Handle mount with FS with 1 device

If the user passes a single device node during the mount and
we have no information for it in the udev db, we read up the
super block.  When we do this, if the FS only has 1 block device
we will simply go ahead and do the mount instead of walking
all the block devices and reading up super blocks looking for
devices with a matching FS UUID.

Signed-off-by: Tony Asleson <tasleson@redhat.com>

bch_bindgen/src/bcachefs.rs

@@ -59,6 +59,10 @@ impl bch_sb {
         uuid::Uuid::from_bytes(self.user_uuid.b)
     }
 
+    pub fn number_of_devices(&self) -> u8 {
+        self.nr_devices
+    }
+
     /// Get the nonce used to encrypt the superblock
     pub fn nonce(&self) -> nonce {
         use byteorder::{LittleEndian, ReadBytesExt};

src/commands/mount.rs

@@ -208,7 +208,7 @@ fn get_devices_by_uuid(
 fn get_uuid_for_dev_node(
     udev_bcachefs: &HashMap<String, Vec<String>>,
     device: &std::path::PathBuf,
-) -> anyhow::Result<Option<Uuid>> {
+) -> anyhow::Result<(Option<Uuid>, Option<(PathBuf, bch_sb_handle)>)> {
     let canonical = fs::canonicalize(device)?;
 
     if !udev_bcachefs.is_empty() {
@@ -216,12 +216,14 @@ fn get_uuid_for_dev_node(
 
         if udev_bcachefs.contains_key(&dev_node_str) && udev_bcachefs[&dev_node_str].len() == 1 {
             let uuid_str = udev_bcachefs[&dev_node_str][0].clone();
-            return Ok(Some(Uuid::parse_str(&uuid_str)?));
+            return Ok((Some(Uuid::parse_str(&uuid_str)?), None));
         }
     } else {
-        return read_super_silent(&canonical).map_or(Ok(None), |sb| Ok(Some(sb.sb().uuid())));
+        return read_super_silent(&canonical).map_or(Ok((None, None)), |sb| {
+            Ok((Some(sb.sb().uuid()), Some((canonical, sb))))
+        });
     }
-    Ok(None)
+    Ok((None, None))
 }
 
 /// Mount a bcachefs filesystem by its UUID.
@@ -289,12 +291,26 @@ fn devs_str_sbs_from_device(
     udev_info: &HashMap<String, Vec<String>>,
     device: &std::path::PathBuf,
 ) -> anyhow::Result<(String, Vec<bch_sb_handle>)> {
-    let uuid = get_uuid_for_dev_node(udev_info, device)?;
+    let (uuid, sb_info) = get_uuid_for_dev_node(udev_info, device)?;
 
-    if let Some(bcache_fs_uuid) = uuid {
-        devs_str_sbs_from_uuid(udev_info, bcache_fs_uuid.to_string())
-    } else {
-        Ok((String::new(), Vec::new()))
+    match (uuid, sb_info) {
+        (Some(uuid), Some((path, sb))) => {
+            // If we have a super block, it implies we aren't using udev db.  If we only need
+            // 1 device to mount, we'll simply return it as we're done, else we'll use the uuid
+            // to walk through all the block devices.
+            debug!(
+                "number of devices in this FS = {}",
+                sb.sb().number_of_devices()
+            );
+            if sb.sb().number_of_devices() == 1 {
+                let dev = path.into_os_string().into_string().unwrap();
+                Ok((dev, vec![sb]))
+            } else {
+                devs_str_sbs_from_uuid(udev_info, uuid.to_string())
+            }
+        }
+        (Some(uuid), None) => devs_str_sbs_from_uuid(udev_info, uuid.to_string()),
+        _ => Ok((String::new(), Vec::new())),
     }
 }