The callback from context.alpn_callback would fire when OpenSSL handles the ALPN extension. Since the callback is triggered in the ClientHello phase of the handshake, you'll be able to replace the socket's context with another context.

The OpenSSL codes work a bit like this:

TLSEXT_INDEX = [
    ...,
    TLSEXT_IDX_server_name,
    ...,
    TLSEXT_IDX_application_layer_protocol_negotiation,
]

for extid in TLSEXT_INDEX:
    if client.has_extension(extid):
        ext = do_stuff(client, extid)
        ext.callback(client)

Any of the callbacks is able to replace the context.

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