Add ability to not save mtime in metadata files

If you're storing metadata in a version control system with multiple
branches, mtime differences are going to produce a whole bunch of
conflicts that you likely don't care about. This allows you to not
save mtime and avoid those.

Note that we use a sentinel value of -1 for the mentry `mtimensec` field
to indicate this in the data, as all values of `mtime` are theoretically
valid, but `mtimensec` must always be between 0 and 999,999,999 in the
real world.

I'm not 100% sure about the mechanism for selecting this feature. The
legacy behaviour for metastore was to save mtimes in the metadata files,
but ignore them for compare/apply by default, with a `--mtime` option to
use the mtime data.

Keeping the legacy behaviour for backwards compatibility, but adding a
`--no-mtime` option to ignore mtimes when saving felt like a reasonable
way of making this happen, but something about it doesn't feel great.

Maybe I just didn't figure out how to make the documentation clear
enough.  ¯\_(ツ)_/¯

1 files changed, 86 insertions, 59 deletions

diff --git a/src/metaentry.c b/src/metaentry.c
index 7f1012f..9956f7f 100644
--- a/src/metaentry.c
+++ b/src/metaentry.c
@@ -193,7 +193,7 @@ mentries_print(const struct metahash *mhash)
 
 /* Creates a metaentry for the file/dir/etc at path */
 struct metaentry *
-mentry_create(const char *path)
+mentry_create(const char *path, int with_mtime)
 {
 #if !defined(NO_XATTR) || !(NO_XATTR+0)
 	ssize_t lsize, vsize;
@@ -233,8 +233,14 @@ mentry_create(const char *path)
 	mentry->owner = xstrdup(pbuf->pw_name);
 	mentry->group = xstrdup(gbuf->gr_name);
 	mentry->mode = sbuf.st_mode & 0177777;
-	mentry->mtime = sbuf.st_mtim.tv_sec;
-	mentry->mtimensec = sbuf.st_mtim.tv_nsec;
+	if (with_mtime) {
+		mentry->mtime = sbuf.st_mtim.tv_sec;
+		mentry->mtimensec = sbuf.st_mtim.tv_nsec;
+	}
+	else {
+		mentry->mtime = 0;
+		mentry->mtimensec = MTIME_NONE;
+	}
 
 	/* symlinks have no xattrs */
 	if (S_ISLNK(mentry->mode))
@@ -364,7 +370,7 @@ mentries_recurse(const char *path, struct metahash *mhash, msettings *st)
 		return;
 	}
 
-	mentry = mentry_create(path);
+	mentry = mentry_create(path, st->do_mtime >= 0);
 	if (!mentry)
 		return;
 
@@ -491,12 +497,17 @@ mentries_tofile_v1(const struct metahash *mhash, FILE * to)
 		fprintf(to, "%.6o", mentry->mode);
 
 		fputc('\t', to);
-		gmtime_r(&mentry->mtime, &tm);
-		strftime(tmbuf, sizeof(tmbuf), "%Y-%m-%dT%H:%M:%S", &tm);
-		fputs(tmbuf, to);
+		if (mentry->mtimensec != MTIME_NONE) {
+			gmtime_r(&mentry->mtime, &tm);
+			strftime(tmbuf, sizeof(tmbuf), "%Y-%m-%dT%H:%M:%S", &tm);
+			fputs(tmbuf, to);
 
-		fputc('.', to);
-		fprintf(to, "%.9ldZ", mentry->mtimensec);
+			fputc('.', to);
+			fprintf(to, "%.9ldZ", mentry->mtimensec);
+		}
+		else {
+			fputc('0', to);
+		}
 
