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ndb/cs/dns: make sure never to block the 9p loop when we run out of processes, fail the request instead

--- a/sys/src/cmd/ndb/cs.c

+++ b/sys/src/cmd/ndb/cs.c

@@ -1654,6 +1654,10 @@

 	qunlock(&dblock);

 	slave(host);

+	if(*isslave == 0){

+		qlock(&dblock);

+		return nil;

+	}

 	if(strcmp(ipattr(host), "ip") == 0)

 		t = dnsquery(mntpt, host, "ptr");

--- a/sys/src/cmd/ndb/dnresolve.c

+++ b/sys/src/cmd/ndb/dnresolve.c

@@ -1603,11 +1603,15 @@

 		return Answnone;

 	slave(qp->req);

+

 	/*

 	 * slave might have forked.  if so, the parent process longjmped to

 	 * req->mret; we're usually the child slave, but if there are too

-	 * many children already, we're still the same process.

+	 * many children already, we're still the same process. under no

+	 * circumstances block the 9p loop.

 	 */

+	if(!qp->req->isslave && strcmp(qp->req->from, "9p") == 0)

+		return Answnone;

 	/*

 	 * don't lock before call to slave so only children can block.

@@ -1636,7 +1640,7 @@

 					" dropping this one; no further logging"

 					" of drops", dp->name);

 			}

-			return 0;

+			return Answnone;

 		}

 		++qlp->Ref.ref;

 		qunlock(qlp);