fast-import: Introduce --import-marks-if-exists

When a frontend uses a marks file to ensure its state persists between
runs, it may represent "clean slate" when bootstrapping with "no marks
yet". In such a case, feeding the last state with --import-marks and
saving the state after the current run with --export-marks would be a
natural thing to do.

The --import-marks option however errors out when the specified marks file
doesn't exist; this makes bootstrapping a bit difficult.  The location of
the marks file becomes backend-dependent when --relative-marks is in
effect, and the frontend cannot check for the existence of the file in
such a case.

The --import-marks-if-exists option does the same thing as --import-marks
but does not flag an error if the named file does not exist yet to help
these frontends.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -78,6 +78,10 @@ OPTIONS
 	set of marks.  If a mark is defined to different values,
 	the last file wins.
 
+--import-marks-if-exists=<file>::
+	Like --import-marks but instead of erroring out, silently
+	skips the file if it does not exist.
+
 --relative-marks::
 	After specifying --relative-marks= the paths specified
 	with --import-marks= and --export-marks= are relative

fast-import.c

@@ -329,6 +329,7 @@ static struct mark_set *marks;
 static const char *export_marks_file;
 static const char *import_marks_file;
 static int import_marks_file_from_stream;
+static int import_marks_file_ignore_missing;
 static int relative_marks_paths;
 
 /* Our last blob */
@@ -1795,7 +1796,11 @@ static void read_marks(void)
 {
 	char line[512];
 	FILE *f = fopen(import_marks_file, "r");
-	if (!f)
+	if (f)
+		;
+	else if (import_marks_file_ignore_missing && errno == ENOENT)
+		return; /* Marks file does not exist */
+	else
 		die_errno("cannot read '%s'", import_marks_file);
 	while (fgets(line, sizeof(line), f)) {
 		uintmax_t mark;
@@ -2861,7 +2866,8 @@ static char* make_fast_import_path(const char *path)
 	return strbuf_detach(&abs_path, NULL);
 }
 
-static void option_import_marks(const char *marks, int from_stream)
+static void option_import_marks(const char *marks,
+					int from_stream, int ignore_missing)
 {
 	if (import_marks_file) {
 		if (from_stream)
@@ -2875,6 +2881,7 @@ static void option_import_marks(const char *marks, int from_stream)
 	import_marks_file = make_fast_import_path(marks);
 	safe_create_leading_directories_const(import_marks_file);
 	import_marks_file_from_stream = from_stream;
+	import_marks_file_ignore_missing = ignore_missing;
 }
 
 static void option_date_format(const char *fmt)
@@ -2974,7 +2981,10 @@ static int parse_one_feature(const char *feature, int from_stream)
 	if (!prefixcmp(feature, "date-format=")) {
 		option_date_format(feature + 12);
 	} else if (!prefixcmp(feature, "import-marks=")) {
-		option_import_marks(feature + 13, from_stream);
+		option_import_marks(feature + 13, from_stream, 0);
+	} else if (!prefixcmp(feature, "import-marks-if-exists=")) {
+		option_import_marks(feature + strlen("import-marks-if-exists="),
+					from_stream, 1);
 	} else if (!prefixcmp(feature, "export-marks=")) {
 		option_export_marks(feature + 13);
 	} else if (!strcmp(feature, "cat-blob")) {

t/t9300-fast-import.sh

@@ -1706,6 +1706,61 @@ test_expect_success \
     'cat input | git fast-import --export-marks=other.marks &&
     grep :1 other.marks'
 
+test_expect_success 'R: catch typo in marks file name' '
+	test_must_fail git fast-import --import-marks=nonexistent.marks </dev/null &&
+	echo "feature import-marks=nonexistent.marks" |
+	test_must_fail git fast-import
+'
+
+test_expect_success 'R: import and output marks can be the same file' '
+	rm -f io.marks &&
+	blob=$(echo hi | git hash-object --stdin) &&
+	cat >expect <<-EOF &&
+	:1 $blob
+	:2 $blob
+	EOF
+	git fast-import --export-marks=io.marks <<-\EOF &&
+	blob
+	mark :1
+	data 3
+	hi
+
+	EOF
+	git fast-import --import-marks=io.marks --export-marks=io.marks <<-\EOF &&
+	blob
+	mark :2
+	data 3
+	hi
+
+	EOF
+	test_cmp expect io.marks
+'
+
+test_expect_success 'R: --import-marks=foo --output-marks=foo to create foo fails' '
+	rm -f io.marks &&
+	test_must_fail git fast-import --import-marks=io.marks --export-marks=io.marks <<-\EOF
+	blob
+	mark :1
+	data 3
+	hi
+
+	EOF
+'
+
+test_expect_success 'R: --import-marks-if-exists' '
+	rm -f io.marks &&
+	blob=$(echo hi | git hash-object --stdin) &&
+	echo ":1 $blob" >expect &&
+	git fast-import --import-marks-if-exists=io.marks --export-marks=io.marks <<-\EOF &&
+	blob
+	mark :1
+	data 3
+	hi
+
+	EOF
+	test_cmp expect io.marks
+'
+
 cat >input << EOF
 feature import-marks=marks.out
 feature export-marks=marks.new