll(key string) []string {
+	return http.Header(hc).Values(key)
+}
+
 // Set stores the key-value pair.
 func (hc HeaderCarrier) Set(key string, value string) {
 	http.Header(hc).Set(key, value)