Move call decorator to interceptors

Instead of a separate CallDecorator abstraction, unify call interception
into ClientInterceptors using a new interceptCall hook and
interceptCalls boolean method. If interceptCalls returns true, then
the interceptors is used in a chain of middleware-style interceptors
that can completely intercept control flow of the call. This can be
used for things like caching, hedging, etc.

A tradeoff of making this an interceptor hook vs a dedicated abstraction
is that the Client, if needed, is not generic and instead stashed in
Context. The cost for making calls is essentially the same when nothing
intercepts. The cost of actually intercepting is essentially the same
(one pre-resolves the chain in a nested call stack, this is iterative).
The advantages are that there's a single abstraction and composition
is the same as before.

Author: mtdowling

client/client-core/src/main/java/software/amazon/smithy/java/client/core/CallDecorator.java

@@ -17,6 +17,7 @@
 import software.amazon.smithy.java.client.core.auth.scheme.AuthSchemeResolver;
 import software.amazon.smithy.java.client.core.interceptors.CallHook;
 import software.amazon.smithy.java.client.core.interceptors.ClientInterceptor;
+import software.amazon.smithy.java.client.core.interceptors.InputHook;
 import software.amazon.smithy.java.client.core.plugins.AutoPlugin;
 import software.amazon.smithy.java.context.Context;
 import software.amazon.smithy.java.core.schema.ApiOperation;
@@ -41,7 +42,6 @@ public abstract class Client implements Closeable {
     private final ClientInterceptor interceptor;
     private final IdentityResolvers identityResolvers;
     private final RetryStrategy retryStrategy;
-    private static final CallDecorator.Invoker CALL_INVOKER = Client::sendCall;
 
     protected Client(Builder<?, ?> builder) {
         ClientConfig.Builder configBuilder = builder.configBuilder();
@@ -72,7 +72,6 @@ protected Client(Builder<?, ?> builder) {
      * @param <O>         Output shape.
      * @return Returns the deserialized output.
      */
-    @SuppressWarnings("unchecked")
     protected <I extends SerializableStruct, O extends SerializableStruct> O call(
             I input,
             ApiOperation<I, O> operation,
@@ -115,15 +114,21 @@ protected <I extends SerializableStruct, O extends SerializableStruct> O call(
                 TypeRegistry.compose(operation.errorRegistry(), typeRegistry),
                 retryStrategy);
 
-        // Resolve the decorator from the (possibly modifyBeforeCall-mutated) config so an
-        // interceptor can install or replace it.
-        var decorator = (CallDecorator<Client>) callConfig.callDecorator();
-        return decorator != null ? decorator.apply(this, call, CALL_INVOKER) : sendCall(call);
-    }
+        // Make the running client available to interceptors that need to re-enter.
+        call.context.put(ClientContext.CLIENT, this);
+
+        if (!callInterceptor.interceptCalls()) {
+            return callPipeline.send(call);
+        }
 
-    private static <I extends SerializableStruct, O extends SerializableStruct> O sendCall(ClientCallView<I, O> call) {
-        ClientCall<I, O> impl = (ClientCall<I, O>) call;
-        return impl.pipeline.send(impl);
+        // Build an InputHook for interceptCall and a terminal Invoker that sends the
+        // (possibly input-substituted) call through the pipeline.
+        InputHook<I, O> hook = new InputHook<>(operation, call.context, input);
+        return callInterceptor.interceptCall(hook, h -> {
+            return h.input() == call.input
+                    ? call.pipeline.send(call)
+                    : call.pipeline.send(new ClientCall<>(call, h.input()));
+        });
     }
 
     /**
@@ -422,19 +427,6 @@ public boolean test(ClientPlugin plugin) {
             }.and(configBuilder.pluginPredicate()));
         }
 
-        /**
-         * Adds a decorator that wraps client call execution. Multiple decorators compose: the first
-         * added is the outermost wrapper.
-         *
-         * @param callDecorator the call decorator to add.
-         * @return the builder.
-         */
-        @SuppressWarnings("unchecked")
-        public B addCallDecorator(CallDecorator<I> callDecorator) {
-            configBuilder.addCallDecorator(callDecorator);
-            return (B) this;
-        }
-
         /**
          * Creates the client.
          *

client/client-core/src/main/java/software/amazon/smithy/java/client/core/Client.java

