fix: Fix MD5 middleware codegen & runtime code to match spec

Sources/ClientRuntime/Networking/Http/Middlewares/ContentMD5Middleware.swift

@@ -15,8 +15,24 @@ public struct ContentMD5Middleware<OperationStackInput, OperationStackOutput> {
     public init() {}
 
     private func addHeaders(builder: HTTPRequestBuilder, attributes: Context) async throws {
-        // Skip MD5 hash if using checksums
-        if builder.headers.exists(name: "x-amz-sdk-checksum-algorithm") {
+        // Initialize logger
+        guard let logger = attributes.getLogger() else {
+            throw ClientError.unknownError("No logger found!")
+        }
+
+        // Skip MD5 hash if using flexible checksum
+        if builder.headers.headers.contains(where: {
+            $0.name.lowercased().starts(with: "x-amz-checksum-")
+        }) {
+            logger.debug("Flexible checksum configured. Skipping MD5 checksum calculation.")
+            return
+        }
+
+        // Skip MD5 hash if it was provided in input by the user
+        if builder.headers.headers.contains(where: {
+            $0.name.lowercased() == "content-md5"
+        }) {
+            logger.debug("MD5 checksum hash provided in the input. Skipping MD5 checksum calculation.")
             return
         }
 
@@ -42,15 +58,9 @@ public struct ContentMD5Middleware<OperationStackInput, OperationStackOutput> {
                 let hashResult = try md5Hasher.digest().toBase64String()
                 builder.updateHeader(name: "Content-MD5", value: hashResult)
             } catch {
-                guard let logger = attributes.getLogger() else {
-                    return
-                }
                 logger.error("Could not compute Content-MD5 of stream due to error \(error)")
             }
         default:
-            guard let logger = attributes.getLogger() else {
-                return
-            }
             logger.error("Unhandled case for Content-MD5")
         }
     }

smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/ContentMD5Middleware.kt

@@ -38,8 +38,18 @@ class ContentMD5Middleware(
 
 /**
  * Check if MD5 checksum is required
- * The Md5 middleware will only be installed if the operation requires a checksum and the user has not opted-in to flexible checksums.
+ * The Md5 middleware will only be generated if the operation is marked with @httpChecksumRequired trait or if
+ *   only the requestChecksumRequired property of the httpChecksum is set.
+ *  https://smithy.io/2.0/aws/aws-core.html#behavior-with-httpchecksumrequired
+ *
+ *  ContentMD5Middleware will skip checksum calculation if flexible checksum was calculated for request already,
+ *    determined by presence of a header with "x-amz-checksum-" prefix.
  */
-private fun OperationShape.isMD5ChecksumRequired(): Boolean =
-    // the checksum requirement can be modeled in either HttpChecksumTrait's `requestChecksumRequired` or the HttpChecksumRequired trait
-    getTrait<HttpChecksumTrait>()?.isRequestChecksumRequired == true || hasTrait<HttpChecksumRequiredTrait>()
+private fun OperationShape.isMD5ChecksumRequired(): Boolean {
+    val httpChecksumTrait = getTrait<HttpChecksumTrait>()
+    val onlyRequestChecksumRequiredIsSetInHttpChecksumTrait = httpChecksumTrait?.isRequestChecksumRequired == true &&
+        httpChecksumTrait.requestAlgorithmMember?.isEmpty == true &&
+        httpChecksumTrait.requestValidationModeMember?.isEmpty == true &&
+        httpChecksumTrait.responseAlgorithms?.isEmpty() == true
+    return onlyRequestChecksumRequiredIsSetInHttpChecksumTrait || hasTrait<HttpChecksumRequiredTrait>()
+}