build: stop turning read failures into silently wrong metadata

Two read_file(...).unwrap_or_default() calls masqueraded IO errors as
empty data: an unreadable debian/files became 'binary build with no
binary artifacts found; cannot distribute', and an unreadable dpkg
status file produced an empty Installed-Build-Depends. Tolerate a
missing debian/files (first build in a fresh tree) but propagate real
read errors, and hard-error on an unreadable status file like the
source-build path does. installed_build_depends_from_content also
returned a bare newline for zero entries, defeating render_buildinfo's
empty-guard and emitting a malformed 'Installed-Build-Depends:' field
with a blank continuation; it now returns an empty string so the field
is omitted.

src/build/binary.rs

@@ -75,9 +75,17 @@ pub fn generate_binary_metadata(
     let control_content = ctx.read_file(&package_dir.join("debian/control"))?;
     let control = ControlInfo::parse_content(&control_content)?;
 
-    let files_content = ctx
-        .read_file(&package_dir.join("debian/files"))
-        .unwrap_or_default();
+    // A missing `debian/files` is tolerated (first binary build in a fresh
+    // tree has nothing registered yet; that surfaces below as the "no binary
+    // artifacts" error), like `FilesList::load`. Any other read failure must
+    // not be silently mistaken for an empty registry.
+    let files_path = package_dir.join("debian/files");
+    let files_content = if ctx.exists(&files_path)? {
+        ctx.read_file(&files_path)
+            .map_err(|e| format!("cannot read '{}': {}", files_path.display(), e))?
+    } else {
+        String::new()
+    };
     let mut files_list = FilesList::parse(&files_content)?;
 
     // ------------------------------------------------------------------
@@ -208,9 +216,13 @@ pub fn generate_binary_metadata(
     // ------------------------------------------------------------------
     // Installed-Build-Depends closure over the context status database
     // ------------------------------------------------------------------
+    // Like the source-build path, a status database that cannot be read is
+    // a hard error: silently treating it as empty would drop (or gut) the
+    // `Installed-Build-Depends` field of the produced metadata.
+    let status_path = Path::new("/var/lib/dpkg/status");
     let status_content = ctx
-        .read_file(Path::new("/var/lib/dpkg/status"))
-        .unwrap_or_default();
+        .read_file(status_path)
+        .map_err(|e| format!("cannot read status file '{}': {}", status_path.display(), e))?;
     let bd_fields = [
         control.source.get("Build-Depends").unwrap_or(""),
         control.source.get("Build-Depends-Arch").unwrap_or(""),
@@ -650,4 +662,65 @@ Description: test package
         assert!(err.contains("previous version"), "{err}");
         assert!(err.contains("unbalanced parenthesis"), "{err}");
     }
+
+    /// An unreadable `debian/files` (e.g. permissions) must fail the
+    /// metadata generation with an error naming the read failure, instead of
+    /// being silently treated as an empty registry and reported as "no
+    /// binary artifacts found". A *missing* file stays tolerated (first
+    /// build in a fresh tree); the distinction matters.
+    #[test]
+    fn unreadable_debian_files_errors_instead_of_empty_registry() {
+        if crate::utils::root::is_root().unwrap_or(false) {
+            // Root can read files regardless of permissions.
+            return;
+        }
+        let changelog = "\
+hello (1.0-1) unstable; urgency=medium
+
+  * Regular build.
+
+ -- A B <a@b.c>  Mon, 01 Jan 2024 00:00:00 +0000
+";
+        let control = "\
+Source: hello
+Section: devel
+Priority: optional
+Maintainer: A B <a@b.c>
+
+Package: hello
+Architecture: all
+Description: test package
+";
+        let base = tempfile::tempdir().expect("tempdir");
+        let tree = base.path().join("hello-1.0");
+        std::fs::create_dir_all(tree.join("debian")).expect("mkdir tree");
+        std::fs::write(tree.join("debian/changelog"), changelog).expect("write changelog");
+        std::fs::write(tree.join("debian/control"), control).expect("write control");
+        let files_path = tree.join("debian/files");
+        std::fs::write(&files_path, "hello_1.0-1_all.deb devel optional\n").expect("write files");
+        #[cfg(unix)]
+        {
+            use std::os::unix::fs::PermissionsExt;
+            std::fs::set_permissions(&files_path, std::fs::Permissions::from_mode(0o000))
+                .expect("chmod files");
+        }
+
+        let ctx = Arc::new(
+            crate::context::Context::new(crate::context::ContextConfig::Local).expect("context"),
+        );
+        let opts = BinaryMetadataOptions {
+            profiles: Vec::new(),
+            vendor: "debian".to_string(),
+            exported_env: BTreeMap::new(),
+            build_arch: "amd64".to_string(),
+            host_arch: "amd64".to_string(),
+        };
+        let err = generate_binary_metadata(&ctx, &tree, base.path(), &opts)
+            .expect_err("unreadable debian/files must fail with a read error");
+        let err = err.to_string();
+        assert!(err.contains("cannot read"), "{err}");
+        assert!(err.contains("debian/files"), "{err}");
+        #[cfg(unix)]
+        assert!(err.contains("Permission denied"), "{err}");
+    }
 }