btrfs-progs: qgroup show: use sysfs to detect inconsistent state early

Currently if "btrfs qgroup show" detects the qgroup is in an
inconsistent state, it will print a warning:

  WARNING: qgroup data inconsistent, rescan recommended

But the detection is based on the tree search ioctl, and qgroup tree is
only updated at transaction commit time.

This means if qgroup is marked inconsistent, and the transaction is not
commit, there can be a time window as long as 30s before "btrfs qgroup
show" gives a proper warning about inconsistent qgroup numbers.

To address this, use the
'/sys/fs/btrfs/<fsid>/qgroup/inconsistent' file to do extra check.

That file is updated at real time, thus there is no delay, and can give
an early warning about inconsistent qgroup numbers.

Link: https://bugzilla.suse.com/show_bug.cgi?id=1235765
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

cmds/qgroup.c

@@ -36,6 +36,7 @@
 #include "kernel-shared/uapi/btrfs.h"
 #include "kernel-shared/ctree.h"
 #include "common/open-utils.h"
+#include "common/sysfs-utils.h"
 #include "common/utils.h"
 #include "common/help.h"
 #include "common/units.h"
@@ -1610,6 +1611,31 @@ static void print_all_qgroups_json(struct qgroup_lookup *qgroup_lookup)
 	fmt_end(&fctx);
 }
 
+/*
+ * Tree search based 'inconsistent' flag is only updated at transaction commit
+ * time.  Thus even if the qgroup_status flag shows consistent, the qgroup may
+ * already be in an inconsistent state.
+ */
+static void check_qgroup_sysfs_inconsistent(int fd, const struct qgroup_lookup *qgroup_lookup)
+{
+	u64 value;
+	int sysfs_fd;
+	int ret;
+
+	if (qgroup_lookup->flags & BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT)
+		return;
+	sysfs_fd = sysfs_open_fsid_file(fd, "qgroups/inconsistent");
+	if (fd < 0)
+		return;
+	ret = sysfs_read_fsid_file_u64(fd, "qgroups/inconsistent", &value);
+	if (ret < 0)
+		goto out;
+	if (value)
+		warning("qgroup data inconsistent, rescan recommended");
+out:
+	close(sysfs_fd);
+}
+
 static int show_qgroups(int fd,
 		       struct btrfs_qgroup_filter_set *filter_set,
 		       struct btrfs_qgroup_comparer_set *comp_set)
@@ -1622,6 +1648,8 @@ static int show_qgroups(int fd,
 	ret = qgroups_search_all(fd, &qgroup_lookup);
 	if (ret)
 		return ret;
+
+	check_qgroup_sysfs_inconsistent(fd, &qgroup_lookup);
 	__filter_and_sort_qgroups(&qgroup_lookup, &sort_tree,
 				  filter_set, comp_set);
 	if (bconf.output_format == CMD_FORMAT_JSON)