deb: hand the staged tree to the build user selectively and in parallel

The handover chowned the whole build root with a serial 'chown -R'.
Through the overlay staging every lower-layer file it touched is
copy-up'd — the full data movement of the staged tree — which the
profile of a kernel build showed as 15m53s in 'Preparing the build
user', 19% of the whole run.

The handover is now a 'find ! -uid | xargs -P chown' pipeline: only
entries the build user does not own yet are chowned (staging and quilt
re-application flip entries back to root; a resumed tree is already
owned and the phase collapses to a metadata-only walk), and the
copy-ups of independent files parallelize across the chroot's cores.
On a 200k-file tree the handover drops from 9.9s to 0.9s.

The ccache handover ran only when the build user was fresh, which is
every build in practice (the chroot is per-run scratch); it now rides
the same selective pipeline on every run, still best-effort — a failed
handover costs cache hits, never the build.

src/deb/local.rs

@@ -116,8 +116,7 @@ fn ensure_build_user(
             .output()
     };
     let probe = id_of(ctx, "-u").map_err(|e| format!("cannot look up {BUILD_USER}: {e}"))?;
-    let fresh = !probe.status.success();
-    if fresh {
+    if !probe.status.success() {
         create_build_user(ctx)?;
     }
 
@@ -135,30 +134,53 @@ fn ensure_build_user(
         }
     }
 
-    // The shared ccache directory moves once, when the user is fresh: it
-    // was root-owned by earlier root builds, and root keeps writing
-    // through the new ownership anyway.
-    if fresh && let Some(dir) = ccache_dir {
-        let _ = ctx
-            .command("chown")
-            .arg("-R")
-            .arg(format!("{}:{}", ids.0, ids.1))
-            .arg(dir)
-            .status();
+    for dir in [Some(workroot), Some(Path::new("/tmp/pkh-home"))]
+        .into_iter()
+        .flatten()
+    {
+        hand_over(ctx, dir, &ids)?;
     }
-    for dir in [workroot, Path::new("/tmp/pkh-home")] {
+    // The shared cache degrades to per-run compiles when it cannot be
+    // handed over: slower, never wrong.
+    if let Some(dir) = ccache_dir {
+        let _ = hand_over(ctx, Path::new(dir), &ids);
+    }
+    Ok(ids)
+}
+
+/// The shell pipeline handing `dir` to the build identity: chown only the
+/// entries it does not own yet, run in parallel.
+///
+/// Through the overlay staging a chown copy-ups every lower-layer file it
+/// touches — the full data movement of the staged tree. The serial
+/// `chown -R` of a kernel tree spends minutes on it; this pipeline pays
+/// the copy-up only for entries a root-side phase flipped back to root
+/// (staging, quilt re-application), and the copy-ups of independent files
+/// parallelize across the chroot's cores. On a resumed tree every entry
+/// is already owned and the phase collapses to a metadata-only walk.
+fn hand_over_script(dir: &str, uid: &str, gid: &str) -> String {
+    format!(
+        "n=$(nproc 2>/dev/null || echo 4); \
+         find {} \\! -uid {uid} -print0 \
+         | xargs -0 -r -P \"$n\" -n 512 chown -- {uid}:{gid}",
+        shlex_quote(dir)
+    )
+}
+
+/// Run [`hand_over_script`] against `dir`, failing the build when the
+/// escalation cannot complete: a tree half-owned by root fails the rules
+/// on their first write into it.
+fn hand_over(ctx: &Arc<Context>, dir: &Path, ids: &(String, String)) -> Result<(), Box<dyn Error>> {
     let status = ctx
-            .command("chown")
-            .arg("-R")
-            .arg(format!("{}:{}", ids.0, ids.1))
-            .arg(dir)
+        .command("sh")
+        .arg("-c")
+        .arg(hand_over_script(&dir.to_string_lossy(), &ids.0, &ids.1))
         .status()
         .map_err(|e| format!("cannot hand {} to {BUILD_USER}: {e}", dir.display()))?;
     if !status.success() {
         return Err(format!("cannot hand '{}' to '{BUILD_USER}'", dir.display()).into());
     }
-    }
-    Ok(ids)
+    Ok(())
 }
 
 /// Create [`BUILD_USER`] in the build context. Minimal mmdebstrap chroots
@@ -691,9 +713,10 @@ pub async fn build(
     // LTO/make -j process flood), and a rules chown that fails for the
     // build user is cosmetic — the .deb is assembled in the binary step's
     // own fakeroot session, where files unknown to its database are
-    // recorded as root-owned anyway. Handing the tree over is a recursive
-    // chown that can take minutes on a large carried tree, so it is a
-    // phase of its own in the status bar and the profile.
+    // recorded as root-owned anyway. Handing the tree over is a selective,
+    // parallel chown (through the overlay staging each chown copy-ups the
+    // file it touches), and it is a phase of its own in the status bar and
+    // the profile.
     if build_user.is_some() {
         enter_phase(view, Phase::PreparingBuildUser, profiler);
     }
@@ -1662,6 +1685,27 @@ mod tests {
         crate::debian::deps::parse_simple(s, true).unwrap()
     }
 
+    /// The handover pipeline chowns only entries the build user does not
+    /// own (a resumed tree needs no re-chown of already-owned files), is
+    /// parallel across the chroot's cores, and survives any spelling of
+    /// the staged path.
+    #[test]
+    fn hand_over_script_selects_and_parallelizes() {
+        let script = hand_over_script("/tmp/pkh-build/tree", "64500", "64500");
+        assert!(
+            script.contains("find '/tmp/pkh-build/tree' \\! -uid 64500 -print0"),
+            "{script}"
+        );
+        assert!(script.contains("xargs -0 -r -P"), "{script}");
+        assert!(script.contains("chown -- 64500:64500"), "{script}");
+
+        let quoted = hand_over_script("/tmp/some'dir", "1000", "1000");
+        assert!(
+            quoted.contains("find '/tmp/some'\\''dir'"),
+            "embedded quotes must be shell-quoted: {quoted}"
+        );
+    }
+
     /// The build env carries the ccache wiring when it is enabled: the
     /// compiler shim first on PATH, the shared cache dir, content-keyed
     /// compiler checks and the size cap.