hakmem/core/slab_handle.h

// slab_handle.h - SlabHandle Box (Ownership + Remote Drain + Metadata Access)
// Purpose: Encapsulate slab ownership acquisition, remote drain, and metadata access
// Invariant: valid==1 ⇔ owner_tid==self && safe to drain/modify
#pragma once
#include <stdint.h>
#include <stdatomic.h>
#include <signal.h>
#include "hakmem_tiny_superslab.h"
#include "tiny_debug_ring.h"
#include "tiny_remote.h"
#include "box/tiny_next_ptr_box.h"  // Box API: next pointer read/write

extern int g_debug_remote_guard;
extern int g_tiny_safe_free_strict;

// Box: SlabHandle
// Capsule: Ownership, remote drain, metadata access
// Invariant: Operations only succeed when valid==1 (owned by current thread)
typedef struct SlabHandle {
    SuperSlab* ss;           // SuperSlab pointer
    TinySlabMeta* meta;      // Cached metadata pointer
    uint8_t slab_idx;        // Slab index within SuperSlab
    uint32_t owner_tid;      // Owner thread ID (cached)
    uint8_t valid;           // 1=owned, 0=invalid/unowned
    uint8_t _pad[3];         // Padding
} SlabHandle;

// Core operations

// Try to acquire ownership of a slab
// Returns valid handle on success, invalid handle on failure
// MUST be called before any drain/modify operations
static inline SlabHandle slab_try_acquire(SuperSlab* ss, int idx, uint32_t tid) {
    SlabHandle h = {0};

    if (!ss || ss->magic != SUPERSLAB_MAGIC) {
        return h;  // Invalid SuperSlab
    }

    int cap = ss_slabs_capacity(ss);
    if (idx < 0 || idx >= cap) {
        return h;  // Invalid index
    }

    TinySlabMeta* m = &ss->slabs[idx];

    // Try to acquire ownership (Box 3: Ownership)
    if (!ss_owner_try_acquire(m, tid)) {
        return h;  // Failed to acquire
    }

    // Success - build valid handle
    h.ss = ss;
    h.meta = m;
    h.slab_idx = (uint8_t)idx;
    h.owner_tid = tid;
    if (__builtin_expect(g_debug_remote_guard, 0)) {
        uint32_t cur = __atomic_load_n(&m->owner_tid, __ATOMIC_RELAXED);
        if (cur != tid || cur == 0) {
            tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                                   (uint16_t)ss->size_class,
                                   m,
                                   ((uintptr_t)cur << 32) | (uintptr_t)tid);
            // Log the error but don't raise signal in debug builds by default to avoid hangs
            #if !HAKMEM_BUILD_RELEASE
            static _Atomic uint64_t g_invalid_owner_count = 0;
            uint64_t count = atomic_fetch_add(&g_invalid_owner_count, 1);
            if (count < 10) {
                fprintf(stderr, "[SLAB_HANDLE] Invalid owner: cur=%u tid=%u (set HAKMEM_SAFE_FREE_STRICT=1 for strict mode)\n",
                        cur, tid);
            }
            #endif
            if (g_tiny_safe_free_strict) {
                raise(SIGUSR2);
            }
            h.valid = 0;
            return h;
        }
        uintptr_t aux = ((uintptr_t)h.slab_idx << 32) | (uintptr_t)tid;
        tiny_debug_ring_record(TINY_RING_EVENT_OWNER_ACQUIRE,
                               (uint16_t)ss->size_class,
                               m,
                               aux);
    }
    h.valid = 1;

    return h;
}

// Forward declaration for internal unsafe drain function
extern void _ss_remote_drain_to_freelist_unsafe(SuperSlab* ss, int slab_idx, TinySlabMeta* meta);

static inline int slab_remote_pending(const SlabHandle* h) {
    if (!h || !h->valid) return 0;
    uintptr_t head = atomic_load_explicit(&h->ss->remote_heads[h->slab_idx], memory_order_acquire);
    return head != 0;
}

