efi: Handle already-mounted ESP in mount_esp_device

When running install-to-filesystem, the host may already have the ESP
mounted (e.g. /dev/sda15 at /boot/efi). Previously mount_esp_device
would unconditionally call mount, which fails with "already mounted".

Check if the mount point already contains a vfat filesystem on a
different device than its parent (i.e. is a real mount point) before
attempting to mount, and reuse the existing mount if so.

Assisted-by: Claude Code (Opus 4)
Signed-off-by: ckyrouac <ckyrouac@redhat.com>

Author: ckyrouac

src/efi.rs

@@ -5,7 +5,6 @@
  */
 
 use std::cell::RefCell;
-use std::os::unix::fs::MetadataExt;
 use std::os::unix::io::AsRawFd;
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -61,6 +60,23 @@ const STUB_INFO_VAR_STR: &str = "StubInfo-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f";
 /// The options of cp command for installation
 const OPTIONS: &[&str] = &["-rp", "--reflink=auto"];
 
+/// Check if the given path is a mount point via statx(MOUNT_ROOT).
+fn is_mount_point(path: &Path) -> Result<bool> {
+    let parent = path.parent().unwrap_or(path);
+    let parent = if parent.as_os_str().is_empty() {
+        Path::new(".")
+    } else {
+        parent
+    };
+    let parent_dir = Dir::open_ambient_dir(parent, cap_std::ambient_authority())?;
+    let basename = path
+        .file_name()
+        .ok_or_else(|| anyhow::anyhow!("path has no file name: {path:?}"))?;
+    parent_dir
+        .is_mountpoint(basename)?
+        .ok_or_else(|| anyhow::anyhow!("could not determine if {path:?} is a mount point"))
+}
+
 /// Return `true` if the system is booted via EFI
 pub(crate) fn is_efi_booted() -> Result<bool> {
     Path::new("/sys/firmware/efi")
@@ -91,9 +107,7 @@ impl Efi {
                 // different device than its parent. Without this check, a vfat
                 // subdirectory (e.g. /boot/efi on a vfat /boot) could be
                 // misidentified as a mounted ESP.
-                let path_dev = std::fs::metadata(&path)?.dev();
-                let parent_dev = std::fs::metadata(path.parent().unwrap_or(&path))?.dev();
-                if path_dev == parent_dev {
+                if !is_mount_point(&path)? {
                     // Same device as parent - this is a subdirectory, not a mount point
                     log::debug!("Skipping {path:?}: vfat but not a mount point");
                     continue;
@@ -123,6 +137,18 @@ impl Efi {
             if !mnt.exists() {
                 continue;
             }
+
+            // Check if the target is already a mounted ESP (e.g. the host
+            // already has the ESP mounted when running install-to-filesystem).
+            let st = rustix::fs::statfs(&mnt)?;
+            if st.f_type == libc::MSDOS_SUPER_MAGIC {
+                if is_mount_point(&mnt)? {
+                    log::debug!("ESP already mounted at {mnt:?}, reusing");
+                    mountpoint = Some(mnt);
+                    break;
+                }
+            }
+
             std::process::Command::new("mount")
                 .arg(&esp_device)
                 .arg(&mnt)