[PATCH/RFC] read-cache: fix file time comparisons with different precisions
- From
Karsten Blees <karsten.blees@gmail.com>
- Date
- Sep 28, 2015, 10:39 UTC
- Message-ID
- <560918F8.1080905@gmail.com>
- In-Reply-To
- <5605D88A.20104@gmail.com>
Different git variants record file times in the index with different precisions, according to their capabilities. E.g. git compiled with NO_NSEC records seconds only, JGit records the mtime in milliseconds, but leaves ctime blank (because ctime is unavailable in Java).
This causes performance issues in git compiled with USE_NSEC, because index entries with such 'incomplete' timestamps are considered dirty, triggering unnecessary content checks.
Add a file time comparison function that auto-detects the precision based on the number of trailing 0 digits, and compares with the lower precision of both values. This initial version supports the known precisions seconds (git + NO_NSEC), milliseconds (JGit) and nanoseconds (git + USE_NSEC), but can be easily extended to e.g. microseconds.
Use the new comparison function in both dirty and racy checks. As a side effect, this fixes racy detection in USE_NSEC-enabled git with core.checkStat=minimal, as the coreStat setting now affects racy checks as well.
Finally, do not check ctime if ctime.sec is 0 (as recorded by JGit).
Signed-off-by: Karsten Blees <blees@dcon.de> --- read-cache.c | 62 ++++++++++++++++++++++++++++++++++++++++-------------------- 1 file changed, 41 insertions(+), 21 deletions(-)
This patch fixes problems 3 and 4, by trying to auto-detect the recorded file time precision.
diff --git a/read-cache.c b/read-cache.c index 87204a5..3a4e6cd 100644 --- a/read-cache.c +++ b/read-cache.c @@ -99,23 +99,50 @@ void fill_stat_data(struct stat_data *sd, struct stat *st) sd->sd_size = st->st_size; } +/* + * Compares two file times. Returns 0 if equal, <0 if t1 < t2, >0 if t1 > t2. + * Auto-detects precision based on trailing 0 digits. Compares seconds only if + * core.checkStat=minimal. + */ +static inline int cmp_filetime(uint32_t t1_sec, uint32_t t1_nsec, + uint32_t t2_sec, uint32_t t2_nsec) { +#ifdef USE_NSEC + /* + * Compare seconds and return result if different, or checkStat=mimimal, + * or one of the time stamps has second precision only (nsec == 0). + */ + int diff = t1_sec - t2_sec; + if (diff || !check_stat || !t1_nsec || !t2_nsec) + return diff; + + /* + * Check if one of the time stamps has millisecond precision only (i.e. + * the trailing 6 digits are 0). First check the trailing 6 bits so that + * we only do (slower) modulo division if necessary. + */ + if ((!(t1_nsec & 0x3f) && !(t1_nsec % 1000000)) || + (!(t2_nsec & 0x3f) && !(t2_nsec % 1000000))) + /* Compare milliseconds. */ + return (t1_nsec - t2_nsec) / 1000000; + + /* Compare nanoseconds */ + return t1_nsec - t2_nsec; +#else + return t1_sec - t2_sec; +#endif +} + int match_stat_data(const struct stat_data *sd, struct stat *st) { int changed = 0; - if (sd->sd_mtime.sec != (unsigned int)st->st_mtime) - changed |= MTIME_CHANGED; - if (trust_ctime && check_stat && - sd->sd_ctime.sec != (unsigned int)st->st_ctime) - changed |= CTIME_CHANGED; - -#ifdef USE_NSEC - if (check_stat && sd->sd_mtime.nsec != ST_MTIME_NSEC(*st)) + if (cmp_filetime(sd->sd_mtime.sec, sd->sd_mtime.nsec, + (unsigned) st->st_mtime, ST_MTIME_NSEC(*st))) changed |= MTIME_CHANGED; - if (trust_ctime && check_stat && - sd->sd_ctime.nsec != ST_CTIME_NSEC(*st)) + if (trust_ctime && check_stat && sd->sd_ctime.sec && + cmp_filetime(sd->sd_ctime.sec, sd->sd_ctime.nsec, + (unsigned) st->st_ctime, ST_CTIME_NSEC(*st))) changed |= CTIME_CHANGED; -#endif if (check_stat) { if (sd->sd_uid != (unsigned int) st->st_uid || @@ -276,16 +303,9 @@ static int ce_match_stat_basic(const struct cache_entry *ce, struct stat *st) static int is_racy_stat(const struct index_state *istate, const struct stat_data *sd) { - return (istate->timestamp.sec && -#ifdef USE_NSEC - /* nanosecond timestamped files can also be racy! */ - (istate->timestamp.sec < sd->sd_mtime.sec || - (istate->timestamp.sec == sd->sd_mtime.sec && - istate->timestamp.nsec <= sd->sd_mtime.nsec)) -#else - istate->timestamp.sec <= sd->sd_mtime.sec -#endif - ); + return istate->timestamp.sec && + (cmp_filetime(istate->timestamp.sec, istate->timestamp.nsec, + sd->sd_mtime.sec, sd->sd_mtime.nsec) <= 0); } static int is_racy_timestamp(const struct index_state *istate,
-- 2.1.0.msysgit.0