Add on_pre_connect() callback.

This allows clients to update usernames/passwords/TLS parameters prior
to reconnecting.

ChangeLog.txt

@@ -11,6 +11,8 @@ Client library:
   feature that allows the network thread to be woken from select() by another
   thread when e.g.  mosquitto_publish() is called. This reduces the number of
   sockets used by each client by two.
+- Add `on_pre_connect()` callback to allow clients to update
+  username/password/TLS parameters before an automatic reconnection.
 
 
 2.0.6 - 2021-01-xx

include/mosquitto.h

@@ -1935,6 +1935,9 @@ libmosq_EXPORT void *mosquitto_ssl_get(struct mosquitto *mosq);
  *        the MQTT protocol version in use.
  *        For MQTT v5.0, look at section 3.2.2.2 Connect Reason code: https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html
  *        For MQTT v3.1.1, look at section 3.2.2.3 Connect Return code: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html
+ *
+ * See Also:
+ *  <mosquitto_pre_connect_callback_set>
  */
 libmosq_EXPORT void mosquitto_connect_callback_set(struct mosquitto *mosq, void (*on_connect)(struct mosquitto *, void *, int));
 
@@ -1957,6 +1960,9 @@ libmosq_EXPORT void mosquitto_connect_callback_set(struct mosquitto *mosq, void
  *        For MQTT v5.0, look at section 3.2.2.2 Connect Reason code: https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html
  *        For MQTT v3.1.1, look at section 3.2.2.3 Connect Return code: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html
  *  flags - the connect flags.
+ *
+ * See Also:
+ *  <mosquitto_pre_connect_callback_set>
  */
 libmosq_EXPORT void mosquitto_connect_with_flags_callback_set(struct mosquitto *mosq, void (*on_connect)(struct mosquitto *, void *, int, int));
 
@@ -1985,9 +1991,30 @@ libmosq_EXPORT void mosquitto_connect_with_flags_callback_set(struct mosquitto *
  *  flags - the connect flags.
  *  props - list of MQTT 5 properties, or NULL
  *
+ * See Also:
+ *  <mosquitto_pre_connect_callback_set>
  */
 libmosq_EXPORT void mosquitto_connect_v5_callback_set(struct mosquitto *mosq, void (*on_connect)(struct mosquitto *, void *, int, int, const mosquitto_property *props));
 
+/*
+ * Function: mosquitto_pre_connect_callback_set
+ *
+ * Set the pre-connect callback. The pre-connect callback is called just before an attempt is made to connect to the broker. This may be useful if you are using <mosquitto_loop_start>, or
+ * <mosquitto_loop_forever>, because when your client disconnects the library
+ * will by default automatically reconnect. Using the pre-connect callback
+ * allows you to set usernames, passwords, and TLS related parameters.
+ *
+ * Parameters:
+ *  mosq -           a valid mosquitto instance.
+ *  on_pre_connect - a callback function in the following form:
+ *                   void callback(struct mosquitto *mosq, void *obj)
+ *
+ * Callback Parameters:
+ *  mosq - the mosquitto instance making the callback.
+ *  obj - the user data provided in <mosquitto_new>
+ */
+libmosq_EXPORT void mosquitto_pre_connect_callback_set(struct mosquitto *mosq, void (*on_pre_connect)(struct mosquitto *, void *));
+
 /*
  * Function: mosquitto_disconnect_callback_set
  *

lib/callbacks.c

@@ -43,6 +43,13 @@ void mosquitto_connect_v5_callback_set(struct mosquitto *mosq, void (*on_connect
 	pthread_mutex_unlock(&mosq->callback_mutex);
 }
 
+void mosquitto_pre_connect_callback_set(struct mosquitto *mosq, void (*on_pre_connect)(struct mosquitto *, void *))
+{
+	pthread_mutex_lock(&mosq->callback_mutex);
+	mosq->on_pre_connect = on_pre_connect;
+	pthread_mutex_unlock(&mosq->callback_mutex);
+}
+
 void mosquitto_disconnect_callback_set(struct mosquitto *mosq, void (*on_disconnect)(struct mosquitto *, void *, int))
 {
 	pthread_mutex_lock(&mosq->callback_mutex);

lib/connect.c

@@ -195,6 +195,14 @@ static int mosquitto__reconnect(struct mosquitto *mosq, bool blocking)
         net__socket_close(mosq); //close socket
     }
 
