Fix an issue with double forward reference

This commit fixes an issue where resolving a forward reference could
throw a ConcurrentModificationException when the resolution would
need to create another forward reference. For example, when applying
a trait to a shape defined in another file without importing the trait
or using its fully qualified shape ID, as done when applying a prelude
trait.

smithy-model/src/main/java/software/amazon/smithy/model/loader/LoaderVisitor.java

@@ -49,6 +49,7 @@
 import software.amazon.smithy.model.validation.ValidatedResult;
 import software.amazon.smithy.model.validation.ValidationEvent;
 import software.amazon.smithy.model.validation.Validator;
+import software.amazon.smithy.utils.ListUtils;
 import software.amazon.smithy.utils.SmithyBuilder;
 
 /**
@@ -436,7 +437,13 @@ public ValidatedResult<Model> onEnd() {
 
     private void finalizeShapeTargets() {
         // Run any finalizers used for things like forward reference resolution.
-        for (ForwardReferenceResolver resolver : forwardReferenceResolvers) {
+        // Do this through a copy to handle forward references that generate more
+        // forward references, like when a Prelude trait is applied to a shape that
+        // is defined in another file.
+        List<ForwardReferenceResolver> resolvers = ListUtils.copyOf(forwardReferenceResolvers);
+        forwardReferenceResolvers.clear();
+
+        for (ForwardReferenceResolver resolver : resolvers) {
             // First, resolve to a shape in the current namespace if one exists.
             if (!hasDefinedShape(resolver.expectedId)) {
                 // Next resolve to a prelude shape if one exists and is public.
@@ -450,7 +457,10 @@ private void finalizeShapeTargets() {
             resolver.consumer.accept(resolver.expectedId);
         }
 
-        forwardReferenceResolvers.clear();
+        // Resolve any new forward references created.
+        if (!forwardReferenceResolvers.isEmpty()) {
+            finalizeShapeTargets();
+        }
     }
 
     private void finalizePendingTraits() {

smithy-model/src/test/resources/software/amazon/smithy/model/loader/apply-use-1.smithy

@@ -7,3 +7,9 @@ apply Foo @bar("hi")
 
 // This applies smithy.example.b#baz to smithy.example#Foo
 apply Foo @baz("hi")
+
+// This applies smithy.api#deprecated to smithy.example#Foo
+apply Foo @deprecated
+
+// This applies smithy.api#sensitive to smithy.example#Foo
+apply Foo @smithy.api#sensitive