Add REAL std.random(REAL low, REAL high) with associated testcase.

Example usage:

    sub vcl_fetch {
	/* Spread TTLs -10%..+5% because CMS timestamps whole minutes */
	set beresp.ttl = beresp.ttl * random(0.90, 1.05);
    }

This usage was one of the very first VCL examples we ever wrote on
a whiteboard, we just never got a round tuit until now.

Fixes: #793



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5421 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishtest/tests/m00002.vtc

+# $Id$
+
+test "Test std.random"
+
+# needs random generator
+random
+
+server s1 {
+	rxreq
+	txresp -proto HTTP/1.0 -nolen -bodylen 9
+} -start
+
+varnish v1 -vcl+backend {
+	import std from "${topbuild}/lib/libvmod_std/.libs/libvmod_std.so.1" ;
+
+	sub vcl_fetch {
+		set beresp.http.rnd1 = std.random(0,1);
+		set beresp.http.rnd2 = std.random(0,10);
+		set beresp.http.rnd3 = std.random(8,10);
+		set beresp.http.rnd4 = std.random(99,100);
+	}
+} -start
+
+# NB: Do not change the number 1
+# NB: Only srandom(1) is standardized as deterministic.
+varnish v1 -cliok "debug.srandom 1"
+
+client c1 {
+	txreq 
+	rxresp
+	expect resp.http.rnd1 == 0.420
+	expect resp.http.rnd2 == 1.972
+	expect resp.http.rnd3 == 8.783
+	expect resp.http.rnd4 == 99.399
+	expect resp.bodylen == 9
+} -run
+
+

lib/libvmod_std/vmod.vcc

@@ -30,3 +30,4 @@ Init init_function
 Function STRING toupper(PRIV_CALL, STRING_LIST)
 Function STRING tolower(PRIV_VCL, STRING_LIST)
 Function VOID set_ip_tos(INT)
+Function REAL random(REAL, REAL)

lib/libvmod_std/vmod_std.c

@@ -120,3 +120,16 @@ init_function(struct vmod_priv *priv, const struct VCL_conf *cfg)
 	priv->free = free;
 	return (0);
 }
+
+double
+vmod_random(struct sess *sp, double lo, double hi)
+{
+	double a;
+
+	(void)sp;
+
+	a = random() / (double)(1ULL<<32);
+	a *= hi - lo;
+	a += lo;
+	return (a);
+}