+	pthread_mutex_lock(&mosq->callback_mutex);
+	if(mosq->on_pre_connect){
+		mosq->in_callback = true;
+		mosq->on_pre_connect(mosq, mosq->userdata);
+		mosq->in_callback = false;
+	}
+	pthread_mutex_unlock(&mosq->callback_mutex);
+
 #ifdef WITH_SOCKS
 	if(mosq->socks5_host){
 		rc = net__socket_connect(mosq, mosq->socks5_host, mosq->socks5_port, mosq->bind_address, blocking);

lib/linker.version

@@ -141,3 +141,8 @@ MOSQ_1.7 {
 		mosquitto_property_next;
 		mosquitto_ssl_get;
 } MOSQ_1.6;
+
+MOSQ_2.1 {
+	global:
+		mosquitto_pre_connect_callback_set;
+} MOSQ_1.7;

lib/mosquitto_internal.h

@@ -310,6 +310,7 @@ struct mosquitto {
 	bool in_callback;
 	struct mosquitto_msg_data msgs_in;
 	struct mosquitto_msg_data msgs_out;
+	void (*on_pre_connect)(struct mosquitto *, void *userdata);
 	void (*on_connect)(struct mosquitto *, void *userdata, int rc);
 	void (*on_connect_with_flags)(struct mosquitto *, void *userdata, int rc, int flags);
 	void (*on_connect_v5)(struct mosquitto *, void *userdata, int rc, int flags, const mosquitto_property *props);

test/lib/01-pre-connect-callback.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+
+# Test whether the pre-connect callback is triggered and allows us to set a username and password.
+
+# The client should connect to port 1888 with keepalive=60, clean session set,
+# client id 01-pre-connect, username set to uname and password set to ;'[08gn=#
+
+from mosq_test_helper import *
+
+port = mosq_test.get_lib_port()
+
+rc = 1
+keepalive = 60
+connect_packet = mosq_test.gen_connect("01-pre-connect", keepalive=keepalive, username="uname", password=";'[08gn=#")
+
+sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+sock.settimeout(10)
+sock.bind(('', port))
+sock.listen(5)
+
+client_args = sys.argv[1:]
+env = dict(os.environ)
+env['LD_LIBRARY_PATH'] = '../../lib:../../lib/cpp'
+try:
+    pp = env['PYTHONPATH']
+except KeyError:
+    pp = ''
+env['PYTHONPATH'] = '../../lib/python:'+pp
+client = mosq_test.start_client(filename=sys.argv[1].replace('/', '-'), cmd=client_args, env=env, port=port)
+
+try:
+    (conn, address) = sock.accept()
+    conn.settimeout(10)
+
+    mosq_test.expect_packet(conn, "connect", connect_packet)
+    rc = 0
+
+    conn.close()
+except mosq_test.TestError:
+    pass
+finally:
+    client.terminate()
+    client.wait()
+    sock.close()
+
+exit(rc)
+

test/lib/Makefile

@@ -27,6 +27,7 @@ c : test-compile
 	./01-keepalive-pingreq.py $@/01-keepalive-pingreq.test
 	./01-no-clean-session.py $@/01-no-clean-session.test
 	./01-server-keepalive-pingreq.py $@/01-server-keepalive-pingreq.test
+	./01-pre-connect-callback.py $@/01-pre-connect-callback.test
 	./01-unpwd-set.py $@/01-unpwd-set.test
 	./01-will-set.py $@/01-will-set.test
 	./01-will-unpwd-set.py $@/01-will-unpwd-set.test

test/lib/c/01-pre-connect-callback.c

@@ -0,0 +1,37 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <mosquitto.h>
+
+void on_pre_connect(struct mosquitto *mosq, void *userdata)
+{
+	mosquitto_username_pw_set(mosq, "uname", ";'[08gn=#");
+}
+
+static int run = -1;
+int main(int argc, char *argv[])
+{
+	int rc;
+	struct mosquitto *mosq;
+
+	int port = atoi(argv[1]);
+
+	mosquitto_lib_init();
+
+	mosq = mosquitto_new("01-pre-connect", true, NULL);
+	if(mosq == NULL){
+		return 1;
+	}
+	mosquitto_pre_connect_callback_set(mosq, on_pre_connect);
+
+	rc = mosquitto_connect(mosq, "localhost", port, 60);
+
+	while(run == -1){
+		mosquitto_loop(mosq, -1, 1);
+	}
+	mosquitto_destroy(mosq);
+
+	mosquitto_lib_cleanup();
+	return run;
+}

test/lib/c/Makefile

@@ -7,6 +7,7 @@ SRC = \
 	01-con-discon-success.c \
 	01-keepalive-pingreq.c \
 	01-no-clean-session.c \
+	01-pre-connect-callback.c \
 	01-server-keepalive-pingreq.c \
 	01-unpwd-set.c \
 	01-will-set.c \