codegen: Limit SDKs API payload ignoring trait to unmodeled request i…

…nputs (#4139)

Updates the SDK's protocol serialization code generation to only ignore
request API payloads for operations without modeled payload input
parameters. The SDKs protocol deserializers did not correctly handle
this value and caused unexpected close_wait states on the HTTP
connection because the response body was not drained.

CHANGELOG_PENDING.md

@@ -3,3 +3,5 @@
 ### SDK Enhancements
 
 ### SDK Bugs
+* `codegen`: Fix SDK not draining HTTP response body for operations without modeled output parameters.
+  * Fixes a bug in the SDK that was causing HTTP response bodies to not be drained, preventing the connection being reused in the connection pool. For some operations this could contributed to an increase in `close_wait` connection states.

private/model/api/shape.go

@@ -535,7 +535,7 @@ func (ref *ShapeRef) GoTags(toplevel bool, isRequired bool) string {
 		if name := ref.Shape.PayloadRefName(); len(name) > 0 {
 			tags = append(tags, ShapeTag{"payload", name})
 		}
-		if !ref.Shape.HasPayloadMembers() && ref.API.Metadata.Protocol == "rest-json" {
+		if ref.Shape.UsedAsInput && !ref.Shape.HasPayloadMembers() && ref.API.Metadata.Protocol == "rest-json" {
 			tags = append(tags, ShapeTag{"nopayload", "true"})
 		}
 	}