exec: Clarify what not to do with exec().

See:
#375 (diff)

modules/exec/README

@@ -160,12 +160,13 @@ modparam("exec", "time_to_kill", 20)
        OSIPS_EXEC_1 the second value.
 
    WARNING: any OpenSIPS pseudo-vars which may contain special
-   bash characters should be placed inside quotes, e.g.
-   exec("update-stats.sh '$ct'");
+   bourne shell (sh/bash) characters should be placed inside
+   quotes, e.g. exec("update-stats.sh '$(ct{re.subst,/'//g})'");
 
    WARNING: "stdin"/"stdout"/"stderr" parameters are not designed
-   for a large amount of data so one should be careful when using
-   them because server could considerably be slowed down.
+   for large amounts of data, so one should be careful when using
+   them. Because of the basic implementation, filled up pipes
+   could cause a read deadlock.
 
    This function can be used from REQUEST_ROUTE, FAILURE_ROUTE,
    LOCAL_ROUTE, STARTUP_ROUTE, TIMER_ROUTE, EVENT_ROUTE,

modules/exec/doc/exec_admin.xml

@@ -200,13 +200,14 @@ modparam("exec", "time_to_kill", 20)
 		</listitem>
 		</itemizedlist>
 		<para>
-		WARNING: any OpenSIPS pseudo-vars which may contain special bash
-		characters should be placed inside quotes, e.g. exec("update-stats.sh '$ct'");
+		WARNING: any OpenSIPS pseudo-vars which may contain special bourne shell (sh/bash)
+		characters should be placed inside quotes, e.g.
+		exec("update-stats.sh '$(ct{re.subst,/'//g})'");
 		</para>
 		<para>
-		WARNING: "stdin"/"stdout"/"stderr" parameters are not designed for a large amount of
-		data so one should be careful when using them because server could considerably be
-		slowed down.
+		WARNING: "stdin"/"stdout"/"stderr" parameters are not designed for large amounts of
+		data, so one should be careful when using them. Because of the basic implementation,
+		filled up pipes could cause a read deadlock.
 		</para>
 		<para>
 		This function can be used from REQUEST_ROUTE, FAILURE_ROUTE,

modules/exec/exec.c

@@ -428,7 +428,7 @@ int exec_sync(struct sip_msg* msg, str* command, str* input, gparam_p outvar, gp
 {
 
 	pid_t pid;
-	int ret;
+	int ret = -1;
 	FILE *pin, *pout, *perr;
 
 	if ((input && input->len && input->s) || outvar || errvar) {
@@ -466,15 +466,13 @@ int exec_sync(struct sip_msg* msg, str* command, str* input, gparam_p outvar, gp
 	if (outvar) {
 		if (read_and_write2var(msg, &pout, outvar) < 0) {
 			LM_ERR("failed reading stdout from pipe\n");
-			ret = -1;
 			goto error;
 		}
 	}
 
 	if (errvar) {
 		if (read_and_write2var(msg, &perr, errvar) < 0) {
 			LM_ERR("failed reading stderr from pipe\n");
-			ret = -1;
 			goto error;
 		}
 	}