Split registration and selection of backend poll functions into two

different functions.

I have not managed to write a vtc case for this one, but I am pretty
sure this:

Fixes:	#945

bin/varnishd/cache_backend.c

@@ -430,7 +430,7 @@ VBE_UseHealth(const struct director *vdi)
 	CAST_OBJ_NOTNULL(vs, vdi->priv, VDI_SIMPLE_MAGIC);
 	if (vs->vrt->probe == NULL)
 		return;
-	VBP_Start(vs->backend, vs->vrt->probe, vs->vrt->hosthdr);
+	VBP_Use(vs->backend, vs->vrt->probe);
 }
 
 /*--------------------------------------------------------------------
@@ -475,7 +475,8 @@ vdi_simple_fini(const struct director *d)
 	CHECK_OBJ_NOTNULL(d, DIRECTOR_MAGIC);
 	CAST_OBJ_NOTNULL(vs, d->priv, VDI_SIMPLE_MAGIC);
 
-	VBP_Stop(vs->backend, vs->vrt->probe);
+	if (vs->vrt->probe != NULL)
+		VBP_Remove(vs->backend, vs->vrt->probe);
 	VBE_DropRefVcl(vs->backend);
 	free(vs->dir.vcl_name);
 	vs->dir.magic = 0;
@@ -506,8 +507,8 @@ VRT_init_dir_simple(struct cli *cli, struct director **bp, int idx,
 	vs->vrt = t;
 
 	vs->backend = VBE_AddBackend(cli, t);
-	if (vs->backend->probe == NULL)
-		VBP_Start(vs->backend, vs->vrt->probe, vs->vrt->hosthdr);
+	if (vs->vrt->probe != NULL)
+		VBP_Insert(vs->backend, vs->vrt->probe, vs->vrt->hosthdr);
 
 	bp[idx] = &vs->dir;
 }

bin/varnishd/cache_backend.h

@@ -145,8 +145,9 @@ void VBE_DropRefVcl(struct backend *);
 void VBE_DropRefLocked(struct backend *b);
 
 /* cache_backend_poll.c */
-void VBP_Start(struct backend *b, struct vrt_backend_probe const *p, const char *hosthdr);
-void VBP_Stop(struct backend *b, struct vrt_backend_probe const *p);
+void VBP_Insert(struct backend *b, struct vrt_backend_probe const *p, const char *hosthdr);
+void VBP_Remove(struct backend *b, struct vrt_backend_probe const *p);
+void VBP_Use(struct backend *b, const struct vrt_backend_probe const *p);
 
 /* Init functions for directors */
 typedef void dir_init_f(struct cli *, struct director **, int , const void*);

bin/varnishd/cache_backend_poll.c

@@ -463,11 +463,11 @@ vbp_new_vcl(const struct vrt_backend_probe *p, const char *hosthdr)
 }
 
 /*--------------------------------------------------------------------
- * Start/Stop called from cache_backend.c
+ * Insert/Remove/Use called from cache_backend.c
  */
 
 void
-VBP_Start(struct backend *b, const struct vrt_backend_probe *p, const char *hosthdr)
+VBP_Insert(struct backend *b, const struct vrt_backend_probe *p, const char *hosthdr)
 {
 	struct vbp_target *vt;
 	struct vbp_vcl *vcl;
@@ -475,9 +475,7 @@ VBP_Start(struct backend *b, const struct vrt_backend_probe *p, const char *host
 	unsigned u;
 
 	ASSERT_CLI();
-
-	if (p == NULL)
-		return;
+	AN(p);
 
 	if (b->probe == NULL) {
 		ALLOC_OBJ(vt, VBP_TARGET_MAGIC);
@@ -493,21 +491,12 @@ VBP_Start(struct backend *b, const struct vrt_backend_probe *p, const char *host
 		vt = b->probe;
 	}
 
-	VTAILQ_FOREACH(vcl, &vt->vcls, list) {
-		if (vcl->probep != p)
-			continue;
-
-		AZ(startthread);
-		Lck_Lock(&vbp_mtx);
-		VTAILQ_REMOVE(&vt->vcls, vcl, list);
-		VTAILQ_INSERT_HEAD(&vt->vcls, vcl, list);
-		Lck_Unlock(&vbp_mtx);
-		return;
-	}
+	VTAILQ_FOREACH(vcl, &vt->vcls, list) 
+		assert (vcl->probep != p);
 
 	vcl = vbp_new_vcl(p, hosthdr);
 	Lck_Lock(&vbp_mtx);
-	VTAILQ_INSERT_HEAD(&vt->vcls, vcl, list);
+	VTAILQ_INSERT_TAIL(&vt->vcls, vcl, list);
 	Lck_Unlock(&vbp_mtx);
 
 	if (startthread) {
@@ -521,21 +510,42 @@ VBP_Start(struct backend *b, const struct vrt_backend_probe *p, const char *host
 }
 
 void
-VBP_Stop(struct backend *b, struct vrt_backend_probe const *p)
+VBP_Use(struct backend *b, const struct vrt_backend_probe *p)
 {
 	struct vbp_target *vt;
 	struct vbp_vcl *vcl;
-	void *ret;
 
 	ASSERT_CLI();
+	AN(p);
+	CHECK_OBJ_NOTNULL(b, BACKEND_MAGIC);
+	AN(b->probe);
+	vt = b->probe;
 
-	if (p == NULL)
+	VTAILQ_FOREACH(vcl, &vt->vcls, list) {
+		if (vcl->probep != p)
+			continue;
+
+		Lck_Lock(&vbp_mtx);
+		VTAILQ_REMOVE(&vt->vcls, vcl, list);
+		VTAILQ_INSERT_HEAD(&vt->vcls, vcl, list);
+		Lck_Unlock(&vbp_mtx);
 		return;
+	}
+}
 
-	CHECK_OBJ_NOTNULL(b, BACKEND_MAGIC);
+void
+VBP_Remove(struct backend *b, struct vrt_backend_probe const *p)
+{
+	struct vbp_target *vt;
+	struct vbp_vcl *vcl;
+	void *ret;
 
+	ASSERT_CLI();
+	AN(p);
+	CHECK_OBJ_NOTNULL(b, BACKEND_MAGIC);
 	AN(b->probe);
 	vt = b->probe;
+
 	VTAILQ_FOREACH(vcl, &vt->vcls, list)
 		if (vcl->probep == p)
 			break;