MINOR: acl: add the new "env()" fetch method to retrieve an environment variable

This is useful in order to take different actions across restarts without
touching the configuration (eg: soft-stop), or to pass some information
such as the local host name to the next hop.

doc/configuration.txt

@@ -8795,6 +8795,20 @@ connslots([<backend>]) : integer
   then this fetch clearly does not make sense, in which case the value returned
   will be -1.
 
+env(<name>) : string
+  Returns a string containing the value of environment variable <name>. As a
+  reminder, environment variables are per-process and are sampled when the
+  process starts. This can be useful to pass some information to a next hop
+  server, or with ACLs to take specific action when the process is started a
+  certain way.
+
+  Examples :
+      # Pass the Via header to next hop with the local hostname in it
+      http-request add-header Via 1.1\ %[env(HOSTNAME)]
+
+      # reject cookie-less requests when the STOP environment variable is set
+      http-request deny if !{ cook(SESSIONID) -m found } { env(STOP) -m found }
+
 fe_conn([<frontend>]) : integer
   Returns the number of currently established connections on the frontend,
   possibly including the connection being evaluated. If no frontend name is

src/acl.c

@@ -1980,6 +1980,26 @@ smp_fetch_false(struct proxy *px, struct session *s, void *l7, unsigned int opt,
 	return 1;
 }
 
+/* retrieve environment variable $1 as a string */
+static int
+smp_fetch_env(struct proxy *px, struct session *s, void *l7, unsigned int opt,
+              const struct arg *args, struct sample *smp)
+{
+	char *env;
+
+	if (!args || args[0].type != ARGT_STR)
+		return 0;
+
+	env = getenv(args[0].data.str.str);
+	if (!env)
+		return 0;
+
+	smp->type = SMP_T_CSTR;
+	smp->data.str.str = env;
+	smp->data.str.len = strlen(env);
+	return 1;
+}
+
 
 /************************************************************************/
 /*      All supported sample and ACL keywords must be declared here.    */
@@ -1993,6 +2013,7 @@ smp_fetch_false(struct proxy *px, struct session *s, void *l7, unsigned int opt,
 static struct sample_fetch_kw_list smp_kws = {{ },{
 	{ "always_false", smp_fetch_false, 0,            NULL, SMP_T_BOOL, SMP_USE_INTRN },
 	{ "always_true",  smp_fetch_true,  0,            NULL, SMP_T_BOOL, SMP_USE_INTRN },
+	{ "env",          smp_fetch_env,   ARG1(1,STR),  NULL, SMP_T_CSTR, SMP_USE_INTRN },
 	{ /* END */ },
 }};