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>

1 files changed, 23 insertions, 33 deletions

diff --git a/src/ipcpd/eth/eth.c b/src/ipcpd/eth/eth.c
index 29c5ff4f..f36e0b13 100644
--- a/src/ipcpd/eth/eth.c
+++ b/src/ipcpd/eth/eth.c
@@ -839,7 +839,7 @@ static void * eth_ipcp_packet_reader(void * o)
 #if defined(HAVE_NETMAP)
         struct nm_pkthdr     hdr;
 #else
-        struct shm_du_buff * sdb;
+        struct ssm_pk_buff * spb;
         fd_set               fds;
         int                  frame_len;
 #endif
@@ -871,21 +871,21 @@ static void * eth_ipcp_packet_reader(void * o)
                 if (select(eth_data.bpf + 1, &fds, NULL, NULL, NULL))
                         continue;
                 assert(FD_ISSET(eth_data.bpf, &fds));
-                if (ipcp_sdb_reserve(&sdb, BPF_LEN))
+                if (ipcp_spb_reserve(&spb, BPF_LEN))
                         continue;
-                buf = shm_du_buff_head(sdb);
+                buf = ssm_pk_buff_head(spb);
                 frame_len = read(eth_data.bpf, buf, BPF_BLEN);
     #elif defined(HAVE_RAW_SOCKETS)
                 FD_SET(eth_data.s_fd, &fds);
                 if (select(eth_data.s_fd + 1, &fds, NULL, NULL, NULL) < 0)
                         continue;
                 assert(FD_ISSET(eth_data.s_fd, &fds));
-                if (ipcp_sdb_reserve(&sdb, ETH_MTU))
+                if (ipcp_spb_reserve(&spb, ETH_MTU))
                         continue;
-                buf = shm_du_buff_head_alloc(sdb, ETH_HEADER_TOT_SIZE);
+                buf = ssm_pk_buff_head_alloc(spb, ETH_HEADER_TOT_SIZE);
                 if (buf == NULL) {
                         log_dbg("Failed to allocate header.");
-                        ipcp_sdb_release(sdb);
+                        ipcp_spb_release(spb);
                         continue;
                 }
                 frame_len = recv(eth_data.s_fd, buf,
@@ -893,7 +893,7 @@ static void * eth_ipcp_packet_reader(void * o)
     #endif
                 if (frame_len <= 0) {
                         log_dbg("Failed to receive frame.");
-                        ipcp_sdb_release(sdb);
+                        ipcp_spb_release(spb);
                         continue;
                 }
 #endif
@@ -935,7 +935,7 @@ static void * eth_ipcp_packet_reader(void * o)
 
                 if (ssap == MGMT_SAP && dsap == MGMT_SAP) {
 #endif
-                        ipcp_sdb_release(sdb); /* No need for the N+1 buffer. */
+                        ipcp_spb_release(spb); /* No need for the N+1 buffer. */
 
                         if (length > MGMT_FRAME_SIZE) {
                                 log_warn("Management frame size %u exceeds %u.",
@@ -981,22 +981,22 @@ static void * eth_ipcp_packet_reader(void * o)
                         pthread_rwlock_unlock(&eth_data.flows_lock);
 
 #ifndef HAVE_NETMAP
-                        shm_du_buff_head_release(sdb, ETH_HEADER_TOT_SIZE);
-                        shm_du_buff_truncate(sdb, length);
+                        ssm_pk_buff_head_release(spb, ETH_HEADER_TOT_SIZE);
+                        ssm_pk_buff_truncate(spb, length);
 #else
-                        if (ipcp_sdb_reserve(&sdb, length))
+                        if (ipcp_spb_reserve(&spb, length))
                                 continue;
 
-                        buf = shm_du_buff_head(sdb);
+                        buf = ssm_pk_buff_head(spb);
                         memcpy(buf, &e_frame->payload, length);
 #endif
-                        if (np1_flow_write(fd, sdb) < 0)
-                                ipcp_sdb_release(sdb);
+                        if (np1_flow_write(fd, spb) < 0)
+                                ipcp_spb_release(spb);
 
                         continue;
  fail_frame:
 #ifndef HAVE_NETMAP
-                        ipcp_sdb_release(sdb);
+                        ipcp_spb_release(spb);
 #endif
                 }
         }
@@ -1012,7 +1012,7 @@ static void cleanup_writer(void * o)
 static void * eth_ipcp_packet_writer(void * o)
 {
         int                  fd;
-        struct shm_du_buff * sdb;
+        struct ssm_pk_buff * spb;
         size_t               len;
 #if defined(BUILD_ETH_DIX)
         uint16_t             deid;
@@ -1040,17 +1040,17 @@ static void * eth_ipcp_packet_writer(void * o)
                         if (fqueue_type(fq) != FLOW_PKT)
                                 continue;
 
-                        if (np1_flow_read(fd, &sdb)) {
+                        if (np1_flow_read(fd, &spb)) {
                                 log_dbg("Bad read from fd %d.", fd);
                                 continue;
                         }
 
-                        len = shm_du_buff_len(sdb);
+                        len = ssm_pk_buff_len(spb);
 
-                        if (shm_du_buff_head_alloc(sdb, ETH_HEADER_TOT_SIZE)
+                        if (ssm_pk_buff_head_alloc(spb, ETH_HEADER_TOT_SIZE)
                             == NULL) {
                                 log_dbg("Failed to allocate header.");
-                                ipcp_sdb_release(sdb);
+                                ipcp_spb_release(spb);
                                 continue;
                         }
 
@@ -1073,10 +1073,10 @@ static void * eth_ipcp_packet_writer(void * o)
 #elif defined(BUILD_ETH_LLC)
                                             dsap, ssap,
 #endif
-                                            shm_du_buff_head(sdb),
+                                            ssm_pk_buff_head(spb),
                                             len))
                                 log_dbg("Failed to send frame.");
-                        ipcp_sdb_release(sdb);
+                        ipcp_spb_release(spb);
                 }
         }
 
@@ -1342,14 +1342,7 @@ static int eth_set_mtu(struct ifreq * ifr)
                          IPCP_ETH_LO_MTU);
                 eth_data.mtu = IPCP_ETH_LO_MTU;
         }
-#ifndef SHM_RDRB_MULTI_BLOCK
-        maxsz = SHM_RDRB_BLOCK_SIZE - 5 * sizeof(size_t) -
-                (DU_BUFF_HEADSPACE + DU_BUFF_TAILSPACE);
-        if ((size_t) eth_data.mtu > maxsz ) {
-                log_dbg("Layer MTU truncated to shm block size.");
-                eth_data.mtu = maxsz;
-        }
-#endif
+
         log_dbg("Layer MTU is %d.", eth_data.mtu);
 
         return 0;
@@ -1503,9 +1496,6 @@ static int eth_ipcp_bootstrap(struct ipcp_config * conf)
         char             ifn[IFNAMSIZ];
 #endif /* HAVE_NETMAP */
 
-#ifndef SHM_RDRB_MULTI_BLOCK
-        size_t           maxsz;
-#endif
         assert(conf);
         assert(conf->type == THIS_TYPE);