Confusion with HTTP_TUNNEL_STATIC_PRODUCER

In the HttpTunnel processing, normally a producer has a VC associated with it. The VC is used to lookup the producer via the HttpTunnel::get_producer method out of the HttpTunnel producer array.

All is well and good, but in the case of a static producer, there is no vc. Rather the constant HTTP_TUNNEL_STATIC_PRODUCER is used in lieu of the vc. If there is only only one static producer all is still well, get_producer will return the one producer associated with HTTP_TUNNEL_STATIC_PRODUCER. But if there is more than one static producer involved with the tunnel, get_producer() will only return one producer. Both static consumers will only interact with one of the static producers and things will go down hill from here.

I ran into this case while chasing down crashes via TS-3105, so this situation does come up in the wild.

I fixed it by clearing out the tunnel before starting a static producer and making some other checks along the way.

We could also avoid some calls to get_producer since it many cases, the caller already has the producer in question, but the callee ends up looking up that value again via the VC.