Added fallback behavior to custom v1 resolvers (#2206)

codegen/smithy-aws-go-codegen/src/main/java/software/amazon/smithy/aws/go/codegen/AddAwsConfigFields.java

@@ -185,6 +185,13 @@ public class AddAwsConfigFields implements GoIntegration {
                     .documentation("API stack mutators")
                     .generatedOnClient(false)
                     .build(),
+            AwsConfigField.builder()
+                    .name(ENDPOINT_RESOLVER_CONFIG_NAME)
+                    .type(getAwsCoreSymbol("EndpointResolver"))
+                    .generatedOnClient(false)
+                    .awsResolveFunction(SymbolUtils.createValueSymbolBuilder(RESOLVE_AWS_CONFIG_ENDPOINT_RESOLVER)
+                            .build())
+                    .build(),
             AwsConfigField.builder()
                     .name(LOGGER_CONFIG_NAME)
                     .type(getAwsCoreSymbol("Logger"))
@@ -263,6 +270,7 @@ private void writeAwsDefaultResolvers(GoWriter writer) {
         writeHttpClientResolver(writer);
         writeRetryerResolvers(writer);
         writeRetryMaxAttemptsFinalizeResolver(writer);
+        writeAwsConfigEndpointResolver(writer);
     }
 
     private void writerAwsDefaultResolversTests(GoWriter writer) {
@@ -581,6 +589,24 @@ private void writeHttpClientResolver(GoWriter writer) {
         writer.popState();
     }
 
+    private void writeAwsConfigEndpointResolver(GoWriter writer) {
+        writer.pushState();
+        writer.putContext("resolverName", RESOLVE_AWS_CONFIG_ENDPOINT_RESOLVER);
+        writer.putContext("clientOption", ENDPOINT_RESOLVER_CONFIG_NAME);
+        writer.putContext("wrapperHelper", EndpointGenerator.AWS_ENDPOINT_RESOLVER_HELPER);
+        writer.putContext("awsResolver", ENDPOINT_RESOLVER_CONFIG_NAME);
+        writer.putContext("awsResolverWithOptions", AWS_ENDPOINT_RESOLVER_WITH_OPTIONS);
+        writer.write("""
+                func $resolverName:L(cfg aws.Config, o *Options) {
+                    if cfg.$awsResolver:L == nil && cfg.$awsResolverWithOptions:L == nil {
+                        return
+                    }
+                    o.$clientOption:L = $wrapperHelper:L(cfg.$awsResolver:L, cfg.$awsResolverWithOptions:L)
+                }
+                """);
+        writer.popState();
+    }
+
     @Override
     public List<RuntimeClientPlugin> getClientPlugins() {
         List<RuntimeClientPlugin> plugins = new ArrayList<>();

codegen/smithy-aws-go-codegen/src/main/java/software/amazon/smithy/aws/go/codegen/EndpointGenerator.java

@@ -394,33 +394,13 @@ private void generateAwsEndpointResolverWrapper(GoWriter writer) {
         writer.write("""
                      type $T struct {
                          awsResolver $T
-                         resolver $T
                      }
-                     """, wrappedResolverSymbol, endpointResolverWithOptions, resolverInterface);
+                     """, wrappedResolverSymbol, endpointResolverWithOptions);
+        var endpointNotFoundError = SymbolUtils.createValueSymbolBuilder("EndpointNotFoundError",
+            AwsGoDependency.AWS_CORE).build();
 
         writeExternalResolveEndpointImplementation(writer, wrappedResolverSymbol, "w", () -> {
-            var endpointNotFoundError = SymbolUtils.createValueSymbolBuilder("EndpointNotFoundError",
-                    AwsGoDependency.AWS_CORE).build();
-            var errorf = SymbolUtils.createValueSymbolBuilder("Errorf",
-                    SmithyGoDependency.FMT).build();
-            writer.write("""
-                         if w.awsResolver == nil {
-                             goto fallback
-                         }
-                         endpoint, err = w.awsResolver.ResolveEndpoint(ServiceID, region, options)
-                         if err == nil {
-                             return endpoint, nil
-                         }
-
-                         if nf := (&$T{}); !errors.As(err, &nf) {
-                             return endpoint, err
-                         }
-
-                         fallback:
-                         if w.resolver == nil {
-                             return endpoint, $T("default endpoint resolver provided was nil")
-                         }
-                         return w.resolver.ResolveEndpoint(region, options)""", endpointNotFoundError, errorf);
+            writer.write("return w.awsResolver.ResolveEndpoint(ServiceID, region, options)");
 
