mount: Factor out a run_findmnt helper

Prep for adding more API.

Signed-off-by: Colin Walters <walters@verbum.org>

lib/src/mount.rs

@@ -22,17 +22,17 @@ pub(crate) struct Findmnt {
     pub(crate) filesystems: Vec<Filesystem>,
 }
 
-#[context("Inspecting filesystem {path}")]
-pub(crate) fn inspect_filesystem(path: &Utf8Path) -> Result<Filesystem> {
+fn run_findmnt(args: &[&str], path: &str) -> Result<Filesystem> {
     let desc = format!("Inspecting {path}");
     let o = Task::new(desc, "findmnt")
         .args([
             "-J",
             "-v",
             // If you change this you probably also want to change the Filesystem struct above
             "--output=SOURCE,FSTYPE,OPTIONS,UUID",
-            path.as_str(),
         ])
+        .args(args)
+        .arg(path)
         .quiet()
         .read()?;
     let o: Findmnt = serde_json::from_str(&o).context("Parsing findmnt output")?;
@@ -42,6 +42,13 @@ pub(crate) fn inspect_filesystem(path: &Utf8Path) -> Result<Filesystem> {
         .ok_or_else(|| anyhow!("findmnt returned no data for {path}"))
 }
 
+#[context("Inspecting filesystem {path}")]
+/// Inspect a target which must be a mountpoint root - it is an error
+/// if the target is not the mount root.
+pub(crate) fn inspect_filesystem(path: &Utf8Path) -> Result<Filesystem> {
+    run_findmnt(&["--mountpoint"], path.as_str())
+}
+
 /// Mount a device to the target path.
 pub(crate) fn mount(dev: &str, target: &Utf8Path) -> Result<()> {
     Task::new_and_run(