Lazy initialize mime accept header (#2629)

## Description

Do not parse and initialize the mime, known at compile time, on every
request.

See
#2607 (comment)


## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/protocols/ServerHttpBoundProtocolGenerator.kt

@@ -178,18 +178,36 @@ class ServerHttpBoundProtocolTraitImplGenerator(
         outputSymbol: Symbol,
         operationShape: OperationShape,
     ) {
+        val operationName = symbolProvider.toSymbol(operationShape).name
+        val staticContentType = "CONTENT_TYPE_${operationName.uppercase()}"
         val verifyAcceptHeader = writable {
             httpBindingResolver.responseContentType(operationShape)?.also { contentType ->
                 rustTemplate(
                     """
-                    if !#{SmithyHttpServer}::protocols::accept_header_classifier(request.headers(), ${contentType.dq()}) {
+                    if !#{SmithyHttpServer}::protocols::accept_header_classifier(request.headers(), &$staticContentType) {
                         return Err(#{RequestRejection}::NotAcceptable);
                     }
                     """,
                     *codegenScope,
                 )
             }
         }
+        val verifyAcceptHeaderStaticContentTypeInit = writable {
+            httpBindingResolver.responseContentType(operationShape)?.also { contentType ->
+                val init = when (contentType) {
+                    "application/json" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_JSON;"
+                    "application/octet-stream" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_OCTET_STREAM;"
+                    "application/x-www-form-urlencoded" -> "const $staticContentType: #{Mime}::Mime = #{Mime}::APPLICATION_WWW_FORM_URLENCODED;"
+                    else ->
+                        """
+                    static $staticContentType: #{OnceCell}::sync::Lazy<#{Mime}::Mime> = #{OnceCell}::sync::Lazy::new(|| {
+                        ${contentType.dq()}.parse::<#{Mime}::Mime>().expect("BUG: MIME parsing failed, content_type is not valid")
+                    });
+                    """
+                }
+                rustTemplate(init, *codegenScope)
+            }
+        }
         val verifyRequestContentTypeHeader = writable {
             operationShape
                 .inputShape(model)
@@ -215,6 +233,7 @@ class ServerHttpBoundProtocolTraitImplGenerator(
         // TODO(https://github.com/awslabs/smithy-rs/issues/2238): Remove the `Pin<Box<dyn Future>>` and replace with thin wrapper around `Collect`.
         rustTemplate(
             """
+            #{verifyAcceptHeaderStaticContentTypeInit:W}
             #{PinProjectLite}::pin_project! {
                 /// A [`Future`](std::future::Future) aggregating the body bytes of a [`Request`] and constructing the
                 /// [`${inputSymbol.name}`](#{I}) using modelled bindings.
@@ -267,6 +286,7 @@ class ServerHttpBoundProtocolTraitImplGenerator(
             "Marker" to protocol.markerStruct(),
             "parse_request" to serverParseRequest(operationShape),
             "verifyAcceptHeader" to verifyAcceptHeader,
+            "verifyAcceptHeaderStaticContentTypeInit" to verifyAcceptHeaderStaticContentTypeInit,
             "verifyRequestContentTypeHeader" to verifyRequestContentTypeHeader,
         )
 

rust-runtime/aws-smithy-http-server/src/protocols.rs

@@ -66,14 +66,10 @@ pub fn content_type_header_classifier(
     Ok(())
 }
 
-pub fn accept_header_classifier(headers: &HeaderMap, content_type: &'static str) -> bool {
+pub fn accept_header_classifier(headers: &HeaderMap, content_type: &mime::Mime) -> bool {
     if !headers.contains_key(http::header::ACCEPT) {
         return true;
     }
-    // Must be of the form: type/subtype
-    let content_type = content_type
-        .parse::<mime::Mime>()
-        .expect("BUG: MIME parsing failed, content_type is not valid");
     headers
         .get_all(http::header::ACCEPT)
         .into_iter()
@@ -195,41 +191,62 @@ mod tests {
     #[test]
     fn valid_accept_header_classifier_multiple_values() {
         let valid_request = req_accept("text/strings, application/json, invalid");
-        assert!(accept_header_classifier(&valid_request, "application/json"));
+        assert!(accept_header_classifier(
+            &valid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn invalid_accept_header_classifier() {
         let invalid_request = req_accept("text/invalid, invalid, invalid/invalid");
-        assert!(!accept_header_classifier(&invalid_request, "application/json"));
+        assert!(!accept_header_classifier(
+            &invalid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn valid_accept_header_classifier_star() {
         let valid_request = req_accept("application/*");
-        assert!(accept_header_classifier(&valid_request, "application/json"));
+        assert!(accept_header_classifier(
+            &valid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn valid_accept_header_classifier_star_star() {
         let valid_request = req_accept("*/*");
-        assert!(accept_header_classifier(&valid_request, "application/json"));
+        assert!(accept_header_classifier(
+            &valid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn valid_empty_accept_header_classifier() {
-        assert!(accept_header_classifier(&HeaderMap::new(), "application/json"));
+        assert!(accept_header_classifier(
+            &HeaderMap::new(),
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn valid_accept_header_classifier_with_params() {
         let valid_request = req_accept("application/json; q=30, */*");
-        assert!(accept_header_classifier(&valid_request, "application/json"));
+        assert!(accept_header_classifier(
+            &valid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 
     #[test]
     fn valid_accept_header_classifier() {
         let valid_request = req_accept("application/json");
-        assert!(accept_header_classifier(&valid_request, "application/json"));
+        assert!(accept_header_classifier(
+            &valid_request,
+            &"application/json".parse().unwrap()
+        ));
     }
 }