Implement "iterate" command in metadata server

Otherwise Dovecot times out when trying to iterate over metadata
of the folder. Apparently it happens when attempting to delete
folder from the server over IMAP.

chatmaild/src/chatmaild/metadata.py

@@ -15,6 +15,7 @@ import requests
 
 
 DICTPROXY_LOOKUP_CHAR = "L"
+DICTPROXY_ITERATE_CHAR = "I"
 DICTPROXY_SET_CHAR = "S"
 DICTPROXY_BEGIN_TRANSACTION_CHAR = "B"
 DICTPROXY_COMMIT_TRANSACTION_CHAR = "C"
@@ -74,6 +75,10 @@ def handle_dovecot_request(msg, transactions, notifier):
     parts = msg[1:].split("\t")
     if short_command == DICTPROXY_LOOKUP_CHAR:
         return "N\n"
+    elif short_command == DICTPROXY_ITERATE_CHAR:
+        # Empty line means ITER_FINISHED.
+        # If we don't return empty line Dovecot will timeout.
+        return "\n"
 
     if short_command not in (DICTPROXY_TRANSACTION_CHARS):
         return

chatmaild/src/chatmaild/tests/test_metadata.py

@@ -70,6 +70,21 @@ def test_handle_dovecot_protocol_set_devicetoken():
     assert wfile.getvalue() == b"O\n"
 
 
+def test_handle_dovecot_protocol_iterate():
+    rfile = io.BytesIO(
+        b"\n".join(
+            [
+                b"H",
+                b"I9\t0\tpriv/5cbe730f146fea6535be0d003dd4fc98/\tci-2dzsrs@nine.testrun.org",
+            ]
+        )
+    )
+    wfile = io.BytesIO()
+    notifier = Notifier()
+    handle_dovecot_protocol(rfile, wfile, notifier)
+    assert wfile.getvalue() == b"\n"
+
+
 def test_handle_dovecot_protocol_messagenew():
     rfile = io.BytesIO(
         b"\n".join(