Ignore double value for grpc-accept-encoding header

CHANGELOG.md

@@ -10,6 +10,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Columns in the HTML Outbounds table on the debug page now properly line up
   with header rows.
 
+### Changed
+- Allow multiple values for the header `grpc-accept-encoding`. Only one (random) value will be used anyway,
+  but no error will be returned.
+
 ## [1.75.3] - 2024-12-04
 ### Added
 - Noop resolver to use in clients with custom load balancing.

transport/grpc/headers.go

@@ -86,6 +86,11 @@ const (
 
 	baseContentType   = "application/grpc"
 	contentTypeHeader = "content-type"
+
+	// grpcAcceptEncodingHeader is the header name that keeps the list of available compressors,
+	// applicable for both request and response headers. Usually consumed by grpc, but the
+	// value is kept in metadata and available in application code.
+	grpcAcceptEncodingHeader = "grpc-accept-encoding"
 )
 
 // TODO: there are way too many repeat calls to strings.ToLower
@@ -128,7 +133,12 @@ func metadataToTransportRequest(md metadata.MD) (*transport.Request, error) {
 		case 1:
 			value = values[0]
 		default:
-			return nil, yarpcerrors.InvalidArgumentErrorf("header has more than one value: %s:%v", header, values)
+			// TODO (https://github.com/yarpc/yarpc-go/issues/2265) don't pass this values to application code
+			if header == grpcAcceptEncodingHeader {
+				value = values[0]
+			} else {
+				return nil, yarpcerrors.InvalidArgumentErrorf("header has more than one value: %s:%v", header, values)
+			}
 		}
 		header = transport.CanonicalizeHeaderKey(header)
 		switch header {
@@ -213,7 +223,12 @@ func getApplicationHeaders(md metadata.MD) (transport.Headers, error) {
 		case 1:
 			value = values[0]
 		default:
-			return headers, yarpcerrors.InvalidArgumentErrorf("header has more than one value: %s:%v", header, values)
+			// TODO (https://github.com/yarpc/yarpc-go/issues/2265) don't pass this values to application code
+			if header == grpcAcceptEncodingHeader {
+				value = values[0]
+			} else {
+				return headers, yarpcerrors.InvalidArgumentErrorf("header has more than one value: %s:%v", header, values)
+			}
 		}
 		headers = headers.With(header, value)
 	}

transport/grpc/headers_test.go

@@ -116,6 +116,22 @@ func TestMetadataToTransportRequest(t *testing.T) {
 				}),
 			},
 		},
+		{
+			Name: "Double grpc-accept-encoding value",
+			MD: metadata.Pairs(
+				grpcAcceptEncodingHeader, "gzip",
+				grpcAcceptEncodingHeader, "zlib",
+				"foo", "bar",
+				"baz", "bat",
+			),
+			TransportRequest: &transport.Request{
+				Headers: transport.HeadersFromMap(map[string]string{
+					"foo":                  "bar",
+					"baz":                  "bat",
+					"grpc-accept-encoding": "gzip",
+				}),
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.Name, func(t *testing.T) {
@@ -253,6 +269,19 @@ func TestGetApplicationHeaders(t *testing.T) {
 			},
 			wantErr: "header has more than one value: test-header-dup:[test-value-1 test-value-2]",
 		},
+		{
+			msg: "Multiple value for grpc-accept-encoding header",
+			meta: metadata.MD{
+				"test-header-valid-1":  []string{"test-value-1"},
+				"test-Header-Valid-2":  []string{"test-value-2"},
+				"grpc-accept-encoding": []string{"gzip", "zlib"},
+			},
+			wantHeaders: map[string]string{
+				"test-header-valid-1":  "test-value-1",
+				"test-header-valid-2":  "test-value-2",
+				"grpc-accept-encoding": "gzip",
+			},
+		},
 	}
 
 	for _, tt := range tests {