If a query sent using the dns_request API times out when the view it was
associated with gets torn down, the dns_dispatch_resume() call in
req_response() may be issued with the 'resp' argument set to NULL,
triggering an assertion failure. Consider the following scenario ([A]
and [B] are thread identifiers):
1. [A] Read timeout for a dispatch query fires.
2. [A] udp_recv() is called. It locks the dispatch, determines it
timed out, prepares for calling the higher-level callback with
ISC_R_TIMEDOUT, and unlocks the dispatch (lib/dns/dispatch.c:633).
3. [B] The last reference to a view is released.
dns_requestmgr_shutdown() is called, canceling all in-flight
requests for that view. (Note that udp_recv() in thread [A] already
unlocked the dispatch, so its state can be modified.) As a part of
this process, request_cancel() calls dns_dispatch_done() on
request->dispentry, setting it to NULL.
4. [A] udp_recv() calls the higher-level callback (req_response()) with
ISC_R_TIMEDOUT.
5. [A] Since the request timed out, req_response() retries sending it.
In the process, it calls dns_dispatch_resume(), passing
request->dispentry as the 'resp' argument.
6. [A] Since 'resp' is NULL, the REQUIRE(VALID_RESPONSE(resp));
assertion in dns_dispatch_resume() fails.
Fix by checking whether the request has been canceled before calling
dns_dispatch_resume(), similarly to how it is done in req_connected()
and req_senddone().
df346b0088