Add script to download a basebackup from pageserver. (#5344)

I used this while investigating a production issue, and seems like it
could come handy in the future, too.

scripts/download_basebackup.py

@@ -0,0 +1,62 @@
+#!/usr/bin/env python3
+#
+# Script to download the basebackup from a pageserver to a tar file.
+#
+# This can be useful in disaster recovery.
+#
+import argparse
+
+import psycopg2
+from psycopg2.extensions import connection as PgConnection
+
+
+def main(args: argparse.Namespace):
+    pageserver_connstr = args.pageserver_connstr
+    tenant_id = args.tenant
+    timeline_id = args.timeline
+    lsn = args.lsn
+    output_path = args.output_path
+
+    psconn: PgConnection = psycopg2.connect(pageserver_connstr)
+    psconn.autocommit = True
+
+    output = open(output_path, "wb")
+
+    with psconn.cursor() as pscur:
+        pscur.copy_expert(f"basebackup {tenant_id} {timeline_id} {lsn}", output)
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--tenant-id",
+        dest="tenant",
+        required=True,
+        help="Id of the tenant",
+    )
+    parser.add_argument(
+        "--timeline-id",
+        dest="timeline",
+        required=True,
+        help="Id of the timeline",
+    )
+    parser.add_argument(
+        "--lsn",
+        dest="lsn",
+        required=True,
+        help="LSN to take the basebackup at",
+    )
+    parser.add_argument(
+        "--pageserver-connstr",
+        dest="pageserver_connstr",
+        required=True,
+        help="libpq connection string of the pageserver",
+    )
+    parser.add_argument(
+        "--output",
+        dest="output_path",
+        required=True,
+        help="output path to write the basebackup to",
+    )
+    args = parser.parse_args()
+    main(args)