tools: introduce a tool for generating diffs for stage tests

There hasn't ever been an official tool for this and everyone was just using
the output from the test to generate the diff. This commit introduces a proper
tool for generating these files.

tools/gen-stage-test-diff

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+import argparse
+import contextlib
+import os
+import subprocess
+import tempfile
+
+
+def run_osbuild(output_directory, store, cache_max_size, libdir, manifest):
+    args = [
+        "osbuild",
+        "--export",
+        "tree",
+        "--output-directory",
+        output_directory,
+        "--store",
+        store,
+        "--cache-max-size",
+        str(cache_max_size),
+        "--checkpoint",
+        "tree",
+        "--checkpoint",
+        "build",
+    ]
+
+    if libdir:
+        args += ["--libdir", libdir]
+
+    args += [manifest]
+
+    try:
+        subprocess.run(
+            args,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            encoding="utf-8",
+            check=True,
+        )
+    except subprocess.CalledProcessError as err:
+        raise RuntimeError(
+            f"osbuild crashed when building {manifest}:\n\nstdout:\n{err.stdout}\n\nstderr:\n{err.stderr}"
+        )
+
+
+epilog = """
+example:
+
+sudo tools/gen-stage-test-diff \\
+  --store ~/osbuild-store \\
+  --libdir . \\
+  test/data/stages/zstd
+"""
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Generator for diff.json files of stage tests",
+        epilog=epilog,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    parser.add_argument(
+        "--store",
+        type=str,
+        help="Specify the osbuild store",
+        default=1024 * 1024 * 1024,
+    )
+    parser.add_argument(
+        "--cache-max-size", type=int, help="Specify the osbuild max cache size"
+    )
+    parser.add_argument("--libdir", type=str, help="Specify the osbuild max cache size")
+    parser.add_argument("stage_test", type=str, help="Specify the stage test")
+    args = parser.parse_args()
+
+    with contextlib.ExitStack() as stack:
+        a = stack.enter_context(tempfile.TemporaryDirectory(dir="/var/tmp"))
+        b = stack.enter_context(tempfile.TemporaryDirectory(dir="/var/tmp"))
+
+        store = args.store
+        if not store:
+            store = stack.enter_context(tempfile.TemporaryDirectory(dir="/var/tmp"))
+
+        run_osbuild(
+            a,
+            store,
+            args.cache_max_size,
+            args.libdir,
+            os.path.join(args.stage_test, "a.json"),
+        )
+        run_osbuild(
+            b,
+            store,
+            args.cache_max_size,
+            args.libdir,
+            os.path.join(args.stage_test, "b.json"),
+        )
+
+        subprocess.run(
+            [
+                os.path.join(os.path.dirname(os.path.abspath(__file__)), "tree-diff"),
+                os.path.join(a, "tree"),
+                os.path.join(b, "tree"),
+            ],
+            check=True,
+        )
+
+
+if __name__ == "__main__":
+    main()