update: Skip bootloader update when no block devices back the root

In environments without block-backed boot filesystems (virtiofs in bcvk
ephemeral, NFS root, ISO boot, etc.) there is no on-disk bootloader to
manage. Previously bootloader-update.service would fail because
get_devices() bailed when it could not find a block device from /boot
or /sysroot.

Change get_devices() to return Ok(None) instead of an error when no
block-backed filesystem is found, and propagate this through
prep_before_update() so the update and adopt-and-update paths exit
cleanly with an informative message. The EFI validate path also skips
gracefully in this case.

This is more general than checking for specific filesystem types: any
environment without backing block devices is handled, which also
prepares for the transition to list_dev_current_root() in PR #1068.

Assisted-by: OpenCode (Claude Opus 4)

Author: cgwalters

src/blockdev.rs

@@ -5,8 +5,13 @@ use anyhow::{Context, Result};
 use bootc_internal_blockdev::PartitionTable;
 use fn_error_context::context;
 
+/// Try to find the block devices backing `/boot` or `/sysroot`.
+///
+/// Returns `Ok(None)` when neither path has a block-backed filesystem (e.g.
+/// virtiofs, NFS, ISO 9660). This is normal for environments without an
+/// on-disk bootloader.
 #[context("get parent devices from mount point boot or sysroot")]
-pub fn get_devices<P: AsRef<Path>>(target_root: P) -> Result<Vec<String>> {
+pub fn get_devices<P: AsRef<Path>>(target_root: P) -> Result<Option<Vec<String>>> {
     let target_root = target_root.as_ref();
     let mut source = None;
 
@@ -26,14 +31,14 @@ pub fn get_devices<P: AsRef<Path>>(target_root: P) -> Result<Vec<String>> {
 
     let source = match source {
         Some(s) => s,
-        None => anyhow::bail!("Failed to inspect filesystem from boot or sysroot"),
+        None => return Ok(None),
     };
 
     // Find the parent devices of the source path
     let parent_devices = bootc_internal_blockdev::find_parent_devices(&source)
         .with_context(|| format!("While looking for backing devices of {}", source))?;
     log::debug!("Found parent devices: {parent_devices:?}");
-    Ok(parent_devices)
+    Ok(Some(parent_devices))
 }
 
 /// Find esp partition on the same device

src/bootupd.rs

@@ -531,17 +531,28 @@ impl RootContext {
     }
 }
 
-/// Initialize parent devices to prepare the update
-fn prep_before_update() -> Result<RootContext> {
+/// Initialize parent devices to prepare the update.
+///
+/// Returns `Ok(None)` when the root has no block-backed boot filesystem,
+/// meaning there is no on-disk bootloader to manage. This happens in
+/// environments like virtiofs (bcvk ephemeral), NFS root, ISO boot, etc.
+fn prep_before_update() -> Result<Option<RootContext>> {
     let path = "/";
     let sysroot = openat::Dir::open(path).context("Opening root dir")?;
-    let devices = crate::blockdev::get_devices(path).context("get parent devices")?;
-    Ok(RootContext::new(sysroot, path, devices))
+    let Some(devices) = crate::blockdev::get_devices(path)? else {
+        println!(
+            "No block-backed boot filesystem found; bootloader update is not applicable, skipping."
+        );
+        return Ok(None);
+    };
+    Ok(Some(RootContext::new(sysroot, path, devices)))
 }
 
 pub(crate) fn client_run_update() -> Result<()> {
     crate::try_fail_point!("update");
-    let rootcxt = prep_before_update()?;
+    let Some(rootcxt) = prep_before_update()? else {
+        return Ok(());
+    };
     let status: Status = status()?;
     if status.components.is_empty() && status.adoptable.is_empty() {
         println!("No components installed.");
@@ -596,7 +607,9 @@ pub(crate) fn client_run_update() -> Result<()> {
 }
 
 pub(crate) fn client_run_adopt_and_update(with_static_config: bool) -> Result<()> {
-    let rootcxt = prep_before_update()?;
+    let Some(rootcxt) = prep_before_update()? else {
+        return Ok(());
+    };
     let status: Status = status()?;
     if status.adoptable.is_empty() {
         println!("No components are adoptable.");

src/efi.rs

@@ -560,7 +560,9 @@ impl Component for Efi {
     }
 
     fn validate(&self, current: &InstalledContent) -> Result<ValidationResult> {
-        let devices = crate::blockdev::get_devices("/").context("get parent devices")?;
+        let Some(devices) = crate::blockdev::get_devices("/")? else {
+            return Ok(ValidationResult::Skip);
+        };
         let esp_devices = blockdev::find_colocated_esps(&devices)?;
         if !is_efi_booted()? && esp_devices.is_none() {
             return Ok(ValidationResult::Skip);