aws/endpoints: Fix SDK endpoint signing name resolution

aws/endpoints.go

@@ -32,8 +32,21 @@ func (v ResolveWithEndpoint) ResolveEndpoint(service, region string) (Endpoint,
 
 // Endpoint represents the endpoint a service client should make requests to.
 type Endpoint struct {
-	URL           string
-	SigningName   string
+	// The URL of the endpoint.
+	URL string
+
+	// The service name that should be used for signing the requests to the
+	// endpoint.
+	SigningName string
+
+	// The region that should be used for signing the request to the endpoint.
 	SigningRegion string
+
+	// States that the signing name for this endpoint was derived from metadata
+	// passed in, but was not explicitly modeled.
+	SigningNameDerived bool
+
+	// The signing method that should be used for signign the requests to the
+	// endpoint.
 	SigningMethod string
 }

aws/endpoints/v3model.go

@@ -229,16 +229,20 @@ func (e endpoint) resolve(service, region, dnsSuffix string, defs []endpoint, op
 	if len(signingRegion) == 0 {
 		signingRegion = region
 	}
+
 	signingName := e.CredentialScope.Service
+	var signingNameDerived bool
 	if len(signingName) == 0 {
 		signingName = service
+		signingNameDerived = true
 	}
 
 	return aws.Endpoint{
-		URL:           u,
-		SigningRegion: signingRegion,
-		SigningName:   signingName,
-		SigningMethod: getByPriority(e.SignatureVersions, signerPriority, defaultSigner),
+		URL:                u,
+		SigningRegion:      signingRegion,
+		SigningName:        signingName,
+		SigningNameDerived: signingNameDerived,
+		SigningMethod:      getByPriority(e.SignatureVersions, signerPriority, defaultSigner),
 	}
 }