Fix for parsing string headers

codegen-test/model/rest-xml-extras.smithy

@@ -17,7 +17,8 @@ service RestXmlExtras {
         XmlMapsFlattenedNestedXmlNamespace,
         EnumKeys,
         PrimitiveIntOpXml,
-        ChecksumRequired
+        ChecksumRequired,
+        StringHeader,
     ]
 }
 
@@ -203,3 +204,32 @@ operation ChecksumRequired {
 structure ChecksumRequiredInput {
     field: String
 }
+
+
+@httpResponseTests([{
+    id: "DeserHeaderStringCommas",
+    code: 200,
+    documentation: """
+    Regression test for https://github.com/awslabs/aws-sdk-rust/issues/122
+    where `,` was eagerly used to split fields in cases where the input was not
+    a list.
+    """,
+    body: "",
+    headers: { "x-field": "a,b,c" },
+    params: {
+        field: "a,b,c"
+    },
+    protocol: "aws.protocols#restXml"
+}])
+@http(uri: "/StringHeader", method: "POST")
+operation StringHeader {
+    output: StringHeaderOutput
+}
+
+structure StringHeaderOutput {
+    @httpHeader("x-field")
+    field: String,
+
+    @httpHeader("x-enum")
+    enumHeader: StringEnum
+}

codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/http/ResponseBindingGenerator.kt

@@ -212,6 +212,13 @@ class ResponseBindingGenerator(protocolConfig: ProtocolConfig, private val opera
      */
     private fun RustWriter.deserializeFromHeader(targetType: Shape, memberShape: MemberShape) {
         val rustType = symbolProvider.toSymbol(targetType).rustType().stripOuter<RustType.Option>()
+        // Normally, we go through a flow that looks for `,`s but that's wrong if the output
+        // is just a single string (which might include `,`s.).
+        // MediaType doesn't include `,` since it's base64, send that through the normal path
+        if (targetType is StringShape && !targetType.hasTrait<MediaTypeTrait>()) {
+            rust("#T::one_or_none(headers)", headerUtil)
+            return
+        }
         val (coreType, coreShape) = if (targetType is CollectionShape) {
             rustType.stripOuter<RustType.Container>() to model.expectShape(targetType.member.target)
         } else {

rust-runtime/smithy-http/src/header.rs

@@ -73,8 +73,25 @@ where
     Ok(out)
 }
 
+/// Read exactly one or none from a headers iterator
+///
+/// This function does not perform comma splitting like `read_many`
+pub fn one_or_none<T: FromStr>(
+    mut values: ValueIter<HeaderValue>,
+) -> Result<Option<T>, ParseError> {
+    let first = match values.next() {
+        Some(v) => v,
+        None => return Ok(None),
+    };
+    let value = std::str::from_utf8(first.as_bytes()).map_err(|_| ParseError)?;
+    match values.next() {
+        None => T::from_str(value.trim()).map_err(|_| ParseError).map(Some),
+        Some(_) => Err(ParseError),
+    }
+}
+
 /// Read one comma delimited value for `FromStr` types
-pub fn read_one<T>(s: &[u8]) -> Result<(T, &[u8]), ParseError>
+fn read_one<T>(s: &[u8]) -> Result<(T, &[u8]), ParseError>
 where
     T: FromStr,
 {