From 7c53b268dd3c025fc972d709746a429b8e82c2dd Mon Sep 17 00:00:00 2001
From: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Date: Wed, 11 Feb 2026 11:58:59 +0100
Subject: [PATCH] deb: use qemu_binfmt on ephemeral/local builds of different
 arch

---
 src/deb/ephemeral.rs | 62 ++++++++++++++++++++++++++++++++------------
 src/deb/mod.rs       |  9 ++++++-
 2 files changed, 54 insertions(+), 17 deletions(-)

diff --git a/src/deb/ephemeral.rs b/src/deb/ephemeral.rs
index c11c125..971013b 100644
--- a/src/deb/ephemeral.rs
+++ b/src/deb/ephemeral.rs
@@ -17,7 +17,12 @@ pub struct EphemeralContextGuard {
 
 impl EphemeralContextGuard {
     /// Create a new ephemeral unshare context for the specified series
-    pub async fn new(series: &str) -> Result<Self, Box<dyn Error>> {
+    ///
+    /// # Arguments
+    /// * `series` - The distribution series (e.g., "noble", "sid")
+    /// * `arch` - Optional target architecture. If provided and different from host,
+    ///   downloads a chroot for that architecture (uses qemu_binfmt transparently)
+    pub async fn new(series: &str, arch: Option<&str>) -> Result<Self, Box<dyn Error>> {
         let current_context_name = context::manager().current_name();
 
         // Create a temporary directory for the chroot
@@ -25,13 +30,14 @@ impl EphemeralContextGuard {
         let chroot_path = PathBuf::from(chroot_path_str);
 
         log::debug!(
-            "Creating new chroot for {} at {}...",
+            "Creating new chroot for {} (arch: {:?}) at {}...",
             series,
+            arch,
             chroot_path.display()
         );
 
         // Download and extract the chroot tarball
-        Self::download_and_extract_chroot(series, &chroot_path).await?;
+        Self::download_and_extract_chroot(series, arch, &chroot_path).await?;
 
         // Switch to an ephemeral context to build the package in the chroot
         context::manager().set_current_ephemeral(Context::new(ContextConfig::Unshare {
@@ -48,6 +54,7 @@ impl EphemeralContextGuard {
 
     async fn download_and_extract_chroot(
         series: &str,
+        arch: Option<&str>,
         chroot_path: &PathBuf,
     ) -> Result<(), Box<dyn Error>> {
         // Get project directories for caching
@@ -56,8 +63,12 @@ impl EphemeralContextGuard {
         let cache_dir = proj_dirs.cache_dir();
         fs::create_dir_all(cache_dir)?;
 
-        // Create tarball filename based on series
-        let tarball_filename = format!("{}-buildd.tar.xz", series);
+        // Create tarball filename based on series and architecture
+        let tarball_filename = if let Some(a) = arch {
+            format!("{}-{}-buildd.tar.xz", series, a)
+        } else {
+            format!("{}-buildd.tar.xz", series)
+        };
         let tarball_path = cache_dir.join(&tarball_filename);
 
         // Check for existing lockfile, and wait for a timeout if it exists
@@ -94,10 +105,18 @@ impl EphemeralContextGuard {
 
         // Download tarball if it doesn't exist
         if !tarball_path.exists() {
-            log::debug!("Downloading chroot tarball for {}...", series);
-            Self::download_chroot_tarball(series, &tarball_path).await?;
+            log::debug!(
+                "Downloading chroot tarball for {} (arch: {:?})...",
+                series,
+                arch
+            );
+            Self::download_chroot_tarball(series, arch, &tarball_path).await?;
         } else {
-            log::debug!("Using cached chroot tarball for {}", series);
+            log::debug!(
+                "Using cached chroot tarball for {} (arch: {:?})",
+                series,
+                arch
+            );
         }
 
         // Extract tarball to chroot directory
@@ -113,6 +132,7 @@ impl EphemeralContextGuard {
 
     async fn download_chroot_tarball(
         series: &str,
+        arch: Option<&str>,
         tarball_path: &Path,
     ) -> Result<(), Box<dyn Error>> {
         let ctx = context::current();
@@ -134,15 +154,21 @@ impl EphemeralContextGuard {
         }
 
         // Use mmdebstrap to download the tarball to the cache directory
-        let status = ctx
-            .command("mmdebstrap")
-            .arg("--variant=buildd")
+        let mut cmd = ctx.command("mmdebstrap");
+        cmd.arg("--variant=buildd")
             .arg("--mode=unshare")
             .arg("--include=mount,curl,ca-certificates")
-            .arg("--format=tar")
-            .arg(series)
-            .arg(tarball_path.to_string_lossy().to_string())
-            .status()?;
+            .arg("--format=tar");
+
+        // Add architecture if specified
+        if let Some(a) = arch {
+            cmd.arg(format!("--arch={}", a));
+        }
+
+        cmd.arg(series)
+            .arg(tarball_path.to_string_lossy().to_string());
+
+        let status = cmd.status()?;
 
         if !status.success() {
             // Remove file on error
@@ -156,7 +182,11 @@ impl EphemeralContextGuard {
                 .arg("-f")
                 .arg(lockfile_path.to_string_lossy().to_string())
                 .status();
-            return Err(format!("Failed to download chroot tarball for series {}", series).into());
+            return Err(format!(
+                "Failed to download chroot tarball for series {} (arch: {:?})",
+                series, arch
+            )
+            .into());
         }
 
         // Remove lockfile: tarball is fully downloaded
diff --git a/src/deb/mod.rs b/src/deb/mod.rs
index f40d44d..33a6e5d 100644
--- a/src/deb/mod.rs
+++ b/src/deb/mod.rs
@@ -49,8 +49,15 @@ pub async fn build_binary_package(
     };
 
     // Create an ephemeral unshare context for all Local builds
+    // Use qemu_binfmt when target architecture differs from host and cross is not requested
+    let chroot_arch = if mode == BuildMode::Local && arch != current_arch && !cross {
+        Some(arch)
+    } else {
+        None
+    };
+
     let mut guard = if mode == BuildMode::Local {
-        Some(ephemeral::EphemeralContextGuard::new(series).await?)
+        Some(ephemeral::EphemeralContextGuard::new(series, chroot_arch).await?)
     } else {
         None
     };