pstream: Support memfd blocks transport

Now that we have the necessary infrastructure to memexport and
mempimport a memfd memblock, extend that support higher up in the
chain with pstreams.

A PA endpoint can now _transparently_ send a memfd memblock to the
other end by simply calling pa_pstream_send_memblock() – provided
the block's memfd pool was earlier registered with the pstream.

If the pipe does not support memfd transfers, we fall back to
sending the block's full data instead of just its reference.

** Further details:

A single pstream connection usually transfers blocks from multiple
pools including the server's srbchannel mempool, the client's
audio data mempool, and the server's global core mempool.

If these mempools are memfd-backed, we now require registering
them with the pstream before sending any blocks they cover. This
is done to minimize fd passing overhead and avoid fd leaks.

Moreover, to support all these pools without hard-coding their
number or nature in the Pulse communication protocol itself, a new
REGISTER_MEMFD_SHMID command is introduced. That command can be
sent _anytime_ during the pstream's lifetime and is used for
creating on demand SHM ID to memfd mappings.

Suggested-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>

src/pulsecore/native-common.h

@@ -24,6 +24,10 @@
 #include <pulse/cdecl.h>
 #include <pulse/def.h>
 
+#include <pulsecore/pdispatch.h>
+#include <pulsecore/pstream.h>
+#include <pulsecore/tagstruct.h>
+
 PA_C_DECL_BEGIN
 
 enum {
@@ -179,6 +183,10 @@ enum {
     PA_COMMAND_ENABLE_SRBCHANNEL,
     PA_COMMAND_DISABLE_SRBCHANNEL,
 
+    /* Supported since protocol v31 (9.0)
+     * BOTH DIRECTIONS */
+    PA_COMMAND_REGISTER_MEMFD_SHMID,
+
     PA_COMMAND_MAX
 };
 
@@ -193,6 +201,9 @@ enum {
 
 #define PA_NATIVE_DEFAULT_UNIX_SOCKET "native"
 
+int pa_common_command_register_memfd_shmid(pa_pstream *p, pa_pdispatch *pd, uint32_t version,
+                                           uint32_t command, pa_tagstruct *t);
+
 PA_C_DECL_END
 
 #endif