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src/subversion/subversion/libsvn_subr/cache-membuffer.c  view on Meta::CPAN

 */
#define DEFAULT_MIN_SEGMENT_SIZE APR_UINT64_C(0x2000000)

/* The minimum segment size we will allow for multi-segmented caches
 */
#define MIN_SEGMENT_SIZE APR_UINT64_C(0x10000)

/* The maximum number of segments allowed. Larger numbers reduce the size
 * of each segment, in turn reducing the max size of a cachable item.
 * Also, each segment gets its own lock object. The actual number supported
 * by the OS may therefore be lower and svn_cache__membuffer_cache_create
 * may return an error.
 */
#define MAX_SEGMENT_COUNT 0x10000

/* As of today, APR won't allocate chunks of 4GB or more. So, limit the
 * segment size to slightly below that.
 */
#define MAX_SEGMENT_SIZE APR_UINT64_C(0xffff0000)

/* We don't mark the initialization status for every group but initialize
 * a number of groups at once. That will allow for a very small init flags
 * vector that is likely to fit into the CPU caches even for fairly large
 * membuffer caches. For instance, the default of 32 means 8x32 groups per
 * byte, i.e. 8 flags/byte x 32 groups/flag x 8 entries/group x 40 index
 * bytes/entry x 8 cache bytes/index byte = 1kB init vector / 640MB cache.
 */
#define GROUP_INIT_GRANULARITY 32

/* Invalid index reference value. Equivalent to APR_UINT32_T(-1)
 */
#define NO_INDEX APR_UINT32_MAX

/* To save space in our group structure, we only use 32 bit size values
 * and, therefore, limit the size of each entry to just below 4GB.
 * Supporting larger items is not a good idea as the data transfer
 * to and from the cache would block other threads for a very long time.
 */
#define MAX_ITEM_SIZE ((apr_uint32_t)(0 - ITEM_ALIGNMENT))

/* A 16 byte key type. We use that to identify cache entries.
 * The notation as just two integer values will cause many compilers
 * to create better code.
 */
typedef apr_uint64_t entry_key_t[2];

/* Debugging / corruption detection support.
 * If you define this macro, the getter functions will performed expensive
 * checks on the item data, requested keys and entry types. If there is
 * a mismatch found in any of them when being compared with the values
 * remembered in the setter function, an error will be returned.
 */
#ifdef SVN_DEBUG_CACHE_MEMBUFFER

/* The prefix passed to svn_cache__create_membuffer_cache() effectively
 * defines the type of all items stored by that cache instance. We'll take
 * the last 7 bytes + \0 as plaintext for easy identification by the dev.
 */
#define PREFIX_TAIL_LEN 8

/* This record will be attached to any cache entry. It tracks item data
 * (content), key and type as hash values and is the baseline against which
 * the getters will compare their results to detect inconsistencies.
 */
typedef struct entry_tag_t
{
  /* MD5 checksum over the serialized the item data.
   */
  unsigned char content_hash [APR_MD5_DIGESTSIZE];

  /* Hash value of the svn_cache_t instance that wrote the item
   * (i.e. a combination of type and repository)
   */
  unsigned char prefix_hash [APR_MD5_DIGESTSIZE];

  /* Note that this only covers the variable part of the key,
   * i.e. it will be different from the full key hash used for
   * cache indexing.
   */
  unsigned char key_hash [APR_MD5_DIGESTSIZE];

  /* Last letters from of the key in human readable format
   * (ends with the type identifier, e.g. "DAG")
   */
  char prefix_tail[PREFIX_TAIL_LEN];

  /* Length of the variable key part.
   */
  apr_size_t key_len;

} entry_tag_t;

/* Per svn_cache_t instance initialization helper.
 */
static void get_prefix_tail(const char *prefix, char *prefix_tail)
{
  apr_size_t len = strlen(prefix);
  apr_size_t to_copy = len > PREFIX_TAIL_LEN-1 ? PREFIX_TAIL_LEN-1 : len;

  memset(prefix_tail, 0, PREFIX_TAIL_LEN);
  memcpy(prefix_tail, prefix + len - to_copy, to_copy);
}

/* Initialize all members of TAG except for the content hash.
 */
static svn_error_t *store_key_part(entry_tag_t *tag,
                                   entry_key_t prefix_hash,
                                   char *prefix_tail,
                                   const void *key,
                                   apr_size_t key_len,
                                   apr_pool_t *pool)
{
  svn_checksum_t *checksum;
  SVN_ERR(svn_checksum(&checksum,
                       svn_checksum_md5,
                       key,
                       key_len,
                       pool));

  memcpy(tag->prefix_hash, prefix_hash, sizeof(tag->prefix_hash));
  memcpy(tag->key_hash, checksum->digest, sizeof(tag->key_hash));