Fix SigV4 Initial Message support

.changelog/1763408225.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["server"]
+authors: ["rcoh"]
+references: ["smithy-rs#4400", "smithy-rs#4397"]
+breaking: true
+new_feature: false
+bug_fix: true
+---
+Fix issue where SigV4 envelopes for EventStreams did not support the initial message. This is _technically_ a breaking change but should not break consumers in practice since the
+resulting type has the same methods.

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/http/HttpBindingGenerator.kt

@@ -96,12 +96,6 @@ sealed class HttpBindingSection(name: String) : Section(name) {
 
     data class AfterDeserializingIntoADateTimeOfHttpHeaders(val memberShape: MemberShape) :
         HttpBindingSection("AfterDeserializingIntoADateTimeOfHttpHeaders")
-
-    data class BeforeCreatingEventStreamReceiver(
-        val operationShape: OperationShape,
-        val unionShape: UnionShape,
-        val unmarshallerVariableName: String,
-    ) : HttpBindingSection("BeforeCreatingEventStreamReceiver")
 }
 
 typealias HttpBindingCustomization = NamedCustomization<HttpBindingSection>
@@ -282,21 +276,11 @@ class HttpBindingGenerator(
             "unmarshallerConstructorFn" to unmarshallerConstructorFn,
         )
 
-        // Allow customizations to wrap the unmarshaller
-        for (customization in customizations) {
-            customization.section(
-                HttpBindingSection.BeforeCreatingEventStreamReceiver(
-                    operationShape,
-                    targetShape,
-                    "unmarshaller",
-                ),
-            )(this)
-        }
-
         rustTemplate(
             """
             let body = std::mem::replace(body, #{SdkBody}::taken());
-            Ok(#{receiver:W})
+            let receiver = #{receiver:W};
+            Ok(receiver)
             """,
             "SdkBody" to RuntimeType.sdkBody(runtimeConfig),
             "receiver" to

codegen-server-test/build.gradle.kts

@@ -116,7 +116,7 @@ val commonCodegenTests = "../codegen-core/common-test-models".let { commonModels
     )
 }
 // When iterating on protocol tests use this to speed up codegen:
-// .filter { it.module == "rpcv2Cbor_extras" || it.module == "rpcv2Cbor_extras_no_initial_response" }
+//    .filter { it.module == "rpcv2Cbor_extras" || it.module == "rpcv2Cbor_extras_no_initial_response" }
 
 val customCodegenTests = "custom-test-models".let { customModels ->
     listOf(

codegen-server-test/integration-tests/eventstreams/Cargo.toml

@@ -18,3 +18,5 @@ aws-smithy-runtime = { workspace = true }
 http-body-util = "0.1.3"
 hyper-util = { version = "0.1.17", features = ["client-legacy", "tokio", "http2", "http1"] }
 tokio-stream = "0.1.17"
+tracing = "0.1.41"
+rstest = "0.23"

codegen-server-test/integration-tests/eventstreams/src/lib.rs

@@ -57,7 +57,7 @@ impl ManualEventStreamClient {
         tokio::spawn(async move {
             while let Some(message) = message_receiver.recv().await {
                 let mut buffer = Vec::new();
-                if let Err(_) = write_message_to(&message, &mut buffer) {
+                if write_message_to(&message, &mut buffer).is_err() {
                     break;
                 }
                 let _ = frame_sender
@@ -131,7 +131,7 @@ impl ManualEventStreamClient {
         self.message_sender
             .send(message)
             .await
-            .map_err(|e| format!("Send failed: {}", e))
+            .map_err(|e| format!("Send failed: {e}"))
     }
 
     /// Receives the next response message.