[master] a469b3de5 round-robin director to consider all backends also when racing

[Nils Goroll]

commit a469b3de517579d6da5f190cd3e853fa6a7d79e8
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Thu Dec 3 15:15:20 2020 +0100

    round-robin director to consider all backends also when racing
    
    When resolve requests race, we were not guaranteed to consider all
    backends because we updated a shared nxt variable.
    
    Fixes #3474

diff --git a/lib/libvmod_directors/round_robin.c b/lib/libvmod_directors/round_robin.c
index 74ec567e0..230bcb02e 100644
--- a/lib/libvmod_directors/round_robin.c
+++ b/lib/libvmod_directors/round_robin.c
@@ -80,14 +80,16 @@ vmod_rr_resolve(VRT_CTX, VCL_BACKEND dir)
 	CHECK_OBJ_NOTNULL(dir, DIRECTOR_MAGIC);
 	CAST_OBJ_NOTNULL(rr, dir->priv, VMOD_DIRECTORS_ROUND_ROBIN_MAGIC);
 	vdir_rdlock(rr->vd);
+	nxt = rr->nxt;
 	for (u = 0; u < rr->vd->n_backend; u++) {
-		nxt = rr->nxt % rr->vd->n_backend;
-		rr->nxt = nxt + 1;
 		be = rr->vd->backend[nxt];
+		nxt++;
+		nxt %= rr->vd->n_backend;
 		CHECK_OBJ_NOTNULL(be, DIRECTOR_MAGIC);
 		if (VRT_Healthy(ctx, be, NULL))
 			break;
 	}
+	rr->nxt = nxt;
 	vdir_unlock(rr->vd);
 	if (u == rr->vd->n_backend)
 		be = NULL;