 		for (i = 0; i < mentry->xattrs; i++) {
 			fputc('\t', to);
@@ -625,50 +636,55 @@ mentries_fromfile_v1(struct metahash **mhash, const char *ptr, const char *max)
 			goto err;
 		if ((mtime = read_string_url(&ptr, max)) == NULL)
 			goto err;
-
-		/* Get the time_t part of `mtime` into mentry->mtime */
-		if ((nsec = strptime(mtime, "%Y-%m-%dT%H:%M:%S", &tm)) == NULL)
-			goto err;
-		tm.tm_isdst = 0;
-		mentry->mtime = timegm(&tm);
-
-		/* Check if there's a nanosecond portion of `mtime` */
-		if (*nsec == '.') {
-			/* There is a decimal point in mtime. Get nanoseconds. */
-			++nsec;
-			mentry->mtimensec = strtol(nsec, &tz, 10);
-			i = tz - nsec;
-			while (i < 9) {
-				/* Too few digits for nanosecond precision.
-				 * e.g. "...T10:15:23.5", which we've parsed as
-				 * 5 nanoseconds, but is actually half a second
-				 * or 500000000 nanoseconds.
-				 * Scale to correct value.
-				 */
-				mentry->mtimensec *= 10;
-				++i;
-			}
-			while (i > 9) {
-				/* Too many digits for nanosecond precision.
-				 * Scale to correct value
-				 */
-				mentry->mtimensec /= 10;
-				--i;
-			}
+		if (strcmp(mtime, "0") == 0) {
+			mentry->mtime = 0;
+			mentry->mtimensec = MTIME_NONE;
 		}
 		else {
-			/* No decimal point. Set nanoseconds to zero */
-			mentry->mtimensec = 0;
-			tz = nsec;
-		}
+			/* Get the time_t part of `mtime` into mentry->mtime */
+			if ((nsec = strptime(mtime, "%Y-%m-%dT%H:%M:%S", &tm)) == NULL)
+				goto err;
+			tm.tm_isdst = 0;
+			mentry->mtime = timegm(&tm);
+
+			/* Check if there's a nanosecond portion of `mtime` */
+			if (*nsec == '.') {
+				/* There is a decimal point in mtime. Get nanoseconds. */
+				++nsec;
+				mentry->mtimensec = strtol(nsec, &tz, 10);
+				i = tz - nsec;
+				while (i < 9) {
+					/* Too few digits for nanosecond precision.
+					 * e.g. "...T10:15:23.5", which we've parsed as
+					 * 5 nanoseconds, but is actually half a second
+					 * or 500000000 nanoseconds.
+					 * Scale to correct value.
+					 */
+					mentry->mtimensec *= 10;
+					++i;
+				}
+				while (i > 9) {
+					/* Too many digits for nanosecond precision.
+					 * Scale to correct value
+					 */
+					mentry->mtimensec /= 10;
+					--i;
+				}
+			}
+			else {
+				/* No decimal point. Set nanoseconds to zero */
+				mentry->mtimensec = 0;
+				tz = nsec;
+			}
 
-		/* Check for a 'Z' (Zulu/UTC) timesone specifier in `mtime`. */
-		if (*tz == 'Z')
-			++tz;
+			/* Check for a 'Z' (Zulu/UTC) timesone specifier in `mtime`. */
+			if (*tz == 'Z')
+				++tz;
 
-		/* Check that we've reached the end of `mtime` */
-		if (*tz != '\0')
-			goto err;
+			/* Check that we've reached the end of `mtime` */
+			if (*tz != '\0')
+				goto err;
+		}
 		free(mtime);
 		mtime = NULL;
 
@@ -842,10 +858,12 @@ mentry_compare(struct metaentry *left, struct metaentry *right, msettings *st)
 	if ((left->mode & S_IFMT) != (right->mode & S_IFMT))
 		retval |= DIFF_TYPE;
 
-	if (st->do_mtime && strcmp(left->path, st->metafile) &&
-	    (   left->mtime     != right->mtime
-	     || left->mtimensec != right->mtimensec)
-	   )
+	if (st->do_mtime > 0
+			&& left->mtimensec != MTIME_NONE
+			&& right->mtimensec != MTIME_NONE
+			&& strcmp(left->path, st->metafile)
+			&& (left->mtime != right->mtime
+				|| left->mtimensec != right->mtimensec))
 		retval |= DIFF_MTIME;
 
 	if (mentry_compare_xattr(left, right)) {
@@ -898,19 +916,28 @@ mentries_dump(struct metahash *mhash)
 	const struct metaentry *mentry;
 	char mode[11 + 1] = "";
 	char date[12 + 2 + 2 + 2*1 + 1 + 2 + 2 + 2 + 2*1 + 1] = "";
-	char zone[5 + 1] = "";
+	char nsec[1 + 9 + 1] = "";
+	char zone[1 + 5 + 1] = "";
 	struct tm cal;
 
 	for (int key = 0; key < HASH_INDEXES; key++) {
 		for (mentry = mhash->bucket[key]; mentry; mentry = mentry->next) {
 			strmode(mentry->mode, mode);
-			localtime_r(&mentry->mtime, &cal);
-			strftime(date, sizeof(date), "%F %T", &cal);
-			strftime(zone, sizeof(zone), "%z", &cal);
-			printf("%s\t%s\t%s\t%s.%09ld %s\t%s%s\n",
+			if (mentry->mtimensec == MTIME_NONE) {
+				snprintf(date, sizeof(date), "%19s", "");
+				snprintf(nsec, sizeof(nsec), "%10s", "");
+				snprintf(zone, sizeof(zone), "%6s", "");
+			}
+			else {
+				localtime_r(&mentry->mtime, &cal);
+				strftime(date, sizeof(date), "%F %T", &cal);
+				snprintf(nsec, sizeof(nsec), ".%09ld", mentry->mtimensec);
+				strftime(zone, sizeof(zone), " %z", &cal);
+			}
+			printf("%s\t%s\t%s\t%s%s%s\t%s%s\n",
 			       mode,
 			       mentry->owner, mentry->group,
-			       date, mentry->mtimensec, zone,
+			       date, nsec, zone,
 			       mentry->path, S_ISDIR(mentry->mode) ? "/" : "");
 			for (unsigned i = 0; i < mentry->xattrs; i++) {
 				printf("\t\t\t\t%s%s\t%s=",