pack-objects: proper pack time stamping with --max-pack-size

Runtime pack access is done in the pack file mtime order since recent
packs are more likely to contain frequently used objects than old packs.
However the --max-pack-size option can produce multiple packs with mtime
in the reversed order as newer objects are always written first.

Let's modify mtime of later pack files (when any) so they appear older
than preceding ones when a repack creates multiple packs.

Signed-off-by: Nicolas Pitre <nico@cam.org>

builtin-pack-objects.c

@@ -454,6 +454,7 @@ static void write_pack_file(void)
 	struct pack_header hdr;
 	int do_progress = progress >> pack_to_stdout;
 	uint32_t nr_remaining = nr_result;
+	time_t last_mtime = 0;
 
 	if (do_progress)
 		progress_state = start_progress("Writing objects", nr_result);
@@ -504,13 +505,15 @@ static void write_pack_file(void)
 
 		if (!pack_to_stdout) {
 			mode_t mode = umask(0);
+			struct stat st;
 			char *idx_tmp_name, tmpname[PATH_MAX];
 
 			umask(mode);
 			mode = 0444 & ~mode;
 
 			idx_tmp_name = write_idx_file(NULL, written_list,
 						      nr_written, sha1);
+
 			snprintf(tmpname, sizeof(tmpname), "%s-%s.pack",
 				 base_name, sha1_to_hex(sha1));
 			if (adjust_perm(pack_tmp_name, mode))
@@ -519,6 +522,28 @@ static void write_pack_file(void)
 			if (rename(pack_tmp_name, tmpname))
 				die("unable to rename temporary pack file: %s",
 				    strerror(errno));
+
+			/*
+			 * Packs are runtime accessed in their mtime
+			 * order since newer packs are more likely to contain
+			 * younger objects.  So if we are creating multiple
+			 * packs then we should modify the mtime of later ones
+			 * to preserve this property.
+			 */
+			if (stat(tmpname, &st) < 0) {
+				warning("failed to stat %s: %s",
+					tmpname, strerror(errno));
+			} else if (!last_mtime) {
+				last_mtime = st.st_mtime;
+			} else {
+				struct utimbuf utb;
+				utb.actime = st.st_atime;
+				utb.modtime = --last_mtime;
+				if (utime(tmpname, &utb) < 0)
+					warning("failed utime() on %s: %s",
+						tmpname, strerror(errno));
+			}
+
 			snprintf(tmpname, sizeof(tmpname), "%s-%s.idx",
 				 base_name, sha1_to_hex(sha1));
 			if (adjust_perm(idx_tmp_name, mode))
@@ -527,6 +552,7 @@ static void write_pack_file(void)
 			if (rename(idx_tmp_name, tmpname))
 				die("unable to rename temporary index file: %s",
 				    strerror(errno));
+
 			free(idx_tmp_name);
 			free(pack_tmp_name);
 			puts(sha1_to_hex(sha1));

git-compat-util.h

@@ -68,6 +68,7 @@
 #include <sys/poll.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <utime.h>
 #ifndef NO_SYS_SELECT_H
 #include <sys/select.h>
 #endif