[PATCH/RFC] read-cache: fix file time comparisons with different precisions

[Karsten Blees <karsten.blees@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