fsck: support demoting errors to warnings

We already have support in `git receive-pack` to deal with some legacy
repositories which have non-fatal issues.

Let's make `git fsck` itself useful with such repositories, too, by
allowing users to ignore known issues, or at least demote those issues
to mere warnings.

Example: `git -c fsck.missingEmail=ignore fsck` would hide
problems with missing emails in author, committer and tagger lines.

In the same spirit that `git receive-pack`'s usage of the fsck machinery
differs from `git fsck`'s – some of the non-fatal warnings in `git fsck`
are fatal with `git receive-pack` when receive.fsckObjects = true, for
example – we strictly separate the fsck.<msg-id> from the
receive.fsck.<msg-id> settings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1242,6 +1242,17 @@ filter.<driver>.smudge::
 	object to a worktree file upon checkout.  See
 	linkgit:gitattributes[5] for details.
 
+fsck.<msg-id>::
+	Allows overriding the message type (error, warn or ignore) of a
+	specific message ID such as `missingEmail`.
++
+For convenience, fsck prefixes the error/warning with the message ID,
+e.g.  "missingEmail: invalid author/committer line - missing email" means
+that setting `fsck.missingEmail = ignore` will hide that issue.
++
+This feature is intended to support working with legacy repositories
+which cannot be repaired without disruptive changes.
+
 gc.aggressiveDepth::
 	The depth parameter used in the delta compression
 	algorithm used by 'git gc --aggressive'.  This defaults

builtin/fsck.c

@@ -46,6 +46,16 @@ static int show_dangling = 1;
 #define DIRENT_SORT_HINT(de) ((de)->d_ino)
 #endif
 
+static int fsck_config(const char *var, const char *value, void *cb)
+{
+	if (skip_prefix(var, "fsck.", &var)) {
+		fsck_set_msg_type(&fsck_obj_options, var, value);
+		return 0;
+	}
+
+	return git_default_config(var, value, cb);
+}
+
 static void objreport(struct object *obj, const char *msg_type,
 			const char *err)
 {
@@ -640,6 +650,8 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 		include_reflogs = 0;
 	}
 
+	git_config(fsck_config, NULL);
+
 	fsck_head_link();
 	fsck_object_dir(get_object_directory());
 

t/t1450-fsck.sh

@@ -287,6 +287,17 @@ test_expect_success 'rev-list --verify-objects with bad sha1' '
 	grep -q "error: sha1 mismatch 63ffffffffffffffffffffffffffffffffffffff" out
 '
 
+test_expect_success 'force fsck to ignore double author' '
+	git cat-file commit HEAD >basis &&
+	sed "s/^author .*/&,&/" <basis | tr , \\n >multiple-authors &&
+	new=$(git hash-object -t commit -w --stdin <multiple-authors) &&
+	test_when_finished "remove_object $new" &&
+	git update-ref refs/heads/bogus "$new" &&
+	test_when_finished "git update-ref -d refs/heads/bogus" &&
+	test_must_fail git fsck &&
+	git -c fsck.multipleAuthors=ignore fsck
+'
+
 _bz='\0'
 _bz5="$_bz$_bz$_bz$_bz$_bz"
 _bz20="$_bz5$_bz5$_bz5$_bz5"