CurlHtttpClient does not fire HeadersReceivedEventHandler on empty body

[sbaluja]

Describe the bug

The HeadersReceivedEventHandler is invoked in the WriteData callback, and therefore it is only invoked if the response body bytes > 0. This is incorrect, and the CrtHttpClient fires the callback regardless of response body size.

// CurlHttpClient.cpp line 197-200
auto& headersHandler = context->m_request->GetHeadersReceivedEventHandler();
if (context->m_numBytesResponseReceived == 0 && headersHandler)
{
    headersHandler(context->m_request, context->m_response);
}

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

The HeadersReceivedEventHandler is invoked regardless of body size

Current Behavior

The HeadersReceivedEventHandler is invoked only if response body size > 0

Reproduction Steps

bool doPutObject(const char* label) {
    std::cout << "\n=== " << label << " ===\n";

    Aws::Client::ClientConfiguration config;
    config.region = REGION;
    Aws::S3::S3Client client(config);

    Aws::S3::Model::PutObjectRequest request;
    request.SetBucket(BUCKET);
    request.SetKey(KEY);

    auto body = Aws::MakeShared<Aws::StringStream>("PutBody");
    *body << "hello";
    request.SetBody(body);

    std::atomic<bool> handlerFired{false};
    request.SetHeadersReceivedEventHandler(
        [&handlerFired](const Aws::Http::HttpRequest*, Aws::Http::HttpResponse* response) {
            handlerFired = true;
            std::cout << "  [HeadersReceivedEventHandler FIRED]\n";
            for (const auto& h : response->GetHeaders()) {
                std::cout << "    " << h.first << ": " << h.second << "\n";
            }
        });

    auto outcome = client.PutObject(request);
    if (outcome.IsSuccess()) {
        std::cout << "  PutObject succeeded. ETag: " << outcome.GetResult().GetETag() << "\n";
    } else {
        std::cout << "  PutObject FAILED: " << outcome.GetError().GetMessage() << "\n";
    }

    std::cout << "  Handler fired: " << (handlerFired.load() ? "YES" : "NO") << "\n";
    return handlerFired.load();
}

Possible Solution

Move to WriteHeader callback

Additional Information/Context

No response

AWS CPP SDK version used

latest (built from source)

Compiler and Version used

clang version 17.0.0

Operating System and version

MacOS