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, 89 insertions, 0 deletions

diff --git a/cmake/lib/ssm.cmake b/cmake/lib/ssm.cmake
new file mode 100644
index 00000000..71790a4a
--- /dev/null
+++ b/cmake/lib/ssm.cmake
@@ -0,0 +1,89 @@
+# Pool size configuration for Ouroboros secure shared memory
+# This file defines the allocation parameters for the
+# secure shared memory pool allocator
+
+# Shared memory pool naming configuration
+set(SSM_PREFIX "o7s.ssm" CACHE STRING
+    "Prefix for secure shared memory pools")
+set(SSM_GSMP_SUFFIX ".gsmp" CACHE STRING
+    "Suffix for Group Shared Memory Pool")
+set(SSM_PPP_SUFFIX ".ppp" CACHE STRING
+    "Suffix for Process Private Pool")
+
+set(SSM_POOL_NAME "/${SHM_PREFIX}.pool" CACHE INTERNAL
+  "Name for the main POSIX shared memory pool")
+set(SSM_POOL_BLOCKS 16384 CACHE STRING
+    "Number of blocks in SSM packet pool, must be a power of 2")
+set(SSM_PK_BUFF_HEADSPACE 256 CACHE STRING
+  "Bytes of headspace to reserve for future headers")
+set(SSM_PK_BUFF_TAILSPACE 32 CACHE STRING
+  "Bytes of tailspace to reserve for future tails")
+set(SSM_RBUFF_SIZE 1024 CACHE STRING
+    "Number of blocks in rbuff buffer, must be a power of 2")
+set(SSM_RBUFF_PREFIX "/${SHM_PREFIX}.rbuff." CACHE INTERNAL
+  "Prefix for rbuff POSIX shared memory filenames")
+set(SSM_FLOW_SET_PREFIX "/${SHM_PREFIX}.set." CACHE INTERNAL
+  "Prefix for the POSIX shared memory flow set")
+
+# Pool blocks per size class
+# This determines how many blocks of each size are preallocated in the pool
+# Higher values reduce allocation failures but increase memory usage
+set(SSM_POOL_256_BLOCKS 1024 CACHE STRING
+    "Number of 256B blocks in pool")
+set(SSM_POOL_512_BLOCKS 768 CACHE STRING
+    "Number of 512B blocks in pool")
+set(SSM_POOL_1K_BLOCKS 512 CACHE STRING
+    "Number of 1KB blocks in pool")
+set(SSM_POOL_2K_BLOCKS 384 CACHE STRING
+    "Number of 2KB blocks in pool")
+set(SSM_POOL_4K_BLOCKS 256 CACHE STRING
+    "Number of 4KB blocks in pool")
+set(SSM_POOL_16K_BLOCKS 128 CACHE STRING
+    "Number of 16KB blocks in pool")
+set(SSM_POOL_64K_BLOCKS 64 CACHE STRING
+    "Number of 64KB blocks in pool")
+set(SSM_POOL_256K_BLOCKS 32 CACHE STRING
+    "Number of 256KB blocks in pool")
+set(SSM_POOL_1M_BLOCKS 16 CACHE STRING
+    "Number of 1MB blocks in pool")
+
+# Number of shards per size class for reducing contention
+set(SSM_POOL_SHARDS 4 CACHE STRING
+    "Number of allocator shards per size class")
+
+# SSM packet buffer overhead - computed at compile time via sizeof()
+# Defined in config.h.in as sizeof(_ssm_memory_block) + sizeof(_ssm_pk_buff)
+# This makes it portable across platforms with different pid_t sizes and padding
+
+# Total shared memory pool size calculation
+math(EXPR SSM_POOL_TOTAL_SIZE
+    "(1 << 8) * ${SSM_POOL_256_BLOCKS} + \
+     (1 << 9) * ${SSM_POOL_512_BLOCKS} + \
+     (1 << 10) * ${SSM_POOL_1K_BLOCKS} + \
+     (1 << 11) * ${SSM_POOL_2K_BLOCKS} + \
+     (1 << 12) * ${SSM_POOL_4K_BLOCKS} + \
+     (1 << 14) * ${SSM_POOL_16K_BLOCKS} + \
+     (1 << 16) * ${SSM_POOL_64K_BLOCKS} + \
+     (1 << 18) * ${SSM_POOL_256K_BLOCKS} + \
+     (1 << 20) * ${SSM_POOL_1M_BLOCKS}")
+
+set(SSM_POOL_TOTAL_SIZE ${SSM_POOL_TOTAL_SIZE} CACHE INTERNAL
+    "Total shared memory pool size in bytes")
+
+include(utils/HumanReadable)
+format_bytes_human_readable(${SSM_POOL_TOTAL_SIZE} SSM_POOL_SIZE_DISPLAY)
+
+# Display configuration summary
+message(STATUS "Secure Shared Memory Pool Configuration:")
+message(STATUS "  Pool prefix: ${SSM_PREFIX}")
+message(STATUS "  Size classes: "
+  "256B, 512B, 1KiB, 2KiB, 4KiB, 16KiB, 64KiB, 256KiB, 1MiB")
+message(STATUS "  Max allocation: 1 MB")
+message(STATUS "  Total pool size: ${SSM_POOL_SIZE_DISPLAY} "
+               "(${SSM_POOL_TOTAL_SIZE} bytes)")
+message(STATUS "  Shards per class: ${SSM_POOL_SHARDS}")
+message(STATUS "  Blocks per class: ${SSM_POOL_256_BLOCKS}, "
+               "${SSM_POOL_512_BLOCKS}, ${SSM_POOL_1K_BLOCKS}, "
+               "${SSM_POOL_2K_BLOCKS}, ${SSM_POOL_4K_BLOCKS}, "
+               "${SSM_POOL_16K_BLOCKS}, ${SSM_POOL_64K_BLOCKS}, "
+               "${SSM_POOL_256K_BLOCKS}, ${SSM_POOL_1M_BLOCKS}")