🐎 Allow xz to use all available CPUs

By default, xz only uses one CPU core even if multiple cores are
available. If xz compression is chosen, allow xz to use all of the
cores available.

Signed-off-by: Major Hayden <major@redhat.com>

assemblers/org.osbuild.tar

@@ -46,8 +46,28 @@ def main(tree, output_dir, options):
             return 1
         extra_args.append(f"--{compression}")
 
-    subprocess.run(["tar", *extra_args, "-cf", f"{output_dir}/{filename}", "-C", tree, "."],
-                   stdout=subprocess.DEVNULL, check=True)
+    # Set environment variables for the tar operation.
+    tar_env = {
+        # Speed up xz by allowing it to use all CPU cores for compression.
+        "XZ_OPT": "--threads 0"
+    }
+
+    # Set up the tar command.
+    tar_cmd = [
+        "tar",
+        *extra_args,
+        "-cf", f"{output_dir}/{filename}",
+        "-C", tree,
+        "."
+    ]
+
+    # Make a tarball of the tree.
+    subprocess.run(
+        tar_cmd,
+        stdout=subprocess.DEVNULL,
+        check=True,
+        env=tar_env
+    )
 
     return 0