Add generic intrusive FIFO queue

We need several FIFO queues (a queue of packets, a queue of messages,
etc.).

Some of them are implemented using cbuf, a generic circular buffer. But
for recording, we need to store the packets in an unbounded queue until
they are written, so the queue was implemented manually.

Create a generic implementation (using macros) to avoid reimplementing
it every time.

app/meson.build

@@ -150,6 +150,9 @@ tests = [
         'tests/test_device_msg_deserialize.c',
         'src/device_msg.c'
     ]],
+    ['test_queue', [
+        'tests/test_queue.c',
+    ]],
     ['test_strutil', [
         'tests/test_strutil.c',
         'src/str_util.c'

app/src/queue.h

@@ -0,0 +1,74 @@
+// generic intrusive FIFO queue
+#ifndef QUEUE_H
+#define QUEUE_H
+
+#include <stdbool.h>
+#include <SDL2/SDL_assert.h>
+
+// To define a queue type of "struct foo":
+//    struct queue_foo QUEUE(struct foo);
+#define QUEUE(TYPE) { \
+    TYPE *first; \
+    TYPE *last; \
+}
+
+#define queue_init(PQ) \
+    (void) ((PQ)->first = NULL)
+
+#define queue_is_empty(PQ) \
+    !(PQ)->first
+
+// NEXTFIELD is the field in the ITEM type used for intrusive linked-list
+//
+// For example:
+//    struct foo {
+//        int value;
+//        struct foo *next;
+//    };
+//
+//    // define the type "struct my_queue"
+//    struct my_queue QUEUE(struct foo);
+//
+//    struct my_queue queue;
+//    queue_init(&queue);
+//
+//    struct foo v1 = { .value = 42 };
+//    struct foo v2 = { .value = 27 };
+//
+//    queue_push(&queue, next, v1);
+//    queue_push(&queue, next, v2);
+//
+//    struct foo *foo;
+//    queue_take(&queue, next, &foo);
+//    assert(foo->value == 42);
+//    queue_take(&queue, next, &foo);
+//    assert(foo->value == 27);
+//    assert(queue_is_empty(&queue));
+//
+
+// push a new item into the queue
+#define queue_push(PQ, NEXTFIELD, ITEM) \
+    (void) ({ \
+        (ITEM)->NEXTFIELD = NULL; \
+        if (queue_is_empty(PQ)) { \
+            (PQ)->first = (PQ)->last = (ITEM); \
+        } else { \
+            (PQ)->last->NEXTFIELD = (ITEM); \
+            (PQ)->last = (ITEM); \
+        } \
+    })
+
+// take the next item and remove it from the queue (the queue must not be empty)
+// the result is stored in *(PITEM)
+// (without typeof(), we could not store a local variable having the correct
+// type so that we can "return" it)
+#define queue_take(PQ, NEXTFIELD, PITEM) \
+    (void) ({ \
+        SDL_assert(!queue_is_empty(PQ)); \
+        *(PITEM) = (PQ)->first; \
+        (PQ)->first = (PQ)->first->NEXTFIELD; \
+    })
+    // no need to update (PQ)->last if the queue is left empty:
+    // (PQ)->last is undefined if !(PQ)->first anyway
+
+#endif

app/tests/test_queue.c

@@ -0,0 +1,38 @@
+#include <assert.h>
+
+#include <queue.h>
+
+struct foo {
+    int value;
+    struct foo *next;
+};
+
+static void test_queue(void) {
+    struct my_queue QUEUE(struct foo) queue;
+    queue_init(&queue);
+
+    assert(queue_is_empty(&queue));
+
+    struct foo v1 = { .value = 42 };
+    struct foo v2 = { .value = 27 };
+
+    queue_push(&queue, next, &v1);
+    queue_push(&queue, next, &v2);
+
+    struct foo *foo;
+
+    assert(!queue_is_empty(&queue));
+    queue_take(&queue, next, &foo);
+    assert(foo->value == 42);
+
+    assert(!queue_is_empty(&queue));
+    queue_take(&queue, next, &foo);
+    assert(foo->value == 27);
+
+    assert(queue_is_empty(&queue));
+}
+
+int main(void) {
+    test_queue();
+    return 0;
+}