alternator: configurable override for DescribeEndpoints

The AWS C++ SDK has a bug (aws/aws-sdk-cpp#2554)
where even if a user specifies a specific enpoint URL, the SDK uses
DescribeEndpoints to try to "refresh" the endpoint. The problem is that
DescribeEndpoints can't return a scheme (http or https) and the SDK
arbitrarily picks https - making it unable to communicate with Alternator
over http. As an example, the new "dynamodb shell" (written in C++)
cannot communicate with Alternator running over http.

This patch adds a configuration option, "alternator_describe_endpoints",
which can be used to override what DescribeEndpoints does:

1. Empty string (the default) leaves the current behavior -
   DescribeEndpoints echos the request's "Host" header.

2. The string "disabled" disables the DescribeEndpoints (it will return
   an UnknownOperationException). This is how DynamoDB Local behaves,
   and the AWS C++ SDK and the Dynamodb Shell work well in this mode.

3. Any other string is a fixed string to be returned by DescribeEndpoints.
   It can be useful in setups that should return a known address.

Note that this patch does not, by default, change the current behaivor
of DescribeEndpoints. But it us the future to override its behavior
in a user experiences problems in the field - without code changes.

Fixes scylladb#14410.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

alternator/executor.cc

@@ -4360,6 +4360,17 @@ future<executor::request_return_type> executor::list_tables(client_state& client
 
 future<executor::request_return_type> executor::describe_endpoints(client_state& client_state, service_permit permit, rjson::value request, std::string host_header) {
     _stats.api_operations.describe_endpoints++;
+    // The alternator_describe_endpoints configuration can be used to disable
+    // the DescribeEndpoints operation, or set it to return a fixed string
+    std::string override = _proxy.data_dictionary().get_config().alternator_describe_endpoints();
+    if (!override.empty()) {
+        if (override == "disabled") {
+            _stats.unsupported_operations++;
+            return make_ready_future<request_return_type>(api_error::unknown_operation(
+                "DescribeEndpoints disabled by configuration (alternator_describe_endpoints=disabled)"));
+        }
+        host_header = std::move(override);
+    }
     rjson::value response = rjson::empty_object();
     // Without having any configuration parameter to say otherwise, we tell
     // the user to return to the same endpoint they used to reach us. The only

db/config.cc

@@ -954,6 +954,13 @@ db::config::config(std::shared_ptr<db::extensions> exts)
     , alternator_ttl_period_in_seconds(this, "alternator_ttl_period_in_seconds", value_status::Used,
         60*60*24,
         "The default period for Alternator's expiration scan. Alternator attempts to scan every table within that period.")
+    , alternator_describe_endpoints(this, "alternator_describe_endpoints", liveness::LiveUpdate, value_status::Used,
+        "",
+        "Overrides the behavior of Alternator's DescribeEndpoints operation. "
+        "An empty value (the default) means DescribeEndpoints will return "
+        "the same endpoint used in the request. The string 'disabled' "
+        "disables the DescribeEndpoints operation. Any other string is the "
+        "fixed value that will be returned by DescribeEndpoints operations.")
     , abort_on_ebadf(this, "abort_on_ebadf", value_status::Used, true, "Abort the server on incorrect file descriptor access. Throws exception when disabled.")
     , redis_port(this, "redis_port", value_status::Used, 0, "Port on which the REDIS transport listens for clients.")
     , redis_ssl_port(this, "redis_ssl_port", value_status::Used, 0, "Port on which the REDIS TLS native transport listens for clients.")

db/config.hh

@@ -392,6 +392,7 @@ public:
     named_value<uint32_t> alternator_streams_time_window_s;
     named_value<uint32_t> alternator_timeout_in_ms;
     named_value<double> alternator_ttl_period_in_seconds;
+    named_value<sstring> alternator_describe_endpoints;
 
     named_value<bool> abort_on_ebadf;