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>

4 files changed, 14 insertions, 14 deletions

diff --git a/src/irmd/reg/flow.c b/src/irmd/reg/flow.c
index d6f6437f..02dc9c99 100644
--- a/src/irmd/reg/flow.c
+++ b/src/irmd/reg/flow.c
@@ -66,11 +66,11 @@ struct reg_flow * reg_flow_create(const struct flow_info * info)
 static void destroy_rbuffs(struct reg_flow * flow)
 {
         if (flow->n_rb != NULL)
-                shm_rbuff_destroy(flow->n_rb);
+                ssm_rbuff_destroy(flow->n_rb);
         flow->n_rb = NULL;
 
         if (flow->n_1_rb != NULL)
-                shm_rbuff_destroy(flow->n_1_rb);
+                ssm_rbuff_destroy(flow->n_1_rb);
         flow->n_1_rb = NULL;
 }
 
@@ -103,28 +103,28 @@ static int create_rbuffs(struct reg_flow *  flow,
         assert(flow != NULL);
         assert(info != NULL);
 
-        flow->n_rb = shm_rbuff_create(info->n_pid, info->id);
+        flow->n_rb = ssm_rbuff_create(info->n_pid, info->id);
         if (flow->n_rb == NULL)
                 goto fail_n_rb;
 
-        if (shm_rbuff_mlock(flow->n_rb) < 0)
+        if (ssm_rbuff_mlock(flow->n_rb) < 0)
                 log_warn("Failed to mlock n_rb for flow %d.", info->id);
 
         assert(flow->info.n_1_pid == 0);
         assert(flow->n_1_rb == NULL);
 
         flow->info.n_1_pid = info->n_1_pid;
-        flow->n_1_rb = shm_rbuff_create(info->n_1_pid, info->id);
+        flow->n_1_rb = ssm_rbuff_create(info->n_1_pid, info->id);
         if (flow->n_1_rb == NULL)
                 goto fail_n_1_rb;
 
-        if (shm_rbuff_mlock(flow->n_1_rb) < 0)
+        if (ssm_rbuff_mlock(flow->n_1_rb) < 0)
                 log_warn("Failed to mlock n_1_rb for flow %d.", info->id);
 
         return 0;
 
  fail_n_1_rb:
-        shm_rbuff_destroy(flow->n_rb);
+        ssm_rbuff_destroy(flow->n_rb);
  fail_n_rb:
         return -ENOMEM;
 }
diff --git a/src/irmd/reg/flow.h b/src/irmd/reg/flow.h
index d1e4811c..b671d486 100644
--- a/src/irmd/reg/flow.h
+++ b/src/irmd/reg/flow.h
@@ -28,7 +28,7 @@
 #include <ouroboros/name.h>
 #include <ouroboros/pthread.h>
 #include <ouroboros/qos.h>
-#include <ouroboros/shm_rbuff.h>
+#include <ouroboros/ssm_rbuff.h>
 #include <ouroboros/utils.h>
 
 #include <sys/types.h>
@@ -45,8 +45,8 @@ struct reg_flow {
 
         char               name[NAME_SIZE + 1];
 
-        struct shm_rbuff * n_rb;
-        struct shm_rbuff * n_1_rb;
+        struct ssm_rbuff * n_rb;
+        struct ssm_rbuff * n_1_rb;
 };
 
 struct reg_flow * reg_flow_create(const struct flow_info * info);
diff --git a/src/irmd/reg/proc.c b/src/irmd/reg/proc.c
index 9bbdf0eb..541731b2 100644
--- a/src/irmd/reg/proc.c
+++ b/src/irmd/reg/proc.c
@@ -75,7 +75,7 @@ struct reg_proc * reg_proc_create(const struct proc_info * info)
                 goto fail_malloc;
         }
 
-        proc->set = shm_flow_set_create(info->pid);
+        proc->set = ssm_flow_set_create(info->pid);
         if (proc->set == NULL) {
                 log_err("Failed to create flow set for %d.", info->pid);
                 goto fail_set;
@@ -99,7 +99,7 @@ void reg_proc_destroy(struct reg_proc * proc)
 {
         assert(proc != NULL);
 
-        shm_flow_set_destroy(proc->set);
+        ssm_flow_set_destroy(proc->set);
 
         __reg_proc_clear_names(proc);
 
diff --git a/src/irmd/reg/proc.h b/src/irmd/reg/proc.h
index 499ecc72..a790e5c9 100644
--- a/src/irmd/reg/proc.h
+++ b/src/irmd/reg/proc.h
@@ -25,7 +25,7 @@
 
 #include <ouroboros/list.h>
 #include <ouroboros/proc.h>
-#include <ouroboros/shm_flow_set.h>
+#include <ouroboros/ssm_flow_set.h>
 
 struct reg_proc {
         struct list_head      next;
@@ -35,7 +35,7 @@ struct reg_proc {
         struct list_head      names;   /* process accepts flows for names */
         size_t                n_names; /* number of names                 */
 
-        struct shm_flow_set * set;
+        struct ssm_flow_set * set;
 };
 
 struct reg_proc * reg_proc_create(const struct proc_info * info);