Memory leak with json-c in multi-threaded C++ program

Question

I have a memory leak in my program and cannot understand why.

My program is multi-threaded and uses json-c and glib.

Thread 1 calls the following method every time it receives data over a UDP stream.

void foo(state_t *self, const char *data)
{
    char *x = strdup(data);
    g_async_queue_push(self->write_queue, x);
}

where write_queue is a GAsyncQueue initialized with g_async_queue_new().

Thread 2 processes the data in an endless loop.

static void bar (state_t *self)
{
    while (1)
    {
        void *msg = g_async_queue_pop(self->write_queue);
        parse_json ((char*)msg);
        free(msg);
    }
}

where

void parse_json (const char *data)
{
    struct json_object *elements;
    elements = json_tokener_parse(data);
    json_object_put(elements);
} 

Threads are created with the g_thread_new() command.

When I run this program, I see memory consumption grow constantly and regularly with htop (roughly 100MB/sec).

When I comment out the call to parse_json(), the memory leak disappears.

When I run parse_json() in a unit test with a large for loop, I notice no memory leak.

What I am missing here?

NB: I notice that I get roughly the same volume of memory leak if I comment out both the call to parse_json() and the free(msg). This leads me to think that my code is keeping a reference count somewhere on the msg string.

I am using this version of Linux

Linux exp-infra 5.11.0-49-generic #55-Ubuntu SMP Wed Jan 12 17:36:34 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

I am compiling my code with g++ --std=c++20 -O2.

My version of g++ is 10.3.0.

Thanks!