[PATCH] btrfs-progs: Fix get_partition_sector_size_sysfs() to handle loopback and device mapper devices

[Zoltan Racz <racz.zoli@gmail.com>]

Commit e39ed66 added get_partition_sector_size_sysfs() used by "btrfs device usage"
which returns the sector size of a partition (or its parent). After more testing 
it turned out it couldn`t handle loopback or mapper devices. This patch adds a fix
for them.

Signed-off-by: Zoltan Racz <racz.zoli@gmail.com>
---
 common/device-utils.c | 48 +++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 15 deletions(-)

diff --git a/common/device-utils.c b/common/device-utils.c
index dd781bc5..a75194bf 100644
--- a/common/device-utils.c
+++ b/common/device-utils.c
@@ -353,26 +353,44 @@ static ssize_t get_partition_sector_size_sysfs(const char *name)
 	char sysfs[PATH_MAX] = {};
 	char sizebuf[128];
 
-	snprintf(link_path, PATH_MAX, "/sys/class/block/%s/..", name);
+	/*
+	 * First we look for hw_sector_size directly directly under 
+	 * /sys/class/block/[partition_name]/queue. In case of loopback and
+	 * device mapper devices there is no parent device (like /dev/sda1 -> /dev/sda), 
+	 * and the partition`s sysfs folder itself contains informations regarding 
+	 * the sector size
+	 */
+	snprintf(sysfs, PATH_MAX, "/sys/class/block/%s/queue/hw_sector_size", name);
+	sysfd = open(sysfs, O_RDONLY);
 
-	if (!realpath(link_path, real_path)) {
-		error("Failed to resolve realpath of %s: %s\n", link_path, strerror(errno));
-		return -1;
-	}
+	if (sysfd < 0) {
+		/*
+		 * If we couldn`t find it, it means our partition is created on a real 
+		 * device and we need to find its parent
+		 */
+		snprintf(link_path, PATH_MAX, "/sys/class/block/%s/..", name);
 
-	dev_name = basename(real_path);
+		if (!realpath(link_path, real_path)) {
+			error("Failed to resolve realpath of %s: %s\n", link_path, strerror(errno));
+			return -1;
+		}
 
-	if (!dev_name) {
-		error("Failed to determine basename for path %s\n", real_path);
-		return -1;
-	}
+		dev_name = basename(real_path);
 
-	snprintf(sysfs, PATH_MAX, "/sys/class/block/%s/queue/hw_sector_size", dev_name);
+		if (!dev_name) {
+			error("Failed to determine basename for path %s\n", real_path);
+			return -1;
+		}
 
-	sysfd = open(sysfs, O_RDONLY);
-	if (sysfd < 0) {
-		error("Error opening %s to determine dev sector size: %s\n", real_path, strerror(errno));
-		return -1;
+		memset(sysfs, 0, PATH_MAX);
+		snprintf(sysfs, PATH_MAX, "/sys/class/block/%s/queue/hw_sector_size", dev_name);
+
+		sysfd = open(sysfs, O_RDONLY);
+
+		if (sysfd < 0) {
+			error("Error opening %s to determine dev sector size: %s\n", real_path, strerror(errno));
+			return -1;
+		}
 	}
 
 	ret = sysfs_read_file(sysfd, sizebuf, sizeof(sizebuf));
-- 
2.48.1