install to-disk: Check for mounts

[cgwalters]

Split from #718 (review)

we definitely need to strengthen the to-disk path to bail hard if the device is mounted for example, even if --wipe is specified.

I looked at implementing this like the below, but it doesn't work because our mount namespace won't have the physical devices mounted.

Instead, I think we can implement this by using findmnt -N 1 --source /path/to/dev for each device...or just scraping the overall findmnt -N 1 and matching up the devices on our own.

From c86a452b05a7b94c341269e147bc31d4c0a82feb Mon Sep 17 00:00:00 2001
From: Colin Walters <walters@verbum.org>
Date: Tue, 23 Jul 2024 12:34:55 -0400
Subject: [PATCH] wip

Signed-off-by: Colin Walters <walters@verbum.org>
---
 lib/src/blockdev.rs         | 17 +++++++++++++++++
 lib/src/install/baseline.rs |  4 ++++
 2 files changed, 21 insertions(+)

diff --git a/lib/src/blockdev.rs b/lib/src/blockdev.rs
index 3c69080..f18e187 100644
--- a/lib/src/blockdev.rs
+++ b/lib/src/blockdev.rs
@@ -36,6 +36,9 @@ pub(crate) struct Device {
     // Filesystem-related properties
     pub(crate) label: Option<String>,
     pub(crate) fstype: Option<String>,
+
+    // VFS properties
+    pub(crate) mountpoints: Option<Vec<String>>,
 }
 
 impl Device {
@@ -49,6 +52,20 @@ impl Device {
         self.children.as_ref().map_or(false, |v| !v.is_empty())
     }
 
+    /// Return the first mounted device (this device, or any of its children) along
+    /// with its mountpoints.
+    pub(crate) fn find_first_mounted(&self) -> Option<(&Device, &[String])> {
+        match self.mountpoints.as_deref() {
+            Some(mounts @ [_, ..]) => return Some((self, mounts)),
+            Some([]) | None => {}
+        }
+        self.children
+            .iter()
+            .flatten()
+            .filter_map(|c| c.find_first_mounted())
+            .next()
+    }
+
     // The "start" parameter was only added in a version of util-linux that's only
     // in Fedora 40 as of this writing.
     fn backfill_start(&mut self) -> Result<()> {
diff --git a/lib/src/install/baseline.rs b/lib/src/install/baseline.rs
index 180a3a1..fb95731 100644
--- a/lib/src/install/baseline.rs
+++ b/lib/src/install/baseline.rs
@@ -191,6 +191,10 @@ pub(crate) fn install_create_rootfs(
             opts.device
         );
     }
+    // Always disallow writing to a mounted device
+    if let Some((dev, mounts)) = device.find_first_mounted() {
+        anyhow::bail!("Device {} is mounted: {:?}", dev.path(), mounts)
+    }
 
     let run_bootc = Utf8Path::new(RUN_BOOTC);
     let mntdir = run_bootc.join("mounts");
-- 
2.44.0