context: stage trees over a caller-supplied overlay upperdir

Resumable build sessions need to re-mount the staged tree over the
same overlay upperdir on every attempt: the build artifacts written
inside the chroot (object files) live there, while the host tree
stays visible as the live lowerdir. Add an optional
ensure_available_with_overlay staging path (OverlayStaging reports
whether the persistent upperdir was actually mounted or a fresh copy
was staged instead) with a default implementation rejecting the
request, so drivers without overlay support degrade cleanly.

The overlay mount itself is factored out of try_overlay_mount into
mount_overlay, taking optional pinned upper/work directories; a
failed mount only cleans up the directories it created, never the
caller-pinned ones.

src/context/api.rs

@@ -29,9 +29,38 @@ use super::schroot::SchrootDriver;
 use super::ssh::SshDriver;
 use super::unshare::UnshareDriver;
 
+/// Outcome of an overlay-aware staging
+/// ([`ContextDriver::ensure_available_with_overlay`]).
+#[derive(Debug, Clone)]
+pub struct OverlayStaging {
+    /// The staged path as it appears inside the context.
+    pub path: PathBuf,
+    /// Whether the staging reused the persistent overlay upperdir (build
+    /// artifacts of a previous attempt are visible); `false` means a fresh
+    /// copy was staged instead.
+    pub overlay: bool,
+}
+
 /// A ContextDriver is the interface for the logic happening inside a context
 pub trait ContextDriver {
     fn ensure_available(&self, src: &Path, dest_root: &str) -> io::Result<PathBuf>;
+    /// Stage `src` at `dest_root` over the given persistent overlay
+    /// upperdir/workdir (host-side paths), for resumable build sessions.
+    ///
+    /// The default implementation rejects the request; drivers that cannot
+    /// overlay-mount fall back to a plain copy in their caller.
+    fn ensure_available_with_overlay(
+        &self,
+        _src: &Path,
+        _dest_root: &str,
+        _upper: &Path,
+        _work: &Path,
+    ) -> io::Result<OverlayStaging> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "overlay staging is not supported by this context driver",
+        ))
+    }
     fn retrieve_path(&self, src: &Path, dest: &Path) -> io::Result<()>;
     fn list_files(&self, path: &Path) -> io::Result<Vec<PathBuf>>;
     fn run(
@@ -281,6 +310,25 @@ impl Context {
             .ensure_available(src, dest_root)
     }
 
+    /// Stage `src` at `dest_root` reusing the given overlay upperdir and
+    /// workdir (host-side paths), so build artifacts written inside the
+    /// context during a previous attempt survive into this one. Falls back
+    /// to a fresh copy when the driver does not support overlay staging or
+    /// the mount fails (the returned [`OverlayStaging::overlay`] says
+    /// which happened).
+    pub fn ensure_available_with_overlay(
+        &self,
+        src: &Path,
+        dest_root: &str,
+        upper: &Path,
+        work: &Path,
+    ) -> io::Result<OverlayStaging> {
+        self.driver()
+            .as_ref()
+            .unwrap()
+            .ensure_available_with_overlay(src, dest_root, upper, work)
+    }
+
     /// Create a temp directory inside context
     pub fn create_temp_dir(&self) -> io::Result<String> {
         self.driver().as_ref().unwrap().create_temp_dir()

src/context/mod.rs

@@ -7,7 +7,7 @@ pub(crate) mod shell;
 mod ssh;
 mod unshare;
 
-pub use api::{Context, ContextCommand, ContextConfig, LineSink, Stream};
+pub use api::{Context, ContextCommand, ContextConfig, LineSink, OverlayStaging, Stream};
 // The driver trait is implementation detail of the context API; it is only
 // needed crate-internally (test-run capture wrapper), so keep it out of the
 // public surface (and its documentation requirement).

src/context/unshare.rs

@@ -176,7 +176,7 @@ impl ContextDriver for UnshareDriver {
 
         // Try overlayfs for directories first — avoids a potentially expensive full copy
         if src.is_dir() && is_overlayfs_available() {
-            match self.try_overlay_mount(src, &dest_path) {
+            match self.mount_overlay(src, &dest_path, None, None) {
                 Ok(()) => {
                     debug!(
                         "Mounted overlay for {} at {}",
@@ -218,6 +218,58 @@ impl ContextDriver for UnshareDriver {
         Ok(Path::new(dest_root).join(filename))
     }
 
