MINOR: Add nbsrv sample converter

This is like the nbsrv() sample fetch function except that it works as
a converter so it can count the number of available servers of a backend
name retrieved using a sample fetch or an environment variable.

Signed-off-by: Nenad Merdanovic <nmerdan@haproxy.com>

doc/configuration.txt

@@ -12530,6 +12530,12 @@ mul(<value>)
   This prefix is followed by a name. The separator is a '.'.  The name may only
   contain characters 'a-z', 'A-Z', '0-9', '.' and '_'.
 
+nbsrv
+  Takes an input value of type string, interprets it as a backend name and
+  returns the number of usable servers in that backend. Can be used in places
+  where we want to look up a backend from a dynamic name, like a result of a
+  map lookup.
+
 neg
   Takes the input value of type signed integer, computes the opposite value,
   and returns the remainder as an signed integer. 0 is identity. This operator

include/proto/backend.h

@@ -46,6 +46,19 @@ struct server *get_server_sh(struct proxy *px, const char *addr, int len);
 struct server *get_server_uh(struct proxy *px, char *uri, int uri_len);
 int be_lastsession(const struct proxy *be);
 
+/* Returns number of usable servers in backend */
+static inline int be_usable_srv(struct proxy *be)
+{
+        if (be->state == PR_STSTOPPED)
+                return 0;
+        else if (be->srv_act)
+                return be->srv_act;
+        else if (be->lbprm.fbck)
+                return 1;
+        else
+                return be->srv_bck;
+}
+
 /* set the time of last session on the backend */
 static void inline be_set_sess_last(struct proxy *be)
 {

src/backend.c

@@ -1821,6 +1821,24 @@ smp_fetch_srv_sess_rate(const struct arg *args, struct sample *smp, const char *
 	return 1;
 }
 
+static int sample_conv_nbsrv(const struct arg *args, struct sample *smp, void *private)
+{
+
+	struct proxy *px;
+
+	if (!smp_make_safe(smp))
+		return 0;
+
+	px = proxy_find_by_name(smp->data.u.str.str, PR_CAP_BE, 0);
+	if (!px)
+		return 0;
+
+	smp->data.type = SMP_T_SINT;
+	smp->data.u.sint = be_usable_srv(px);
+
+	return 1;
+}
+
 
 /* Note: must not be declared <const> as its list will be overwritten.
  * Please take care of keeping this list alphabetically sorted.
@@ -1841,6 +1859,12 @@ static struct sample_fetch_kw_list smp_kws = {ILH, {
 	{ /* END */ },
 }};
 
+/* Note: must not be declared <const> as its list will be overwritten */
+static struct sample_conv_kw_list sample_conv_kws = {ILH, {
+	{ "nbsrv", sample_conv_nbsrv, 0, NULL, SMP_T_STR, SMP_T_SINT },
+	{ /* END */ },
+}};
+
 
 /* Note: must not be declared <const> as its list will be overwritten.
  * Please take care of keeping this list alphabetically sorted.
@@ -1854,6 +1878,7 @@ __attribute__((constructor))
 static void __backend_init(void)
 {
 	sample_register_fetches(&smp_kws);
+	sample_register_convs(&sample_conv_kws);
 	acl_register_keywords(&acl_kws);
 }