deb: scope the arch-indep build-dep pass to the host arch in cross builds

Without --host-architecture, the second build-dep pass re-resolves the
whole Build-Depends field for the native architecture: apt swaps
host-arch -dev packages for native ones (e.g. libcurl4-gnutls-dev,
whose arch-differing curl-config makes dpkg refuse the co-install) and
breaks the cross build environment.

Per dpkg-checkbuilddeps, both Build-Depends and Build-Depends-Indep
resolve for the host architecture in cross mode, so pass
--host-architecture to the second pass as well. Skip the pass entirely
when the source declares no Build-Depends-Indep.

Add an end-to-end regression test building a package that declares
libdb-dev in both fields and links a host-arch binary against it: the
test only passes if the arch-indep pass did not swap the arm64 -dev
packages for native ones.

src/deb/local.rs

@@ -276,26 +276,39 @@ pub async fn build(
         return Err("Could not install build-dependencies for the build".into());
     }
 
-    // Install arch-independant build dependencies
-    log::debug!("Installing arch-independant build dependencies...");
-    let status = cap(
-        ctx.command("apt-get")
-            .current_dir(package_dir_str)
+    // Install arch-independant build dependencies, only if the source declares
+    // any: without --arch-only this pass resolves the whole Build-Depends field
+    // too, which is redundant after the first pass and breaks cross builds.
+    let has_indep_deps = match ctx.read_file(&package_dir.join("debian/control")) {
+        Ok(control) => control
+            .lines()
+            .any(|l| l.to_ascii_lowercase().starts_with("build-depends-indep:")),
+        Err(e) => {
+            log::debug!("cannot read debian/control for indep build-deps: {}", e);
+            true
+        }
+    };
+    if has_indep_deps {
+        log::debug!("Installing arch-independant build dependencies...");
+        let mut cmd = ctx.command("apt-get");
+        cmd.current_dir(package_dir_str)
             .envs(env.clone())
             .arg("-y")
-            .arg("build-dep")
-            .arg("./"),
-        &sink,
-    )
-    .status()?;
+            .arg("build-dep");
+        if cross {
+            cmd.arg(format!("--host-architecture={arch}"));
+        }
+        cmd.arg("./");
+        let status = cap(&mut cmd, &sink).status()?;
 
-    // If build-dep fails, we try to explain the failure using dose-debcheck
-    if !status.success() {
-        if let Some(u) = &ui {
-            u.suspend();
+        // If build-dep fails, we try to explain the failure using dose-debcheck
+        if !status.success() {
+            if let Some(u) = &ui {
+                u.suspend();
+            }
+            dose3_explain_dependencies(package, version, arch, build_root, cross, ctx.clone())?;
+            return Err("Could not install build-dependencies for the build".into());
         }
-        dose3_explain_dependencies(package, version, arch, build_root, cross, ctx.clone())?;
-        return Err("Could not install build-dependencies for the build".into());
     }
 
