fsck: make fsck_config() re-usable

Move the fsck_config() function from builtin/fsck.c to fsck.[ch]. This
allows for re-using it in other tools that expose fsck logic and want
to support its configuration variables.

A logical continuation of this change would be to use a common
function for all of {fetch,receive}.fsck.* and fsck.*. See
5d477a334a (fsck (receive-pack): allow demoting errors to warnings,
2015-06-22) and my own 1362df0d41 (fetch: implement fetch.fsck.*,
2018-07-27) for the relevant code.

However, those routines want to not parse the fsck.skipList into OIDs,
but rather pass them along with the --strict option to another
process. It would be possible to refactor that whole thing so we
support e.g. a "fetch." prefix, then just keep track of the skiplist
as a filename instead of parsing it, and learn to spew that all out
from our internal structures into something we can append to the
--strict option.

But instead I'm planning to re-use this in "mktag", which'll just
re-use these "fsck.*" variables as-is.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fsck.c

@@ -73,25 +73,7 @@ static const char *printable_type(const struct object_id *oid,
 
 static int fsck_config(const char *var, const char *value, void *cb)
 {
-	if (strcmp(var, "fsck.skiplist") == 0) {
-		const char *path;
-		struct strbuf sb = STRBUF_INIT;
-
-		if (git_config_pathname(&path, var, value))
-			return 1;
-		strbuf_addf(&sb, "skiplist=%s", path);
-		free((char *)path);
-		fsck_set_msg_types(&fsck_obj_options, sb.buf);
-		strbuf_release(&sb);
-		return 0;
-	}
-
-	if (skip_prefix(var, "fsck.", &var)) {
-		fsck_set_msg_type(&fsck_obj_options, var, value);
-		return 0;
-	}
-
-	return git_default_config(var, value, cb);
+	return fsck_config_internal(var, value, cb, &fsck_obj_options);
 }
 
 static int objerror(struct object *obj, const char *err)

fsck.c

@@ -1310,3 +1310,27 @@ int fsck_finish(struct fsck_options *options)
 	oidset_clear(&gitmodules_done);
 	return ret;
 }
+
+int fsck_config_internal(const char *var, const char *value, void *cb,
+			 struct fsck_options *options)
+{
+	if (strcmp(var, "fsck.skiplist") == 0) {
+		const char *path;
+		struct strbuf sb = STRBUF_INIT;
+
+		if (git_config_pathname(&path, var, value))
+			return 1;
+		strbuf_addf(&sb, "skiplist=%s", path);
+		free((char *)path);
+		fsck_set_msg_types(options, sb.buf);
+		strbuf_release(&sb);
+		return 0;
+	}
+
+	if (skip_prefix(var, "fsck.", &var)) {
+		fsck_set_msg_type(options, var, value);
+		return 0;
+	}
+
+	return git_default_config(var, value, cb);
+}

fsck.h

@@ -103,4 +103,11 @@ void fsck_put_object_name(struct fsck_options *options,
 const char *fsck_describe_object(struct fsck_options *options,
 				 const struct object_id *oid);
 
+/*
+ * git_config() callback for use by fsck-y tools that want to support
+ * fsck.<msg> fsck.skipList etc.
+ */
+int fsck_config_internal(const char *var, const char *value, void *cb,
+			 struct fsck_options *options);
+
 #endif