MQTTAsync_reconnect() doesn't reset serverUri index to 0. The client can be stuck on the last server uri of the configuration list. #1661

/*
 * Reproducer: MQTTAsync_reconnect does not restart the server URI cycling from the beginning.
 *
 * Expected behavior: when a client is configured with serverURIs=[A, B] and
 * MQTTAsync_reconnect is called, it should iterate over all configured URIs starting
 * from the first one, retrying until one succeeds.
 *
 * Actual behavior: MQTTAsync_reconnect doesnt modify the serverUri index, 
 * and reconnecting is only attempted again the last serverUri in the list. 
 *
 * Workaround: MQTTAsync_disconnect followed by MQTTAsync_connect resets the URI
 * index to 0 and iterates from the beginning.
 *
 * Dependencies: paho-mqtt-c (async), mosquitto broker, toxiproxy 2.x
 *
 * Setup (docker/podman):
 *   docker run -d --name mosquitto -p 1883:1883 eclipse-mosquitto:1.6
 *   docker run -d --name toxiproxy -p 8474:8474 -p 1884:1884 -p 1885:1885 \
 *     ghcr.io/shopify/toxiproxy:2.9.0
 *   curl -s -X POST http://localhost:8474/populate -H 'Content-Type: application/json' \
 *     -d '[{"name":"mqtt","listen":"0.0.0.0:1884","upstream":"mosquitto:1883","enabled":true},
 *          {"name":"mqtt2","listen":"0.0.0.0:1885","upstream":"mosquitto:1883","enabled":true}]'
 */

#include <MQTTAsync.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <unistd.h>

/* ---- assert ---- */

#define ASSERT(cond, msg) do { \
    if (!(cond)) { fprintf(stderr, "FAIL [%s:%d]: %s\n", __FILE__, __LINE__, msg); exit(1); } \
    else { printf("PASS: %s\n", msg); } \
} while(0)

/* ---- toxiproxy control via curl(1) ---- */

static void toxi(const char *method, const char *path, const char *body) {
    char cmd[512];
    if (body)
        snprintf(cmd, sizeof(cmd),
            "curl -sf -X %s http://localhost:8474%s "
            "-H 'Content-Type: application/json' -d '%s' > /dev/null", method, path, body);
    else
        snprintf(cmd, sizeof(cmd),
            "curl -sf -X %s http://localhost:8474%s > /dev/null", method, path);
    if (system(cmd) != 0) { fprintf(stderr, "toxiproxy request failed: %s\n", cmd); exit(1); }
}

/* ---- callbacks ---- */

typedef struct {
    volatile bool connected;
    volatile bool connect_failed;
} conn_ctx_t;

static void on_connect(void *ctx, MQTTAsync_successData *r) {
    (void)r; ((conn_ctx_t *)ctx)->connected = true;
}
static void on_connect_fail(void *ctx, MQTTAsync_failureData *r) {
    (void)r; ((conn_ctx_t *)ctx)->connect_failed = true;
}
static void on_conn_lost(void *ctx, char *cause) {
    (void)ctx; (void)cause;
}
static void on_disconnect(void *ctx, MQTTAsync_successData *r) {
    (void)r; *(int *)ctx = 1;
}

/* ---- wait helpers ---- */

#define WAIT_TRUE(expr, ms) do { \
    for (int _i = 0; _i < (ms)/10 && !(expr); _i++) usleep(10000); \
} while(0)

/* ---- main ---- */

int main(void) {
    const char *uris[] = {"tcp://localhost:1884", "tcp://localhost:1885"};
    conn_ctx_t conn = {0};

    MQTTAsync client;
    MQTTAsync_create(&client, uris[0], "repro_client", MQTTCLIENT_PERSISTENCE_NONE, NULL);
    MQTTAsync_setCallbacks(client, &conn, on_conn_lost, NULL, NULL);

    MQTTAsync_connectOptions co = MQTTAsync_connectOptions_initializer;
    co.cleansession     = 1;
    co.automaticReconnect = 0;
    co.serverURIs       = (char **)uris;
    co.serverURIcount   = 2;
    co.onSuccess        = on_connect;
    co.onFailure        = on_connect_fail;
    co.context          = &conn;

    /* start with only 1884 up */
    toxi("POST", "/proxies/mqtt",  "{\"enabled\":true}");
    toxi("POST", "/proxies/mqtt2", "{\"enabled\":false}");

    MQTTAsync_connect(client, &co);
    WAIT_TRUE(conn.connected || conn.connect_failed, 3000);
    ASSERT(MQTTAsync_isConnected(client), "initial connect on 1884");

    /* take 1884 down */
    toxi("POST", "/proxies/mqtt", "{\"enabled\":false}");
    WAIT_TRUE(!MQTTAsync_isConnected(client), 3000);
    ASSERT(!MQTTAsync_isConnected(client), "disconnected after 1884 went down");

    /* bring 1885 up, reconnect — paho tries next URI (1885), succeeds */
    toxi("POST", "/proxies/mqtt2", "{\"enabled\":true}");
    conn.connected = false; conn.connect_failed = false;
    MQTTAsync_reconnect(client);
    WAIT_TRUE(conn.connected || conn.connect_failed, 3000);
    ASSERT(MQTTAsync_isConnected(client), "reconnected via 1885");

    /* take 1885 down, bring 1884 up */
    toxi("POST", "/proxies/mqtt2", "{\"enabled\":false}");
    toxi("POST", "/proxies/mqtt",  "{\"enabled\":true}");
    WAIT_TRUE(!MQTTAsync_isConnected(client), 3000);
    ASSERT(!MQTTAsync_isConnected(client), "disconnected after 1885 went down");

    /* reconnect — expected to restart cycle from 1884, but URI index is not reset:
       paho tries 1885 again (stuck at last index), fails, stays disconnected */
    conn.connected = false; conn.connect_failed = false;
    MQTTAsync_reconnect(client);
    WAIT_TRUE(conn.connected || conn.connect_failed, 3000);
    ASSERT(!MQTTAsync_isConnected(client),
        "BUG: reconnect did not restart cycle from 1884 — stayed stuck on 1885");

    /* bring 1885 back, reconnect — succeeds on 1885 */
    toxi("POST", "/proxies/mqtt2", "{\"enabled\":true}");
    conn.connected = false; conn.connect_failed = false;
    MQTTAsync_reconnect(client);
    WAIT_TRUE(conn.connected || conn.connect_failed, 3000);
    ASSERT(MQTTAsync_isConnected(client), "reconnected via 1885 after it came back");

    /* disconnect + connect (not reconnect) — resets URI index to 0, connects to 1884 */
    int disc = 0;
    MQTTAsync_disconnectOptions dco = MQTTAsync_disconnectOptions_initializer;
    dco.onSuccess = on_disconnect; dco.context = &disc;
    MQTTAsync_disconnect(client, &dco);
    WAIT_TRUE(disc, 3000);

    toxi("POST", "/proxies/mqtt2", "{\"enabled\":false}"); /* only 1884 available */
    toxi("POST", "/proxies/mqtt",  "{\"enabled\":true}");
    conn.connected = false; conn.connect_failed = false;
    MQTTAsync_connect(client, &co);
    WAIT_TRUE(conn.connected || conn.connect_failed, 3000);
    ASSERT(MQTTAsync_isConnected(client),
        "WORKAROUND: disconnect+connect resets URI cycle, reconnects to 1884");

    MQTTAsync_destroy(&client);
    printf("done.\n");
    return 0;
}