lib: Replace rdrbuff with a proper slab allocatorbe

This is a first step towards the Secure Shared Memory (SSM)
infrastructure for Ouroboros, which will allow proper resource
separation for non-privileged processes.

This replaces the rdrbuff (random-deletion ring buffer) PoC allocator
with a sharded slab allocator for the packet buffer pool to avoid the
head-of-line blocking behaviour of the rdrb and reduce lock contention
in multi-process scenarios. Each size class contains multiple
independent shards, allowing parallel allocations without blocking.

- Configurable shard count per size class (default: 4, set via
  SSM_POOL_SHARDS in CMake). The configured number of blocks are
  spread over the number of shards. As an example:

  	 SSM_POOL_512_BLOCKS = 768 blocks total
	 These 768 blocks are shared among 4 shards
	       (not 768 × 4 = 3072 blocks)

- Lazy block distribution: all blocks initially reside in shard 0
  and naturally migrate to process-local shards upon first
  allocation and subsequent free operations

- Fallback with work stealing: processes attempt allocation from
  their local shard (pid % SSM_POOL_SHARDS) first, then steal
  from other shards if local is exhausted, eliminating
  fragmentation while maintaining low contention

- Round-robin condvar signaling: blocking allocations cycle
  through all shard condition variables to ensure fairness

- Blocks freed to allocator's shard: uses allocator_pid to
  determine target shard, enabling natural load balancing as
  process allocation patterns stabilize over time

Maintains existing robust mutex semantics including EOWNERDEAD
handling for dead process recovery. Internal structures exposed in
ssm.h for testing purposes. Adds some tests (pool_test,
pool_sharding_test.c. etc) verifying lazy distribution, migration,
fallback stealing, and multiprocess behavior.

Updates the ring buffer (rbuff) to use relaxed/acquire/release
ordering on atomic indices. The ring buffer requires the (robust)
mutex to ensure cross-structure synchronization between pool buffer
writes and ring buffer index publication.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

3 files changed, 4 insertions, 114 deletions

diff --git a/src/lib/tests/CMakeLists.txt b/src/lib/tests/CMakeLists.txt
index 6ab69bd1..d2535bc2 100644
--- a/src/lib/tests/CMakeLists.txt
+++ b/src/lib/tests/CMakeLists.txt
@@ -16,7 +16,6 @@ create_test_sourcelist(${PARENT_DIR}_tests test_suite.c
   kex_test_pqc.c
   md5_test.c
   sha3_test.c
-  shm_rbuff_test.c
   sockets_test.c
   time_test.c
   tpm_test.c
diff --git a/src/lib/tests/shm_rbuff_test.c b/src/lib/tests/shm_rbuff_test.c
deleted file mode 100644
index e36c3229..00000000
--- a/src/lib/tests/shm_rbuff_test.c
+++ /dev/null
@@ -1,113 +0,0 @@
-/*
- * Ouroboros - Copyright (C) 2016 - 2024
- *
- * Test of the shm_rbuff
- *
- *    Dimitri Staessens <dimitri@ouroboros.rocks>
- *    Sander Vrijders   <sander@ouroboros.rocks>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., http://www.fsf.org/about/contact/.
- */
-
-#define _POSIX_C_SOURCE 200112L
-
-#include "config.h"
-
-#include <ouroboros/shm_rbuff.h>
-
-#include <errno.h>
-#include <stdio.h>
-#include <unistd.h>
-
-int shm_rbuff_test(int     argc,
-                   char ** argv)
-{
-        struct shm_rbuff * rb;
-        size_t             i;
-
-        (void) argc;
-        (void) argv;
-
-        printf("Test: create rbuff...");
-
-        rb = shm_rbuff_create(getpid(), 1);
-        if (rb == NULL)
-                goto err;
-
-        printf("success.\n\n");
-        printf("Test: write a value...");
-
-        if (shm_rbuff_write(rb, 1) < 0)
-                goto error;
-
-        printf("success.\n\n");
-        printf("Test: check queue length is 1...");
-
-        if (shm_rbuff_queued(rb) != 1)
-                goto error;
-
-        printf("success.\n\n");
-        printf("Test: read a value...");
-
-        if (shm_rbuff_read(rb) != 1)
-                goto error;
-
-        printf("success.\n\n");
-        printf("Test: check queue is empty...");
-
-        if (shm_rbuff_read(rb) != -EAGAIN)
-                goto error;
-
-        printf("success.\n\n");
-        printf("Test: fill the queue...");
-
-        for (i = 0; i < SHM_RBUFF_SIZE - 1; ++i) {
-                if (shm_rbuff_queued(rb) != i)
-                        goto error;
-                if (shm_rbuff_write(rb, 1) < 0)
-                        goto error;
-        }
-
-        printf("success.\n\n");
-        printf("Test: check queue is full...");
-
-        if (shm_rbuff_queued(rb) != SHM_RBUFF_SIZE - 1)
-                goto error;
-
-        printf("success [%zd entries].\n\n", shm_rbuff_queued(rb));
-
-        printf("Test: check queue is full by writing value...");
-        if (!(shm_rbuff_write(rb, 1) < 0))
-                goto error;
-
-        printf("success [%zd entries].\n\n", shm_rbuff_queued(rb));
-
-        /* empty the rbuff */
-        while (shm_rbuff_read(rb) >= 0)
-                ;
-
-        shm_rbuff_destroy(rb);
-
-        return 0;
-
- error:
-        /* empty the rbuff */
-        while (shm_rbuff_read(rb) >= 0)
-                ;
-
-        shm_rbuff_destroy(rb);
- err:
-        printf("failed.\n\n");
-        return -1;
-}
diff --git a/src/lib/tests/sockets_test.c b/src/lib/tests/sockets_test.c
index 952f9529..ce9051b6 100644
--- a/src/lib/tests/sockets_test.c
+++ b/src/lib/tests/sockets_test.c
@@ -20,7 +20,11 @@
  * Foundation, Inc., http://www.fsf.org/about/contact/.
  */
 
+#if defined(__linux__) || defined(__CYGWIN__)
+#define _DEFAULT_SOURCE
+#else
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #include <ouroboros/sockets.h>
 #include <test/test.h>