bcachefs: Add error code to defer option parsing

This introduces a new error code, option_needs_open_fs, which is used to
indicate that an attempt was made to parse a mount option prior to
opening a filesystem, when that mount option requires an open filesystem
in order to be validated.

Returning this error results in bch2_parse_one_mount_opt() saving that
option for later parsing, after the filesystem is opened.
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/disk_groups.c

@@ -511,7 +511,7 @@ int bch2_opt_target_parse(struct bch_fs *c, const char *val, u64 *res,
 		return -EINVAL;
 
 	if (!c)
-		return 0;
+		return -BCH_ERR_option_needs_open_fs;
 
 	if (!strlen(val) || !strcmp(val, "none")) {
 		*res = 0;

fs/bcachefs/errcode.h

@@ -257,7 +257,8 @@
 	x(BCH_ERR_nopromote,		nopromote_no_writes)			\
 	x(BCH_ERR_nopromote,		nopromote_enomem)			\
 	x(0,				need_inode_lock)			\
-	x(0,				invalid_snapshot_node)
+	x(0,				invalid_snapshot_node)			\
+	x(0,				option_needs_open_fs)
 
 enum bch_errcode {
 	BCH_ERR_START		= 2048,

fs/bcachefs/opts.c

@@ -378,6 +378,10 @@ int bch2_opt_parse(struct bch_fs *c,
 		break;
 	case BCH_OPT_FN:
 		ret = opt->fn.parse(c, val, res, err);
+
+		if (ret == -BCH_ERR_option_needs_open_fs)
+			return ret;
+
 		if (ret < 0) {
 			if (err)
 				prt_printf(err, "%s: parse error",
@@ -495,6 +499,17 @@ int bch2_parse_one_mount_opt(struct bch_fs *c, struct bch_opts *opts,
 		goto bad_opt;
 
 	ret = bch2_opt_parse(c, &bch2_opt_table[id], val, &v, &err);
+	if (ret == -BCH_ERR_option_needs_open_fs && parse_later) {
+		prt_printf(parse_later, "%s=%s,", name, val);
+		if (parse_later->allocation_failure) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		ret = 0;
+		goto out;
+	}
+
 	if (ret < 0)
 		goto bad_val;