+    fn ensure_available_with_overlay(
+        &self,
+        src: &Path,
+        dest_root: &str,
+        upper: &Path,
+        work: &Path,
+    ) -> io::Result<super::api::OverlayStaging> {
+        let dest_dir = Path::new(&self.path).join(dest_root.trim_start_matches('/'));
+        std::fs::create_dir_all(&dest_dir)?;
+        let filename = src
+            .file_name()
+            .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, "Invalid source path"))?;
+        let dest_path = dest_dir.join(filename);
+
+        // The upper/work dirs are the persistent ones of a build session:
+        // mounting them again exposes the previous attempt's build
+        // artifacts over the (live) host tree.
+        if src.is_dir() && is_overlayfs_available() {
+            match self.mount_overlay(src, &dest_path, Some(upper), Some(work)) {
+                Ok(()) => {
+                    debug!(
+                        "Mounted overlay for {} at {} (reused upper {})",
+                        src.display(),
+                        dest_path.display(),
+                        upper.display()
+                    );
+                    return Ok(super::api::OverlayStaging {
+                        path: Path::new(dest_root).join(filename),
+                        overlay: true,
+                    });
+                }
+                Err(e) => {
+                    debug!(
+                        "Overlay remount failed for '{}', falling back to copy: {}",
+                        src.display(),
+                        e
+                    );
+                }
+            }
+        }
+
+        if src.is_dir() {
+            copy_dir_recursive(src, &dest_path)?;
+        } else {
+            copy_file_with_times(src, &dest_path)?;
+        }
+        Ok(super::api::OverlayStaging {
+            path: Path::new(dest_root).join(filename),
+            overlay: false,
+        })
+    }
+
     fn cleanup(&self) -> io::Result<()> {
         let mounts = self.overlay_mounts.lock().unwrap();
         for mount_path in mounts.iter() {
@@ -370,19 +422,33 @@ impl UnshareDriver {
             .expect("UnshareDriver requires a parent context")
     }
 
-    /// Try to mount `src` as an overlay at `dest_path` inside the chroot.
+    /// Mount `src` as an overlay at `dest_path` inside the chroot.
     ///
-    /// On success, the overlay is tracked in `overlay_mounts` for later cleanup.
-    /// On failure, the caller should fall back to the copy-based approach.
-    fn try_overlay_mount(&self, src: &Path, dest_path: &Path) -> io::Result<()> {
-        // Create unique upper/work dirs inside the chroot
+    /// `upper`/`work` override the per-mount random directories: the
+    /// resumable-session staging passes its persistent upperdir so build
+    /// artifacts survive across attempts. On success, the overlay is
+    /// tracked in `overlay_mounts` for later cleanup. On failure, the
+    /// caller should fall back to the copy-based approach.
+    fn mount_overlay(
+        &self,
+        src: &Path,
+        dest_path: &Path,
+        upper: Option<&Path>,
+        work: Option<&Path>,
+    ) -> io::Result<()> {
+        // Create unique upper/work dirs inside the chroot unless the caller
+        // pinned them (session reuse).
         let overlay_base = Path::new(&self.path).join("pkh-overlay");
         let id = std::time::SystemTime::now()
             .duration_since(std::time::UNIX_EPOCH)
             .unwrap()
             .as_nanos();
-        let upper_dir = overlay_base.join(format!("upper-{}", id));
-        let work_dir = overlay_base.join(format!("work-{}", id));
+        let upper_dir = upper
+            .map(Path::to_path_buf)
+            .unwrap_or_else(|| overlay_base.join(format!("upper-{}", id)));
+        let work_dir = work
+            .map(Path::to_path_buf)
+            .unwrap_or_else(|| overlay_base.join(format!("work-{}", id)));
 
         fs::create_dir_all(&upper_dir)?;
         fs::create_dir_all(&work_dir)?;
@@ -421,9 +487,13 @@ impl UnshareDriver {
             .output()?;
 
         if !output.status.success() {
-            // Clean up dirs we created
+            // Clean up dirs we created (never the caller-pinned persistent
+            // upper/work of a session: they hold the previous attempt's
+            // build artifacts)
+            if upper.is_none() {
                 let _ = fs::remove_dir_all(&upper_dir);
                 let _ = fs::remove_dir_all(&work_dir);
+            }
             let stderr = String::from_utf8_lossy(&output.stderr);
             return Err(io::Error::other(format!(
                 "Overlay mount of '{}' at '{}' failed: {}",