Improve reuse of outgoing TCP connections
The dns_dispatch_gettcp() function is used for finding an existing TCP
connection that can be reused for sending a query from a specified local
address to a specified remote address. The logic for matching the
provided <local address, remote address> tuple to one of the existing
TCP connections is implemented in the dispatch_match() function:
-
if the examined TCP connection already has a libuv handle assigned, it means the connection has already been established; therefore, compare the provided
<local address, remote address>tuple against the corresponding address tuple for the libuv handle associated with the connection, -
if the examined TCP connection does not yet have a libuv handle assigned, it means the connection has not yet been established; therefore, compare the provided
<local address, remote address>tuple against the corresponding address tuple that the TCP connection was originally created for.
This logic limits TCP connection reuse potential as the libuv handle
assigned to an existing dispatch object may have a more specific local
<address, port> tuple associated with it than the local <address, port> tuple that the dispatch object was originally created for.
That's because the local address for outgoing connections can be set to
a wildcard <address, port> tuple (indicating that the caller does not
care which source <address, port> tuple will be used for establishing
the connection, thereby delegating the task of picking it to the
operating system) and then get "upgraded" to a specific <address, port> tuple when the socket is bound (and a libuv handle gets
associated with it). When another dns_dispatch_gettcp() caller then
tries to look for an existing TCP connection to the same peer and passes
a wildcard address in the local part of the tuple, the function will not
match that request to a previously-established TCP connection (unless
isc_nmhandle_localaddr() returns a wildcard address as well).
See !8348 for prior discussion.