[PATCH v2 2/2] hex: make hash_to_hex_algop() and friends thread-safe
- From
Matheus Tavares <matheus.bernardino@usp.br>
- Date
- Jun 26, 2020, 21:54 UTC
- Message-ID
- <b47445fa1cef6d4523dd0ca336f7ee22bce89466.1593208411.git.matheus.bernardino@usp.br>
- In-Reply-To
- <cover.1593208411.git.matheus.bernardino@usp.br>
hash_to_hex_algop() returns a static buffer, relieving callers from the responsibility of freeing memory after use. But the current implementation uses the same static data for all threads and, thus, is not thread-safe. We could avoid using this function and its wrappers in threaded code, but they are sometimes too deep in the call stack to be noticed or even avoided.
grep.c:grep_source_load_oid(), for example, uses the thread-unsafe oid_to_hex() (on errors) despite being called in threaded code. And oid_to_hex() -- which calls hash_to_hex_algop() -- is used in many other places, as well:
$ git grep 'oid_to_hex(' | wc -l 818
Although hash_to_hex_algop() and its wrappers don't seem to be causing problems out there for now (at least not reported), making them thread-safe makes the codebase more robust against race conditions. We can easily do that by replicating the static buffer in each thread's local storage.
Original-patch-by: Fredrik Kuivinen <frekui@gmail.com> Signed-off-by: Fredrik Kuivinen <frekui@gmail.com> Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> --- hex.c | 46 ++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 42 insertions(+), 4 deletions(-)
diff --git a/hex.c b/hex.c index da51e64929..4f2f163d5e 100644 --- a/hex.c +++ b/hex.c @@ -1,4 +1,5 @@ #include "cache.h" +#include "thread-utils.h" const signed char hexval_table[256] = { -1, -1, -1, -1, -1, -1, -1, -1, /* 00-07 */ @@ -136,12 +137,49 @@ char *oid_to_hex_r(char *buffer, const struct object_id *oid) return hash_to_hex_algop_r(buffer, oid->hash, the_hash_algo); } +struct hexbuf_array { + int idx; + char bufs[4][GIT_MAX_HEXSZ + 1]; +}; + +#ifdef HAVE_THREADS +static pthread_key_t hexbuf_array_key; +static pthread_once_t hexbuf_array_once = PTHREAD_ONCE_INIT; + +static void init_hexbuf_array_key(void) +{ + if (pthread_key_create(&hexbuf_array_key, free)) + die(_("failed to initialize threads' key for hash to hex conversion")); +} + +#else +static struct hexbuf_array default_hexbuf_array; +#endif + char *hash_to_hex_algop(const unsigned char *hash, const struct git_hash_algo *algop) { - static int bufno; - static char hexbuffer[4][GIT_MAX_HEXSZ + 1]; - bufno = (bufno + 1) % ARRAY_SIZE(hexbuffer); - return hash_to_hex_algop_r(hexbuffer[bufno], hash, algop); + struct hexbuf_array *ha; + +#ifdef HAVE_THREADS + void *value; + + if (pthread_once(&hexbuf_array_once, init_hexbuf_array_key)) + die(_("failed to initialize threads' key for hash to hex conversion")); + + value = pthread_getspecific(hexbuf_array_key); + if (value) { + ha = (struct hexbuf_array *) value; + } else { + ha = xmalloc(sizeof(*ha)); + if (pthread_setspecific(hexbuf_array_key, (void *)ha)) + die(_("failed to set thread buffer for hash to hex conversion")); + } +#else + ha = &default_hexbuf_array; +#endif + + ha->idx = (ha->idx + 1) % ARRAY_SIZE(ha->bufs); + return hash_to_hex_algop_r(ha->bufs[ha->idx], hash, algop); } char *hash_to_hex(const unsigned char *hash)
-- 2.26.2