// Drain remote queue to freelist (Box 2: Remote Queue boundary)
// Requires: h->valid == 1 (ownership verified)
// Effect: Merges remote queue into freelist, resets remote_counts
static inline void slab_drain_remote(SlabHandle* h) {
    if (!h || !h->valid) {
        #ifdef HAKMEM_DEBUG_VERBOSE
        fprintf(stderr, "[SLAB_HANDLE] drain_remote: invalid handle\n");
        #endif
        return;  // Invalid handle - no-op
    }

    if (__builtin_expect(g_debug_remote_guard, 0)) {
        uint32_t cur_owner = __atomic_load_n(&h->meta->owner_tid, __ATOMIC_RELAXED);
        if (cur_owner != h->owner_tid || cur_owner == 0) {
            uintptr_t aux = ((uintptr_t)cur_owner << 32) | (uintptr_t)h->owner_tid;
            tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                                   (uint16_t)h->ss->size_class,
                                   h->meta,
                                   aux);
            #if !HAKMEM_BUILD_RELEASE
            static _Atomic uint64_t g_drain_invalid_count = 0;
            uint64_t count = atomic_fetch_add(&g_drain_invalid_count, 1);
            if (count < 10) {
                fprintf(stderr, "[SLAB_HANDLE] Drain invalid owner: cur=%u expected=%u\n",
                        cur_owner, h->owner_tid);
            }
            #endif
            if (g_tiny_safe_free_strict) {
                raise(SIGUSR2);
                return;
            }
        }
    }

    // Ownership is guaranteed by valid==1
    // Safe to call internal unsafe version (ownership already verified)
    _ss_remote_drain_to_freelist_unsafe(h->ss, h->slab_idx, h->meta);
}

static inline void slab_drain_remote_full(SlabHandle* h) {
    if (!h || !h->valid) return;
    for (int attempt = 0; attempt < 32; attempt++) {
        if (!slab_remote_pending(h)) break;
        slab_drain_remote(h);
    }
    if (__builtin_expect(g_debug_remote_guard, 0)) {
        uintptr_t head = atomic_load_explicit(&h->ss->remote_heads[h->slab_idx], memory_order_relaxed);
        if (head != 0) {
            tiny_remote_watch_note("drain_pending",
                                   h->ss,
                                   h->slab_idx,
                                   (void*)head,
                                   0xA242u,
                                   h->owner_tid,
                                   0);
        }
    }
}

// Get metadata pointer (read-only access)
// Returns: Cached metadata pointer if valid, NULL otherwise
static inline TinySlabMeta* slab_meta(SlabHandle* h) {
    return (h && h->valid) ? h->meta : NULL;
}

// Release ownership (optional - for explicit cleanup)
// Effect: Resets owner_tid to 0, invalidates handle
static inline void slab_release(SlabHandle* h) {
    if (!h || !h->valid) {
        return;  // Already invalid
    }

    if (__builtin_expect(g_debug_remote_guard, 0)) {
        uint32_t cur_owner = __atomic_load_n(&h->meta->owner_tid, __ATOMIC_RELAXED);
        uintptr_t aux = ((uintptr_t)h->slab_idx << 32) | (uintptr_t)cur_owner;
        tiny_debug_ring_record(TINY_RING_EVENT_OWNER_RELEASE,
                               (uint16_t)(h->ss ? h->ss->size_class : 0u),
                               h->meta,
                               aux);
        if (cur_owner != h->owner_tid || cur_owner == 0) {
            tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                                   (uint16_t)(h->ss ? h->ss->size_class : 0u),
                                   h->meta,
                                   ((uintptr_t)cur_owner << 32) | (uintptr_t)h->owner_tid);
            #if !HAKMEM_BUILD_RELEASE
            static _Atomic uint64_t g_release_invalid_count = 0;
            uint64_t count = atomic_fetch_add(&g_release_invalid_count, 1);
            if (count < 10) {
                fprintf(stderr, "[SLAB_HANDLE] Release invalid owner: cur=%u expected=%u\n",
                        cur_owner, h->owner_tid);
            }
            #endif
            if (g_tiny_safe_free_strict) {
                raise(SIGUSR2);
            }
        }
    }

