share one hive-priv request helper across the vm tests

nix/vm-tests/priv-request.nix

@@ -0,0 +1,28 @@
+# Python for a VM test script: `priv_request`, the one way a test talks to
+# hive-priv. Splice it into `testScript` after `import json`. The VM it runs
+# against must have `socat` installed.
+{ pkgs }:
+''
+  import shlex
+
+  def priv_journal(node):
+      return node.execute("journalctl -u hive-priv --no-pager | tail -n 30")[1]
+
+  def priv_request(node, req, timeout=120):
+      """Send one request to hive-priv, wait for its reply, assert it is ok.
+
+      `-t` is how long socat waits for the reply after stdin ends; its default
+      is 0.5s, which drops the answer of any op that takes longer. Returns the
+      parsed reply.
+      """
+      op = req["op"]
+      out = node.succeed(
+          f"echo {shlex.quote(json.dumps(req))} | "
+          f"${pkgs.socat}/bin/socat -t {timeout} - UNIX-CONNECT:/run/hive/priv.sock"
+      )
+      lines = out.strip().splitlines()
+      assert lines, f"hive-priv sent no response to {op}; journal:\n{priv_journal(node)}"
+      reply = json.loads(lines[-1])
+      assert reply.get("ok") is True, f"{op}: response={out!r}; journal:\n{priv_journal(node)}"
+      return reply
+''