vatrog-vm-signaling/src/discovery/slot.c

/* slot.c — vmid <-> endpoint allocator (see slot.h). Pure logic + a tiny pointer-free
 * on-disk format; no core dependency. */
#define _GNU_SOURCE
#include "slot.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>

void slot_init(slot_table* t) {
    memset(t, 0, sizeof *t);
}

int slot_lookup(const slot_table* t, uint32_t vmid) {
    if (!vmid) return -1;
    for (int e = 0; e < VMSIG_SLOT_COUNT; e++)
        if (t->ent[e].vmid == vmid) return e;
    return -1;
}

int slot_alloc(slot_table* t, uint32_t vmid) {
    if (!vmid) return -1;
    int e = slot_lookup(t, vmid);
    if (e >= 0) return e;                       /* idempotent pin */
    /* lowest free bit: ffsll of the complement (1-based; 0 => none free) */
    int b = __builtin_ffsll((long long)~t->used_mask);
    if (b == 0) return -1;                      /* table full (64-VM ceiling) */
    e = b - 1;
    t->ent[e].vmid = vmid;
    t->used_mask  |= (1ull << e);
    return e;
}

void slot_free(slot_table* t, uint32_t vmid) {
    int e = slot_lookup(t, vmid);
    if (e < 0) return;
    t->ent[e].vmid = 0;
    t->used_mask  &= ~(1ull << e);
}

/* ---- persistence: magic + version + 64 * uint32 vmid (native byte order, tmpfs-local) ---- */
#define SLOT_MAGIC   0x534C4F54u   /* "SLOT" */
#define SLOT_VERSION 1u

typedef struct {
    uint32_t magic;
    uint32_t version;
    uint32_t vmid[VMSIG_SLOT_COUNT];
} slot_blob;

int slot_save(const slot_table* t, const char* path) {
    if (!path) return -1;
    slot_blob b;
    memset(&b, 0, sizeof b);
    b.magic = SLOT_MAGIC; b.version = SLOT_VERSION;
    for (int e = 0; e < VMSIG_SLOT_COUNT; e++) b.vmid[e] = t->ent[e].vmid;

    char tmp[512];
    int n = snprintf(tmp, sizeof tmp, "%s.tmp", path);
    if (n < 0 || (size_t)n >= sizeof tmp) return -1;

    int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0600);
    if (fd < 0) return -1;
    ssize_t w = write(fd, &b, sizeof b);
    int rc = (w == (ssize_t)sizeof b) ? 0 : -1;
    if (close(fd) != 0) rc = -1;
    if (rc == 0 && rename(tmp, path) != 0) rc = -1;
    if (rc != 0) unlink(tmp);
    return rc;
}

int slot_load(slot_table* t, const char* path) {
    slot_init(t);
    if (!path) return 0;
    int fd = open(path, O_RDONLY | O_CLOEXEC);
    if (fd < 0) return 0;                        /* no file => fresh start (valid) */
    slot_blob b;
    ssize_t r = read(fd, &b, sizeof b);
    close(fd);
    if (r != (ssize_t)sizeof b || b.magic != SLOT_MAGIC || b.version != SLOT_VERSION) {
        slot_init(t);                            /* corrupt/old => fresh start */
        return 0;
    }
    for (int e = 0; e < VMSIG_SLOT_COUNT; e++) {
        t->ent[e].vmid = b.vmid[e];
        if (b.vmid[e]) t->used_mask |= (1ull << e);
    }
    return 0;
}