fix(extract): name the unsupported bzip2 format instead of misparsing it

A .tar.bz2 tarball fell through extract's compression dispatch to
the magic-byte fallback and died as garbage tar errors; a bzip2
distro URL would spend the whole transfer first (download now
rejects it before spending the transfer). No bzip2 decoder exists
in the tree, and none is planned.

crates/ecrou/src/download.rs

@@ -113,6 +113,14 @@ pub fn download_image(
             if !url.starts_with("http://") && !url.starts_with("https://") {
                 return Err(anyhow!("Unsupported URL scheme: {}", url).into());
             }
+            if url.ends_with(".tar.bz2") || url.ends_with(".tbz2") {
+                return Err(anyhow!(
+                    "bzip2-compressed tarballs are not supported: {} (use a \
+                     .tar.gz, .tar.xz or .tar.zst source)",
+                    url
+                )
+                .into());
+            }
             // Verify the tarball against the checksum the distro publishes
             // next to it.  A fetch failure (missing artifact, network error)
             // degrades to an unverified download with a warning rather than

crates/ecrou/src/extract.rs

@@ -45,6 +45,14 @@ fn extract_single_archive(
         extract_xz(reader, dest, &mut fixups, progress)?;
     } else if filename.ends_with(".tar.zst") || filename.ends_with(".tar.zstd") {
         extract_zst(reader, dest, &mut fixups, progress)?;
+    } else if filename.ends_with(".tar.bz2") || filename.ends_with(".tbz2") {
+        // Named before the magic-byte fallback misreads a bzip2
+        // stream as an uncompressed tar.
+        return Err(anyhow::anyhow!(
+            "bzip2-compressed tarballs are not supported: {} (use a \
+             .tar.gz, .tar.xz or .tar.zst rootfs)",
+            tarball.display()
+        ));
     } else if filename.ends_with(".tar") {
         extract_tar(reader, dest, &mut fixups, progress)?;
     } else {
@@ -802,4 +810,19 @@ mod tests {
             Path::new("etc/dir/absent.pem")
         );
     }
+    #[test]
+    fn a_bzip2_tarball_is_a_named_error() {
+        let dir = tempfile::tempdir().unwrap();
+        let tarball = dir.path().join("rootfs.tar.bz2");
+        std::fs::write(&tarball, b"BZh9-not-really").unwrap();
+        let out = tempfile::tempdir().unwrap();
+        let err = format!(
+            "{:#}",
+            extract_tarball(&tarball, out.path(), &NoProgress).unwrap_err()
+        );
+        assert!(
+            err.contains("bzip2-compressed tarballs are not supported"),
+            "{err}"
+        );
+    }
 }