rootmap: use full path for 'root=' karg when rootfs is directly on multipath

Issue: coreos/fedora-coreos-tracker#1980

This was first observed on s390x builders under high system load, where `ext.config.multipath.resilient`
would intermittently fail during subsequent boot:
```
[    2.781559] multipathd[321]: sdd [8:48]: path added to devmap 0xcadf6fadb3ee446d
[    2.853163] multipathd[321]: sdb [8:16]: path added to devmap 0x000000000000000b
[    3.012431] systemd[1]: Reached target coreos-multipath-wait.target - CoreOS Wait For Multipathed Boot.
[    3.139605] systemd[1]: Mounting sysroot.mount - /sysroot...
[    3.450666] mount[806]: mount: /sysroot: fsconfig system call failed: /dev/sdd4: Can't open blockdev.
```

It looks like a race condition between multipathd taking ownership of the root device and systemd trying to
mount /sysroot. This might happen because the udev database isn't ready in time.

Signed-off-by: Nikita Dubrovskii <nikita@linux.ibm.com>

Author: nikita-dubrovskii

docs/release-notes.md

@@ -17,6 +17,7 @@ Internal changes:
 - Add initial TMT tests and a new workflow to execute tests on PRs
 - Use release profile for smaller binaries in CI testing
 - Support cryptsetup-2.8.0's UUIDs naming of dm-integrity devices
+- rootmap: Inject `root=/dev/disk/by-uuid/dm-mpath-$UUID` when on multipath
 
 Packaging changes:
 

src/bin/rdcore/rootmap.rs

@@ -41,6 +41,8 @@ pub fn rootmap(config: RootmapConfig) -> Result<()> {
 
     // and from that we can collect all the parent backing devices too
     let mut backing_devices = get_blkdev_deps_recursing(&device)?;
+    // check if device's parent (last in the list) is a mpath device
+    let on_multipath = is_on_multipath(&backing_devices)?;
     backing_devices.push(device);
 
     // for each of those, convert them to kargs
@@ -51,13 +53,22 @@ pub fn rootmap(config: RootmapConfig) -> Result<()> {
         }
     }
 
+    // use full path when the rootfs is directly on multipath;
+    // this prevents race condition between systemd's autogenerated sysroot.mount and multipathd/udev,
+    // see also: https://github.com/coreos/fedora-coreos-tracker/issues/1980
+    let root = if on_multipath {
+        // https://github.com/coreos/fedora-coreos-config/blob/testing-devel/overlay.d/05core/usr/lib/udev/rules.d/90-coreos-device-mapper.rules#L25
+        format!(
+            "root=/dev/disk/by-uuid/dm-mpath-{}",
+            physical_mount.get_filesystem_uuid()?
+        )
+    } else {
+        format!("root=UUID={}", physical_mount.get_filesystem_uuid()?)
+    };
     // we push the root kargs last, this has the nice property that the final order of kargs goes
     // from lowest level to highest; see also
     // https://github.com/coreos/fedora-coreos-tracker/issues/465
-    kargs.push(format!(
-        "root=UUID={}",
-        physical_mount.get_filesystem_uuid()?
-    ));
+    kargs.push(root);
 
     // we need this because with root= it's systemd that takes care of mounting via
     // systemd-fstab-generator, and it defaults to read-only otherwise
@@ -307,3 +318,12 @@ fn write_boot_uuid_grub2_dropin<P: AsRef<Path>>(uuid: &str, p: P) -> Result<()>
     std::fs::write(p, format!("set BOOT_UUID=\"{uuid}\"\n"))
         .with_context(|| format!("writing {}", p.display()))
 }
+
+fn is_on_multipath(backing_devices: &[PathBuf]) -> Result<bool> {
+    let blkinfo = match backing_devices.last() {
+        Some(p) => lsblk_single(p)?,
+        _ => return Ok(false),
+    };
+
+    Ok(blkinfo.get("TYPE").is_some_and(|t| t == "mpath"))
+}