aws/endpoints: Fix SDK endpoint signing name resolution (#181)

Fixes how the SDK derives service signing names. If the signing name is
not modeled in the endpoints package the service will fallback to the
signing name modeled in the service model.

Fix #163
Fix #153
V2 Port of aws/aws-sdk-go#1854

Author: jasdel

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),
 	}
 }