Some TCP error conditions are handled incorrectly

In lib/dns/dispatch.c:tcp_recv(), the default case in the second switch statement is overly broad and causes some result codes provided by netmgr to be incorrectly overridden with ISC_R_SHUTTINGDOWN. This enables e.g. shutting down resolving ... messages to be logged in the middle of normal resolver operation (which is confusing as such messages are only supposed to be logged when the netmgr is being shut down entirely) and causes some error conditions to be mishandled by resolver code (resquery_response()).

As an example, consider what happens if tcp_recv() gets invoked with the ISC_R_EOF result code:

1. The first switch statement calls tcp_recv_shutdown() to prepare the dispatch for cleanup.

2. The result variable remains untouched.

3. The second switch statement matches ISC_R_EOF to the default case.

4. tcp_recv_cancelall() gets called, which causes the higher-level dispatch callback (resquery_response()) to be invoked with the ISC_R_SHUTTINGDOWN result code.

5. Instead of returning early (which should happen for ISC_R_EOF), resquery_response() continues to process the ISC_R_SHUTTINGDOWN result code, eventually reaching rctx_dispfail() and marking the server as bad.

At least two other error conditions (ISC_R_CANCELED and ISC_R_CONNECTIONRESET) can be similarly overridden with ISC_R_SHUTTINGDOWN.

AIUI, the original netmgr error code should be propagated to the resolver code so that it can be handled accordingly. Perhaps something like this could work?

diff --git a/lib/dns/dispatch.c b/lib/dns/dispatch.c
index c13d91d0642..81e2f0658d7 100644
--- a/lib/dns/dispatch.c
+++ b/lib/dns/dispatch.c
@@ -713,13 +713,14 @@ tcp_recv_done(dns_dispentry_t *resp, isc_result_t eresult,
 }
 
 static void
-tcp_recv_cancelall(dns_displist_t *resps, isc_region_t *region) {
+tcp_recv_cancelall(dns_displist_t *resps, isc_region_t *region,
+		   isc_result_t result) {
 	dns_dispentry_t *resp = NULL, *next = NULL;
 
 	for (resp = ISC_LIST_HEAD(*resps); resp != NULL; resp = next) {
 		next = ISC_LIST_NEXT(resp, rlink);
 		ISC_LIST_UNLINK(*resps, resp, rlink);
-		resp->response(ISC_R_SHUTTINGDOWN, region, resp->arg);
+		resp->response(result, region, resp->arg);
 		dispentry_detach(&resp);
 	}
 }
@@ -831,7 +832,7 @@ tcp_recv(isc_nmhandle_t *handle, isc_result_t result, isc_region_t *region,
 		break;
 	default:
 		/* We're being shut down; cancel all outstanding resps. */
-		tcp_recv_cancelall(&resps, region);
+		tcp_recv_cancelall(&resps, region, result);
 	}
 
 	dns_dispatch_detach(&disp);