clone: repo synchronization based on Gerrit topic

If the CI job was triggered by a Gerrit commit which carries a topic,
then the clone.sh script will attempt to synchronize other repositories
to the branch that matches the Gerrit topic name. [1]

[1]: https://linaro.atlassian.net/browse/TFC-75

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I7ed181cfb02669d03c84ab90ebdc38f57eaeda80
diff --git a/scripts/parse_refspec.py b/scripts/parse_refspec.py
new file mode 100755
index 0000000..83fd30e
--- /dev/null
+++ b/scripts/parse_refspec.py
@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+#
+# Copyright (c) 2019-2022 Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+
+# This script parse the gerrit query results from the stdin
+# and return the correct refspec
+
+import sys
+import json
+
+def print_topic_tip(query_results):
+    patchsets = []
+    parents = []
+    project = query_results[0]["project"]
+    topic = query_results[0]["topic"]
+
+    # For each change, get its most recent patchset
+    for change in query_results:
+        patchsets.append(change["patchSets"][-1])
+
+    # For each patchset, get its parent commit
+    for patchset in patchsets:
+        parents.append(patchset["parents"][0])
+
+    # If a patchset's revision is NOT in the list of parents then it should
+    # be the tip commit
+    tips = list(filter(lambda x: x["revision"] not in parents, patchsets))
+
+    # There must be only one patchset remaining, otherwise the tip is ambiguous
+    if len(tips) > 1:
+        raise Exception("{} in {} has no unique tip commit.".format(topic, project))
+    if len(tips) == 0:
+        raise Exception("No tip commit found for {} in {}.".format(topic, project))
+    # Print the reference of the topic tip patchset
+    print(tips[0]["ref"])
+
+try:
+    changes = [json.loads(resp_line) for resp_line in sys.stdin]
+except:
+    raise Exception("Input error, it's not a JSON string!")
+
+# The last object is a summary; drop it as it's not of interest to us.
+changes.pop()
+
+if not changes:
+    raise Exception("Can not find anything.")
+
+if len(changes) > 1:
+   print_topic_tip(changes)
+else:
+    print(changes[0]["currentPatchSet"]["ref"])