sources: add org.ostree.ostree source

This source can be used to fetch ostree commits. The commits are
accessed via their commit is. The only option currently is `url`,
given for each commit, that will be used as the location of the
remote. A cache repository, that will be created if necessary,
acts as an intermediary, so remotes will be added with `name` as
the identifier to it and commits are pulled into that. In the
output directory another repository will be created as 'repo' and
the requested commit pulled into that from the cache repository via
a local pull.

sources/org.osbuild.ostree

@@ -0,0 +1,64 @@
+#!/usr/bin/python3
+
+import json
+import os
+import sys
+import subprocess
+import uuid
+
+
+def ostree(*args, **kwargs):
+    args = list(args) + [f'--{k}={v}' for k, v in kwargs.items()]
+    print(f"ostree " + " ".join(args), file=sys.stderr)
+    subprocess.run(["ostree"] + args,
+                   encoding="utf-8",
+                   stdout=sys.stderr,
+                   check=True)
+
+
+def main(options, checksums, cache, output):
+    commits = options["commits"]
+
+    os.makedirs(output, exist_ok=True)
+    os.makedirs(cache, exist_ok=True)
+
+    # Prepare the cache and the output repo
+    repo_cache = os.path.join(cache, "repo")
+    ostree("init", mode="archive", repo=repo_cache)
+
+    repo_out = os.path.join(output, "repo")
+    ostree("init", mode="archive", repo=repo_out)
+
+    for commit in checksums:
+        remote = commits[commit]["remote"]
+        url = remote["url"]
+        uid = str(uuid.uuid4())
+
+        ostree("remote", "add",
+               "--no-gpg-verify",
+               uid, url,
+               repo=repo_cache)
+
+        # Transfer the commit: remote → cache
+        print(f"pulling {commit}", file=sys.stderr)
+        ostree("pull", uid, commit, repo=repo_cache)
+
+        # Transfer the commit: cache → output
+        ostree("pull-local", repo_cache, commit,
+               repo=repo_out)
+
+        # Remove the temporary remote again
+        ostree("remote", "delete", uid,
+               repo=repo_cache)
+
+    json.dump({}, sys.stdout)
+    return 0
+
+
+if __name__ == '__main__':
+    source_args = json.load(sys.stdin)
+    r = main(source_args["options"],
+             source_args["checksums"],
+             source_args["cache"],
+             source_args["output"])
+    sys.exit(r)