    // Release ownership (Box 3: Ownership)
    __atomic_store_n(&h->meta->owner_tid, 0u, __ATOMIC_RELEASE);
    h->valid = 0;
    h->owner_tid = 0;
}

// Check if handle is valid (owned and safe to use)
static inline int slab_is_valid(SlabHandle* h) {
    return (h && h->valid) ? 1 : 0;
}

// Get freelist pointer (convenience accessor)
static inline void* slab_freelist(SlabHandle* h) {
    if (!h || !h->valid) return NULL;
    return h->meta->freelist;
}

// Get used count (convenience accessor)
static inline uint16_t slab_used(SlabHandle* h) {
    if (!h || !h->valid) return 0;
    return h->meta->used;
}

// Get capacity (convenience accessor)
static inline uint16_t slab_capacity(SlabHandle* h) {
    if (!h || !h->valid) return 0;
    return h->meta->capacity;
}

// ========== FreeList Box Operations ==========
// Box Invariant: All freelist operations REQUIRE valid==1 (ownership guaranteed)

// Push to freelist (called during free)
// Returns: 1 on success, 0 on failure (no ownership)
static inline int slab_freelist_push(SlabHandle* h, void* ptr) {
    if (!h || !h->valid) {
#ifdef HAKMEM_DEBUG_VERBOSE
        fprintf(stderr, "[SLAB_HANDLE] freelist_push: invalid handle (no ownership)\n");
#endif
        return 0;  // Box: No ownership → FAIL
    }

    extern int g_debug_remote_guard;
    if (__builtin_expect(g_debug_remote_guard, 0)) {
        uintptr_t pval = (uintptr_t)ptr;
        uintptr_t fval = (uintptr_t)h->meta->freelist;
        if ((pval & (sizeof(void*) - 1)) != 0 || (fval && (fval & (sizeof(void*) - 1)) != 0)) {
            fprintf(stderr,
                    "[SLAB_HANDLE] FREELIST_ALIGN cls=%u slab=%u ptr=%p freelist=%p owner=%u used=%u\n",
                    h->ss ? h->ss->size_class : 0u,
                    (unsigned)h->slab_idx,
                    ptr,
                    h->meta->freelist,
                    h->meta->owner_tid,
                    (unsigned)h->meta->used);
        }
    }

    // Ownership guaranteed by valid==1 → safe to modify freelist
    void* old_freelist = h->meta->freelist;  // Store for empty→non-empty detection
    void* prev = h->meta->freelist;
    tiny_next_write(h->ss->size_class, ptr, prev);  // Box API: next pointer write
   h->meta->freelist = ptr;
   // Optional freelist mask update (opt-in via env HAKMEM_TINY_FREELIST_MASK)
   do {
       static int g_mask_en = -1;
       if (__builtin_expect(g_mask_en == -1, 0)) {
           const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");
           g_mask_en = (e && *e && *e != '0') ? 1 : 0;
       }
       if (__builtin_expect(g_mask_en, 0) && prev == NULL && h->ss) {
           uint32_t bit = (1u << h->slab_idx);
           atomic_fetch_or_explicit(&h->ss->freelist_mask, bit, memory_order_release);
       }
   } while (0);
   if (h->meta->used > 0) h->meta->used--;
   // Phase 6-2.2: Update nonempty_mask if transition empty→non-empty
   // BUGFIX: Use atomic OR to prevent bit loss in concurrent pushes
   if (old_freelist == NULL) {
       uint32_t bit = (1u << h->slab_idx);
       atomic_fetch_or_explicit(&h->ss->nonempty_mask, bit, memory_order_release);
   }
   tiny_remote_watch_note("freelist_push", h->ss, h->slab_idx, ptr, 0xA236u, h->owner_tid, 0);
    tiny_remote_track_on_local_free(h->ss, h->slab_idx, ptr, "freelist_push", h->owner_tid);
   return 1;
}

