Allow fast-import frontends to reload the marks table

I'm giving fast-import a lesson on how to reload the marks table
using the same format it outputs with --export-marks.  This way
a frontend can reload the marks table from a prior import, making
incremental imports less painful.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-fast-import.txt

@@ -62,7 +62,18 @@ OPTIONS
 	Dumps the internal marks table to <file> when complete.
 	Marks are written one per line as `:markid SHA-1`.
 	Frontends can use this file to validate imports after they
-	have been completed.
+	have been completed, or to save the marks table across
+	incremental runs.  As <file> is only opened and truncated
+	at checkpoint (or completion) the same path can also be
+	safely given to \--import-marks.
+
+--import-marks=<file>::
+	Before processing any input, load the marks specified in
+	<file>.  The input file must exist, must be readable, and
+	must use the same format as produced by \--export-marks.
+	Multiple options may be supplied to import more than one
+	set of marks.  If a mark is defined to different values,
+	the last file wins.
 
 --export-pack-edges=<file>::
 	After creating a packfile, print a line of data to

fast-import.c

@@ -1997,6 +1997,40 @@ static void cmd_checkpoint(void)
 	read_next_command();
 }
 
+static void import_marks(const char *input_file)
+{
+	char line[512];
+	FILE *f = fopen(input_file, "r");
+	if (!f)
+		die("cannot read %s: %s", input_file, strerror(errno));
+	while (fgets(line, sizeof(line), f)) {
+		uintmax_t mark;
+		char *end;
+		unsigned char sha1[20];
+		struct object_entry *e;
+
+		end = strchr(line, '\n');
+		if (line[0] != ':' || !end)
+			die("corrupt mark line: %s", line);
+		*end = 0;
+		mark = strtoumax(line + 1, &end, 10);
+		if (!mark || end == line + 1
+			|| *end != ' ' || get_sha1(end + 1, sha1))
+			die("corrupt mark line: %s", line);
+		e = find_object(sha1);
+		if (!e) {
+			enum object_type type = sha1_object_info(sha1, NULL);
+			if (type < 0)
+				die("object not found: %s", sha1_to_hex(sha1));
+			e = insert_object(sha1);
+			e->type = type;
+			e->pack_id = MAX_PACK_ID;
+		}
+		insert_mark(mark, e);
+	}
+	fclose(f);
+}
+
 static const char fast_import_usage[] =
 "git-fast-import [--date-format=f] [--max-pack-size=n] [--depth=n] [--active-branches=n] [--export-marks=marks.file]";
 
@@ -2034,6 +2068,8 @@ int main(int argc, const char **argv)
 			max_depth = strtoul(a + 8, NULL, 0);
 		else if (!prefixcmp(a, "--active-branches="))
 			max_active_branches = strtoul(a + 18, NULL, 0);
+		else if (!prefixcmp(a, "--import-marks="))
+			import_marks(a + 15);
 		else if (!prefixcmp(a, "--export-marks="))
 			mark_file = a + 15;
 		else if (!prefixcmp(a, "--export-pack-edges=")) {

t/t9300-fast-import.sh

@@ -111,6 +111,14 @@ test_expect_success \
 	'A: verify marks output' \
 	'diff -u expect marks.out'
 
+test_expect_success \
+	'A: verify marks import' \
+	'git-fast-import \
+		--import-marks=marks.out \
+		--export-marks=marks.new \
+		</dev/null &&
+	diff -u expect marks.new'
+
 ###
 ### series B
 ###