             writer.addUseImports(SmithyGoDependency.ERRORS);
         });
@@ -441,28 +421,31 @@ private void generateAwsEndpointResolverWrapper(GoWriter writer) {
         // Generate exported helper for constructing a wrapper around the AWS EndpointResolver type that is compatible
         // with the clients EndpointResolver interface.
         writer.write("""
-                     // $L returns an EndpointResolver that first delegates endpoint resolution to the awsResolver.
-                     // If awsResolver returns aws.EndpointNotFoundError error, the resolver will use the the provided
-                     // fallbackResolver for resolution.
+                     // $1L returns an $3T that first delegates endpoint resolution to the awsResolver.
+                     // If awsResolver returns $7T error, the v1 resolver middleware will swallow the error, 
+                     // and set an appropriate context flag such that fallback will occur when $8L is invoked 
+                     // via its middleware.
                      //
-                     // fallbackResolver must not be nil
-                     func $L(awsResolver $T, awsResolverWithOptions $T, fallbackResolver $T) $T {
-                         var resolver $T
+                     // If another error (besides $7T) is returned, then that error will be propagated.
+                     func $1L(awsResolver $2T, awsResolverWithOptions $3T) $6L {
+                         var resolver $3T
 
                          if awsResolverWithOptions != nil {
                              resolver = awsResolverWithOptions
                          } else if awsResolver != nil {
-                             resolver = $L(awsResolver.ResolveEndpoint)
+                             resolver = $4L(awsResolver.ResolveEndpoint)
                          }
 
-                         return &$T{
+                         return &$5T{
                              awsResolver: resolver,
-                             resolver: fallbackResolver,
                          }
                      }
-                     """, AWS_ENDPOINT_RESOLVER_HELPER, AWS_ENDPOINT_RESOLVER_HELPER,
-                endpointResolver, endpointResolverWithOptions, resolverInterface,
-                resolverInterface, endpointResolverWithOptions, AWS_ENDPOINT_RESOLVER_ADAPTOR, wrappedResolverSymbol);
+                     """, AWS_ENDPOINT_RESOLVER_HELPER,
+                endpointResolver, endpointResolverWithOptions,
+                AWS_ENDPOINT_RESOLVER_ADAPTOR, wrappedResolverSymbol,
+                resolverInterface, endpointNotFoundError,
+                EndpointResolutionGenerator.RESOLVER_INTERFACE_NAME
+                );
     }
 
     private void generateMiddleware(GoWriter writer) {
@@ -532,9 +515,22 @@ private void generateMiddlewareResolverBody(GoStackStepMiddlewareGenerator g, Go
         w.write("var endpoint $T", SymbolUtils.createValueSymbolBuilder("Endpoint", AwsGoDependency.AWS_CORE)
                 .build());
         w.write("endpoint, err = m.Resolver.ResolveEndpoint(awsmiddleware.GetRegion(ctx), eo)");
-        w.openBlock("if err != nil {", "}", () -> {
-            w.write("return out, metadata, fmt.Errorf(\"failed to resolve service endpoint, %w\", err)");
-        });
+        w.write(
+            """
+                if err != nil {
+                    nf := (&$1T{})
+                    if $2T(err, &nf) {
+                        ctx = $3T(ctx, false)
+                        return next.HandleSerialize(ctx, in)
+                    }
+                    return out, metadata, $4T(\"failed to resolve service endpoint, %w\", err)
+                }
+            """,
+            SymbolUtils.createValueSymbolBuilder("EndpointNotFoundError", AwsGoDependency.AWS_CORE).build(),
+            SymbolUtils.createValueSymbolBuilder("As", SmithyGoDependency.ERRORS).build(),
+            SymbolUtils.createValueSymbolBuilder("SetRequiresLegacyEndpoints", AwsGoDependency.AWS_MIDDLEWARE).build(),
+            SymbolUtils.createValueSymbolBuilder("Errorf", SmithyGoDependency.FMT).build()
+        );
         w.write("");
 
         w.write("req.URL, err = url.Parse(endpoint.URL)");
@@ -625,7 +621,6 @@ private void generatePublicResolverTypes(GoWriter writer) {
             writer.write("return fn(region, options)");
         });
 
-
         // Generate EndpointResolverFromURL helper
         writer.writeDocs(String.format("%s returns an EndpointResolver configured using the provided endpoint url. "
                                        + "By default, the resolved endpoint resolver uses the client region as signing region, and  "

service/s3/internal/customizations/update_endpoint_test.go

@@ -957,6 +957,18 @@ func TestVPC_CustomEndpoint(t *testing.T) {
 			expectedSigningName:   "s3",
 			expectedSigningRegion: "us-west-2",
 		},
+		"custom resolver to v2 fallback": {
+			bucket: "bucketname",
+			options: s3.Options{
+				EndpointResolver: EndpointResolverFunc(func(region string, options s3.EndpointResolverOptions) (aws.Endpoint, error) {
+					return aws.Endpoint{}, &aws.EndpointNotFoundError{}
+				}),
+				Region: "us-west-2",
+			},
+			expectedReqURL:        "https://bucketname.s3.us-west-2.amazonaws.com/",
+			expectedSigningName:   "s3",
+			expectedSigningRegion: "us-west-2",
+		},
 		"AccessPoint with custom endpoint url": {
 			bucket: "arn:aws:s3:us-west-2:123456789012:accesspoint:myendpoint",
 			options: s3.Options{