// Pop from freelist (called during alloc)
// Returns: pointer on success, NULL on failure/empty
static inline void* slab_freelist_pop(SlabHandle* h) {
    if (!h || !h->valid) {
        #ifdef HAKMEM_DEBUG_VERBOSE
        fprintf(stderr, "[SLAB_HANDLE] freelist_pop: invalid handle (no ownership)\n");
        #endif
        return NULL;  // Box: No ownership → FAIL
    }

    void* ptr = h->meta->freelist;
    // Option B: Defense-in-depth against sentinel leakage
    if (__builtin_expect((uintptr_t)ptr == TINY_REMOTE_SENTINEL, 0)) {
        if (__builtin_expect(g_debug_remote_guard, 0)) {
            fprintf(stderr, "[FREELIST_POP] sentinel detected in freelist (cls=%u slab=%u) -> break chain\n",
                    h->ss ? h->ss->size_class : 0u,
                    (unsigned)h->slab_idx);
        }
        h->meta->freelist = NULL;  // break the chain to avoid propagating corruption
        if (__builtin_expect(g_tiny_safe_free_strict, 0)) { raise(SIGUSR2); }
        return NULL;
    }
    if (ptr) {
        void* next = tiny_next_read(h->ss->size_class, ptr);  // Box API: next pointer read
        h->meta->freelist = next;
        h->meta->used++;
        // Optional freelist mask clear when freelist becomes empty
        do {
            static int g_mask_en2 = -1;
            if (__builtin_expect(g_mask_en2 == -1, 0)) {
                const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");
                g_mask_en2 = (e && *e && *e != '0') ? 1 : 0;
            }
            if (__builtin_expect(g_mask_en2, 0) && next == NULL && h->ss) {
                uint32_t bit = (1u << h->slab_idx);
                atomic_fetch_and_explicit(&h->ss->freelist_mask, ~bit, memory_order_release);
            }
        } while (0);
        // Keep nonempty_mask sticky to ensure subsequent frees remain discoverable.
        // Do NOT clear nonempty_mask on transient empty; adopt gate will verify safety.
        tiny_remote_watch_note("freelist_pop", h->ss, h->slab_idx, ptr, 0xA237u, h->owner_tid, 0);
        tiny_remote_assert_not_remote(h->ss, h->slab_idx, ptr, "freelist_pop_ret", h->owner_tid);
        tiny_remote_track_on_alloc(h->ss, h->slab_idx, ptr, "freelist_pop", h->owner_tid);
    }
    return ptr;
}

// Check if freelist is non-empty (read-only, safe without ownership)
static inline int slab_has_freelist(SlabHandle* h) {
    if (!h || !h->valid) return 0;
    return (h->meta->freelist != NULL);
}

// ========== Box 4 Boundary: Adopt/Bind Safe Guard ==========
// Box Invariant: bind は remote_head==0 を保証する必要がある
// TOCTOU Race 防止: drain 後から bind 前の間に別スレッドが remote push する可能性

// Check if slab is safe to bind (TOCTOU-safe check)
// Returns: 1 if safe (freelist exists AND remote_head==0), 0 otherwise
// Usage: Must be called immediately before bind to prevent TOCTOU race
//
// Example (correct usage - TOCTOU-safe):
//   SlabHandle h = slab_try_acquire(ss, idx, tid);
//   if (slab_is_valid(&h)) {
//       slab_drain_remote_full(&h);
//       if (slab_is_safe_to_bind(&h)) {  // ← bind 直前に再チェック
//           tiny_tls_bind_slab(tls, h.ss, h.slab_idx);
//           return h.ss;
//       }
//       slab_release(&h);
//   }
//
// Example (incorrect - TOCTOU race):
//   slab_drain_remote_full(&h);
//   if (!slab_remote_pending(&h)) {  // ← チェック
//       // ★ここで別スレッドが remote push できる！
//       tiny_tls_bind_slab(...);     // ← bind時に remote pending かも
//   }
static inline int slab_is_safe_to_bind(SlabHandle* h) {
    if (!h || !h->valid) return 0;
    if (!slab_freelist(h)) return 0;
    // Box 4 Boundary: bind 直前の最終確認（TOCTOU 防止）
    return !slab_remote_pending(h);
}