aws/endpoints: Fix SDK resolving endpoint without region

Fixes the SDK's endpoint resolve incorrectly resolving endpoints for a
service when the region is empty. Also fixes the SDK attempting to
resolve a service when the service value is empty.

Related to: aws/aws-sdk-go#2911

aws/endpoints/v3model.go

@@ -74,16 +74,29 @@ func (p partition) canResolveEndpoint(service, region string, strictMatch bool)
 	return p.RegionRegex.MatchString(region)
 }
 
+func allowLegacyEmptyRegion(service string) bool {
+	legacy := map[string]struct{}{
+		"ec2metadata": {},
+	}
+
+	_, allowed := legacy[service]
+	return allowed
+}
+
 func (p partition) EndpointFor(service, region string, opts ResolveOptions) (resolved aws.Endpoint, err error) {
 	s, hasService := p.Services[service]
-	if !hasService && opts.StrictMatching {
+	if len(service) == 0 || (!hasService && opts.StrictMatching) {
 		// Only return error if the resolver will not fallback to creating
 		// endpoint based on service endpoint ID passed in.
 		return resolved, NewUnknownServiceError(p.ID, service, serviceList(p.Services))
 	}
 
+	if len(region) == 0 && allowLegacyEmptyRegion(service) && len(s.PartitionEndpoint) != 0 {
+		region = s.PartitionEndpoint
+	}
+
 	e, hasEndpoint := s.endpointForRegion(region)
-	if !hasEndpoint && opts.StrictMatching {
+	if len(region) == 0 || (!hasEndpoint && opts.StrictMatching) {
 		return resolved, NewUnknownEndpointError(p.ID, service, region, endpointList(s.Endpoints))
 	}
 

aws/endpoints/v3model_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"reflect"
 	"regexp"
+	"strings"
 	"testing"
 )
 
@@ -522,3 +523,46 @@ func TestResolveEndpoint_AwsGlobal(t *testing.T) {
 		t.Errorf("expect the signing name to be derived")
 	}
 }
+
+func TestEndpointFor_EmptyRegion(t *testing.T) {
+	cases := map[string]struct {
+		Service    string
+		Region     string
+		RealRegion string
+		ExpectErr  string
+	}{
+		// Legacy services that previous accepted empty region
+		"ec2metadata": {Service: "ec2metadata", RealRegion: "aws-global"},
+
+		// Other services
+		"s3":           {Service: "s3", Region: "us-east-1", RealRegion: "us-east-1"},
+		"s3 no region": {Service: "s3", ExpectErr: "could not resolve endpoint"},
+	}
+
+	for name, c := range cases {
+		t.Run(name, func(t *testing.T) {
+			actual, err := NewDefaultResolver().ResolveEndpoint(c.Service, c.Region)
+			if len(c.ExpectErr) != 0 {
+				if e, a := c.ExpectErr, err.Error(); !strings.Contains(a, e) {
+					t.Errorf("expect %q error in %q", e, a)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("expect no error got, %v", err)
+			}
+
+			expect, err := NewDefaultResolver().ResolveEndpoint(c.Service, c.RealRegion)
+			if err != nil {
+				t.Fatalf("failed to get endpoint for default resolver")
+			}
+			if e, a := expect.URL, actual.URL; e != a {
+				t.Errorf("expect %v URL, got %v", e, a)
+			}
+			if e, a := expect.SigningRegion, actual.SigningRegion; e != a {
+				t.Errorf("expect %v signing region, got %v", e, a)
+			}
+
+		})
+	}
+}