Remove support for Alternator DescribeEndpoints request

[nyh]

The short story is that although Alternator has some rudimentary support for the DescribeEndpoints request, I think it should be removed completely - or alternatively be made optional: A configuration option determines what value DescribeEndpoints should return, and if the configuration is unset, this request won't be supported.

It turns out that DynamoDB Local also doesn't support this request, so I think Alternator shouldn't either.

The long story is as follows:

We first saw this problem in the context of the new AWS DynamoDB Shell , which is written using the AWS C++ SDK, but the same problem may affect any application written with the AWS C++ SDK:

When you ask the C++ SDK to use a specific endpoint URL, e.g., http://localhost:8000, it turns out that the C++ SDK doesn't use this endpoint URL immediately, but rather on the first request it tries to run a DescribeEndpoints request on this URL, to possibly get a better endpoint. The relevant code in the C++ SDK source code looks like this (it's repeated many times in https://github.com/aws/aws-sdk-cpp/blob/main/generated/src/aws-cpp-sdk-dynamodb/source/DynamoDBClient.cpp):

      DescribeEndpointsRequest endpointRequest;
      auto endpointOutcome = DescribeEndpoints(endpointRequest);
      if (endpointOutcome.IsSuccess() && !endpointOutcome.GetResult().GetEndpoi
nts().empty())
      {
        const auto& item = endpointOutcome.GetResult().GetEndpoints()[0];
        ...
        endpoint = Aws::String(SchemeMapper::ToString(m_clientConfiguration.scheme)) + "://" + item.GetAddress();

The problem in this code is that no matter what the original endpoint URL was (in my case http://localhost:8000, the new endpoint built by this code will always use the default scheme ("https"), regardless of which scheme the original URL used. This will make this code try to use https://localhost:8000, which won't work because Alternator was listening on http, not https.

The "original sin" is that DescribeEndpoints is documented to return just the endpoint's "IP address", not a full URL. Although Alternator actually deviates from this and return an address and port - e.g., localhost:8000, it can't return the scheme of the URL, and in any case code such as the above just tacks on "https://" in front of the returned string, which is wrong.

There is no real way to fix this problem - it's a bug in Amazon's documentation (which doesn't say that DescribeEndpoints should return URLs) as well as the SDK implementations (this C++ SDK assumes the return value is not a URL).

The only way I can think of fixing this problem is to simply remove the DescribeEndpoints implementation. It turns out that DynamoDB Local doesn't support this request, so I would assume all official Amazon drivers will know how to handle this case. In the C++ SDK, if DescribeEndpoints doesn't work, the error is ignored and the request is sent to the original URL. In particular, if I comment out DescribeEndpoints handling in Alternator, the dynamodb-shell tool (written in the C++ SDK) starts to work.

Instead outright removing this request, we can also make it configurable - the configuration variable can list a specific string to return from DescribeEndpoints (or if missing, the operation won't be supported). Of course setting this will only make sense if the Scylla installation uses the default scheme (https).

I also reported this issue to the C++ SDK developers: aws/aws-sdk-cpp#2554

[nyh]

I think I want to do the configuration-option solution described above instead of disabling this feature completely. It will allow us to have some reasonable behavior by default (probably just the behavior we have today) plus be able to immediately fix a problem that a user might have with this feature - in the field without code changes.

[nyh]

The bug in the AWS C++ SDK was apparently fixed, aws/aws-sdk-cpp#2554.
The fix they chose is to not call DescribeEndpoints at all when the endpoint was explicitly given.

So the fix we added (a configurable workaround) will not be necessary for new versions of the SDK, which suggests maybe it's not important to backport this fix - users can upgrade their SDK instead of Scylla.

[denesb]

@nyh does this need to be backported?