There is a connection leak in the THttpClient when using the Apache HttpClient with the PoolingClientConnectionManager. Without calling releaseConnection on the HttpPost object, the connections are never returned to the pool. Under heavy load, this can lead to both failures for subsequent calls to be able to get a connection from the pool and connections being held by the underlying OS, eventually resulting in the inability to grab another client port for outgoing connections. Per the Apache HttpClient examples/documentation:
"In order to ensure correct deallocation of system resources
the user MUST either fully consume the response content or abort request
execution by calling HttpGet#releaseConnection()."
This might have not been an issue when using the 3.x version of the HttpClient, but it's definitely an issue in the 4.x line. See https://hc.apache.org/httpcomponents-client-4.2.x/quickstart.html for more details.