pulsecore: Specially mark global mempools

Color global mempools with a special mark. This special marking
is needed for handling memfd-backed pools.

To avoid fd leaks, memfd pools are registered with the connection
pstream to create an ID<->memfd mapping on both PA endpoints.
Such memory regions are then always referenced by their IDs and
never by their fds, and so their fds can be safely closed later.

Unfortunately this scheme cannot work with global pools since the
registration ID<->memfd mechanism needs to happen for each newly
connected client, and thus the need for a more special handling.
That is, for the pool's fd to be always open :-(

Almost all mempools are now created on a per-client basis. The
only exception is the pa_core's mempool which is still shared
between all clients of the system.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>

src/pulsecore/shm.h

@@ -41,7 +41,11 @@ typedef struct pa_shm {
      *
      * When we don't have ownership for the memfd fd in question (e.g.
      * pa_shm_attach()), or the file descriptor has now been closed,
-     * this is set to -1. */
+     * this is set to -1.
+     *
+     * For the special case of a global mempool, we keep this fd
+     * always open. Check comments on top of pa_mempool_new() for
+     * rationale. */
     int fd;
 } pa_shm;