     // Run the build step

src/deb/mod.rs

@@ -492,4 +492,123 @@ mod tests {
     async fn test_deb_gcc_debian_end_to_end() {
         test_build_end_to_end("gcc-15", "sid", None, None, false).await;
     }
+
+    /// Create a synthetic source package that discriminates which architecture
+    /// is used to resolve Build-Depends-Indep during cross builds:
+    ///
+    /// - 'libdb-dev' is an arch:any package that is not Multi-Arch: same, so an
+    ///   amd64 copy can only be installed by replacing the arm64 one
+    /// - the arch-specific binary links against libdb for the host
+    ///   architecture, so the build only succeeds if the arm64 libdb-dev was
+    ///   left in place by the arch-independant build-dep pass
+    fn create_indep_cross_test_source(parent: &Path) -> PathBuf {
+        let pkg_dir = parent.join("pkh-crosstest");
+        std::fs::create_dir_all(pkg_dir.join("debian/source")).unwrap();
+
+        std::fs::write(
+            pkg_dir.join("debian/changelog"),
+            "pkh-crosstest (1.0) noble; urgency=medium\n\n  \
+             * Synthetic package exercising Build-Depends-Indep in cross builds.\n\n \
+             -- pkh tests <pkh@example.com>  Tue, 15 Sep 2026 08:00:00 +0000\n",
+        )
+        .unwrap();
+
+        std::fs::write(
+            pkg_dir.join("debian/control"),
+            "Source: pkh-crosstest\n\
+             Section: devel\n\
+             Priority: optional\n\
+             Maintainer: pkh tests <pkh@example.com>\n\
+             Standards-Version: 4.7.4\n\
+             Build-Depends: debhelper-compat (= 13), libdb-dev\n\
+             Build-Depends-Indep: libdb-dev\n\
+             Architecture: any all\n\
+             \n\
+             Package: pkh-crosstest\n\
+             Architecture: any\n\
+             Depends: ${misc:Depends}, ${shlibs:Depends}\n\
+             Description: Cross-build regression package for build-dep resolution\n \
+              Builds a host-architecture binary against libdb to detect a cross\n \
+              build environment damaged by a wrongly-scoped build-dep pass.\n\
+             \n\
+             Package: pkh-crosstest-data\n\
+             Architecture: all\n\
+             Description: Cross-build regression package data (arch-indep)\n \
+              Arch-indep binary so the indep build path is exercised.\n",
+        )
+        .unwrap();
+
+        std::fs::write(
+            pkg_dir.join("debian/rules"),
+            "#!/usr/bin/make -f\n\
+             %:\n\
+             \tdh $@\n\
+             \n\
+             override_dh_auto_build:\n\
+             \tprintf '#include <db.h>\\nint main(void){DB *d; return db_create(&d, NULL, 0);}\\n' > main.c\n\
+             \t$(DEB_HOST_GNU_TYPE)-gcc main.c -ldb -o pkh-crosstest\n",
+        )
+        .unwrap();
+
+        use std::os::unix::fs::PermissionsExt;
+        let rules = pkg_dir.join("debian/rules");
+        let mut perms = std::fs::metadata(&rules).unwrap().permissions();
+        perms.set_mode(0o755);
+        std::fs::set_permissions(&rules, perms).unwrap();
+
+        std::fs::write(pkg_dir.join("debian/source/format"), "3.0 (native)\n").unwrap();
+
+        pkg_dir
+    }
+
+    /// This ensures the arch-independant build-dep pass of a cross build
+    /// resolves dependencies for the host architecture, like dpkg-checkbuilddeps
+    /// does, instead of re-resolving the whole Build-Depends field for the
+    /// native architecture, which swaps host-arch -dev packages for native ones
+    /// and breaks the cross build environment.
+    #[tokio::test]
+    #[test_log::test]
+    #[cfg(target_arch = "x86_64")]
+    async fn test_deb_cross_indep_host_arch_end_to_end() {
+        let temp_dir = tempfile::tempdir().unwrap();
+        let pkg_dir = create_indep_cross_test_source(temp_dir.path());
+
+        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local));
+
+        crate::deb::build_binary_package(
+            Some("arm64"),
+            Some("noble"),
+            None,
+            Some(&pkg_dir),
+            true,
+            None,
+            None,
+            None,
+            Some(ctx),
+            None,
+            None,
+        )
+        .await
+        .expect("Cannot cross-build package declaring Build-Depends-Indep");
+
+        // Both binary packages must have been produced, including the
+        // arch-independant one
+        let deb_files: Vec<String> = std::fs::read_dir(temp_dir.path())
+            .unwrap()
+            .filter_map(|e| e.ok())
+            .map(|e| e.file_name().to_string_lossy().to_string())
+            .collect();
+        assert!(
+            deb_files
+                .iter()
+                .any(|f| f.starts_with("pkh-crosstest_1.0_arm64.deb")),
+            "arch-specific .deb not produced, got: {deb_files:?}"
+        );
+        assert!(
+            deb_files
+                .iter()
+                .any(|f| f.starts_with("pkh-crosstest-data_1.0_all.deb")),
+            "arch-independant .deb not produced, got: {deb_files:?}"
+        );
+    }
 }