S3 HeadObject fails to deserialize 404s

[jdisanti]

I encountered this on both v0.0.5-alpha and v0.0.6-alpha.

The S3 HeadObject operation fails with the following when an object doesn't exist:

Error parsing XML: no root element

Trace:

Jun 02 21:05:57.853 TRACE smithy_http_tower::dispatch: request=Request { method: HEAD, uri: https://s3.us-west-2.amazonaws.com/test-bucket-4d39b6e4-9e3d-4496-a8c8-a6a4ceea620e/test-wedger-s3/893faf94bac34607b188e8598507865d01b5900b562d4ac3e53e53691f32c743.gz.encrypted, version: HTTP/1.1, headers: {"content-type": "application/xml", "content-length": "0", "host": "s3.us-west-2.amazonaws.com", "user-agent": "aws-sdk-rust/0.1.0 os/linux lang/rust/1.52.1", "x-amz-user-agent": "aws-sdk-rust/0.1.0 api/s3/0.0.5-alpha os/linux lang/rust/1.52.1", "authorization": Sensitive, "x-amz-date": "20210603T040557Z", "x-amz-content-sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"}, body: SdkBody { inner: Once(Some(b"")), retryable: true } }

Jun 02 21:05:58.073 TRACE smithy_http::middleware: response=Response { 
  status: 404, version: HTTP/1.1, 
  headers: {
     "x-amz-request-id": "GRZ6BZ468DF52F2E", "x-amz-id-2": 
     "UTniwu6QmCIjVeuK2ZfeWBOnu7SqMQOS3Vac6B/K4H2ZCawYUl+nDbhGTImuyhZ5DFiojR3Kcz4=", 
     "content-type": "application/xml", 
     "date": "Thu, 03 Jun 2021 04:05:52 GMT", 
     "server": "AmazonS3"
   }, 
   body: SdkBody { inner: Streaming(Body(Empty)), retryable: false } 
}

It looks like the generated code is expecting a body when S3 doesn't respond with one:
https://github.com/awslabs/aws-sdk-rust/blob/main/sdk/s3/src/operation_deser.rs#L1916

[rcoh]

oh good catch. I think our empty body logic for XML must be wrong.

[rcoh]

this is actually a bit more subtle, it happens because S3 is returning an empty 404 error and I'm not sure how the SDK is supposed to know that's going to happen