private/protocol/query: Add parsing of ServiceUnavailableException

Adds support to fallback and attempt to parse the
ServiceUnavailableException out of the response body. This is helpful
when the request failed and the service returned unavailable error.
Previously the error unmarshaller would fail with a serialization error.

Fix #331

private/protocol/query/unmarshal_error.go

@@ -2,7 +2,7 @@ package query
 
 import (
 	"encoding/xml"
-	"io"
+	"io/ioutil"
 
 	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/aws/request"
@@ -15,18 +15,27 @@ type xmlErrorResponse struct {
 	RequestID string   `xml:"RequestId"`
 }
 
+type xmlServiceUnavailableResponse struct {
+	XMLName xml.Name `xml:"ServiceUnavailableException"`
+}
+
 // UnmarshalErrorHandler is a name request handler to unmarshal request errors
 var UnmarshalErrorHandler = request.NamedHandler{Name: "awssdk.query.UnmarshalError", Fn: UnmarshalError}
 
 // UnmarshalError unmarshals an error response for an AWS Query service.
 func UnmarshalError(r *request.Request) {
 	defer r.HTTPResponse.Body.Close()
 
-	resp := &xmlErrorResponse{}
-	err := xml.NewDecoder(r.HTTPResponse.Body).Decode(resp)
-	if err != nil && err != io.EOF {
-		r.Error = awserr.New("SerializationError", "failed to decode query XML error response", err)
-	} else {
+	bodyBytes, err := ioutil.ReadAll(r.HTTPResponse.Body)
+	if err != nil {
+		r.Error = awserr.New("SerializationError", "failed to read from query HTTP response body", err)
+		return
+	}
+
+	// First check for specific error
+	resp := xmlErrorResponse{}
+	decodeErr := xml.Unmarshal(bodyBytes, &resp)
+	if decodeErr == nil {
 		reqID := resp.RequestID
 		if reqID == "" {
 			reqID = r.RequestID
@@ -36,5 +45,22 @@ func UnmarshalError(r *request.Request) {
 			r.HTTPResponse.StatusCode,
 			reqID,
 		)
+		return
+	}
+
+	// Check for unhandled error
+	servUnavailResp := xmlServiceUnavailableResponse{}
+	unavailErr := xml.Unmarshal(bodyBytes, &servUnavailResp)
+	if unavailErr == nil {
+		r.Error = awserr.NewRequestFailure(
+			awserr.New("ServiceUnavailableException", "service is unavailable", nil),
+			r.HTTPResponse.StatusCode,
+			r.RequestID,
+		)
+		return
 	}
+
+	// Failed to retrieve any error message from the response body
+	r.Error = awserr.New("SerializationError",
+		"failed to decode query XML error response", decodeErr)
 }