build: fail binary-only metadata when the previous version is unparseable

The binNMU path swallowed parse errors with a let-chain: a changelog
that could not yield the previous version silently produced a .changes
with plain 'Source: pkg', no Binary-Only-Changes and no redistributed
previous .dsc. Propagate the parse error like the source-build path
does (a single-entry changelog stays tolerated), and reuse the
changelog already read instead of reading the file a second time.

src/build/binary.rs

@@ -130,22 +130,31 @@ pub fn generate_binary_metadata(
     let mut source_display = entry.source.clone();
     let mut binary_only_changes = None;
 
-    if entry.binary_only
-        && let Ok(prev_entry) = crate::debian::changelog::parse_previous_version_from_str(
-            &ctx.read_file(&package_dir.join("debian/changelog"))?,
-        )
-        && let Some(prev) = prev_entry
-    {
-        source_display = format!("{} ({})", entry.source, prev);
-        binary_only_changes = Some(format!(
-            "{}\n\n -- {} <{}>  {}",
-            entry.changes_field, entry.maintainer_name, entry.maintainer_email, entry.date_raw
-        ));
-        let prev_version = crate::debian::DebianVersion::parse(&prev)?;
-        let dsc_name = format!("{}_{}.dsc", entry.source, prev_version.no_epoch());
-        let dsc_path = upload_dir.join(&dsc_name);
-        if ctx.exists(&dsc_path)? {
-            include_dsc_artifacts(ctx, upload_dir, &dsc_name, &mut checksums)?;
+    if entry.binary_only {
+        // A binary-only upload must reference the previous source version;
+        // a changelog that cannot yield it is a hard error, like in the
+        // source-build path. Reuse the changelog read above instead of
+        // reading the file a second time.
+        let changelog_path = package_dir.join("debian/changelog");
+        let prev = crate::debian::changelog::parse_previous_version_from_str(&changelog_content)
+            .map_err(|e| {
+                format!(
+                    "cannot parse the previous version from '{}': {e}",
+                    changelog_path.display()
+                )
+            })?;
+        if let Some(prev) = prev {
+            source_display = format!("{} ({})", entry.source, prev);
+            binary_only_changes = Some(format!(
+                "{}\n\n -- {} <{}>  {}",
+                entry.changes_field, entry.maintainer_name, entry.maintainer_email, entry.date_raw
+            ));
+            let prev_version = crate::debian::DebianVersion::parse(&prev)?;
+            let dsc_name = format!("{}_{}.dsc", entry.source, prev_version.no_epoch());
+            let dsc_path = upload_dir.join(&dsc_name);
+            if ctx.exists(&dsc_path)? {
+                include_dsc_artifacts(ctx, upload_dir, &dsc_name, &mut checksums)?;
+            }
         }
     }
 
@@ -580,4 +589,65 @@ Files:
         assert_eq!(tar_entry.sha1, "aaa111");
         assert_eq!(tar_entry.sha256, "bbb222");
     }
+
+    /// A binary-only (binNMU) build whose changelog cannot yield the
+    /// previous version (malformed second header, unbalanced parenthesis)
+    /// must fail the metadata generation with a diagnostic naming the
+    /// changelog, instead of silently emitting a plain `Source:` `.changes`
+    /// with no `Binary-Only-Changes` and no redistributed previous `.dsc`.
+    #[test]
+    fn binary_only_prev_version_parse_failure_errors_instead_of_wrong_metadata() {
+        let changelog = "\
+hello (1.0-1+b1) unstable; urgency=medium, binary-only=yes
+
+  * Binary-only rebuild.
+
+ -- A B <a@b.c>  Mon, 01 Jan 2024 00:00:00 +0000
+
+hello (1.0-1 unstable; urgency=medium
+
+  * Previous entry with an unbalanced parenthesis.
+
+ -- A B <a@b.c>  Sun, 31 Dec 2023 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");
+        std::fs::write(
+            tree.join("debian/files"),
+            "hello_1.0-1+b1_all.deb devel optional\n",
+        )
+        .expect("write files");
+        std::fs::write(base.path().join("hello_1.0-1+b1_all.deb"), "deb payload")
+            .expect("write deb");
+
+        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("binary-only build with an unparseable changelog must fail");
+        let err = err.to_string();
+        assert!(err.contains("debian/changelog"), "{err}");
+        assert!(err.contains("previous version"), "{err}");
+        assert!(err.contains("unbalanced parenthesis"), "{err}");
+    }
 }