@@ -32,7 +32,7 @@
  * @param <I> Input to send.
  * @param <O> Output to return.
  */
-final class ClientCall<I extends SerializableStruct, O extends SerializableStruct> implements ClientCallView<I, O> {
+final class ClientCall<I extends SerializableStruct, O extends SerializableStruct> {
 
     final I input;
     final ApiOperation<I, O> operation;
@@ -91,7 +91,7 @@ final class ClientCall<I extends SerializableStruct, O extends SerializableStruc
      * resolved configuration. Retry state is shared too — decorators run before the retry loop starts,
      * so {@code attemptCount}/{@code retryToken} are at their initial values either way.
      */
-    private ClientCall(ClientCall<I, O> source, I newInput) {
+    ClientCall(ClientCall<I, O> source, I newInput) {
         this.input = Objects.requireNonNull(newInput, "input is null");
         this.operation = source.operation;
         this.context = source.context;
@@ -109,51 +109,6 @@ private ClientCall(ClientCall<I, O> source, I newInput) {
                 : null;
     }
 
-    @Override
-    public I input() {
-        return input;
-    }
-
-    @Override
-    public ApiOperation<I, O> operation() {
-        return operation;
-    }
-
-    @Override
-    public Context context() {
-        return context;
-    }
-
-    @Override
-    public EndpointResolver endpointResolver() {
-        return endpointResolver;
-    }
-
-    @Override
-    public TypeRegistry typeRegistry() {
-        return typeRegistry;
-    }
-
-    @Override
-    public AuthSchemeResolver authSchemeResolver() {
-        return authSchemeResolver;
-    }
-
-    @Override
-    public Map<ShapeId, AuthScheme<?, ?>> supportedAuthSchemes() {
-        return supportedAuthSchemes;
-    }
-
-    @Override
-    public IdentityResolvers identityResolvers() {
-        return identityResolvers;
-    }
-
-    @Override
-    public ClientCallView<I, O> withInput(I newInput) {
-        return new ClientCall<>(this, newInput);
-    }
-
     /**
      * Check if a retry is disallowed for this call.
      *

client/client-core/src/main/java/software/amazon/smithy/java/client/core/ClientCall.java

@@ -53,7 +53,6 @@ public final class ClientConfig {
     private final RetryStrategy retryStrategy;
     private final String retryScope;
     private final Set<Class<? extends ClientPlugin>> appliedPluginClasses;
-    private final CallDecorator<?> callDecorator;
 
     private ClientConfig(Builder builder) {
         // Collect and apply plugins, updating builder.appliedPluginClasses as we go
@@ -96,7 +95,6 @@ private ClientConfig(Builder builder) {
 
         this.context = Context.unmodifiableCopy(builder.context);
         this.service = Objects.requireNonNull(builder.service, "Missing required service schema");
-        this.callDecorator = builder.callDecorator;
     }
 
     private static List<ClientPlugin> collectPlugins(
@@ -225,10 +223,6 @@ String retryScope() {
         return retryScope;
     }
 
-    CallDecorator<?> callDecorator() {
-        return callDecorator;
-    }
-
     /**
      * Create a new builder to build {@link ClientConfig}.
      *
@@ -328,7 +322,6 @@ public static final class Builder {
         private final Map<Class<? extends ClientPlugin>, ClientPlugin> plugins = new LinkedHashMap<>();
         // Mutable set that tracks which plugin classes have been applied to this builder
         private final Set<Class<? extends ClientPlugin>> appliedPluginClasses = new HashSet<>();
-        private CallDecorator<?> callDecorator;
 
         public Builder() {
             plugins.put(DefaultPlugin.class, DefaultPlugin.INSTANCE);
@@ -350,7 +343,6 @@ private Builder copyBuilder() {
             builder.plugins.putAll(plugins);
             builder.pluginPredicate = pluginPredicate;
             builder.appliedPluginClasses.addAll(appliedPluginClasses);
-            builder.callDecorator = callDecorator;
             return builder;
         }
 
@@ -667,20 +659,6 @@ public Builder addPluginPredicate(Predicate<ClientPlugin> pluginPredicate) {
             return pluginPredicate(this.pluginPredicate.and(pluginPredicate));
         }
 
-        /**
-         * Adds a decorator that wraps client call execution. Multiple decorators compose: the first
-         * added is the outermost wrapper.
-         *
-         * @param callDecorator the call decorator to add.
-         * @return the builder.
-         */
-        @SuppressWarnings({"unchecked", "rawtypes"})
-        public Builder addCallDecorator(CallDecorator<?> callDecorator) {
-            this.callDecorator = CallDecorator.chain((CallDecorator) this.callDecorator,
-                    (CallDecorator) callDecorator);
-            return this;
-        }
-
         /**
          * Get the plugin predicate of the builder.
          *