Harden email IMAP polling retries

tests/test_email_channel.py

@@ -1,5 +1,6 @@
 from email.message import EmailMessage
 from datetime import date
+import imaplib
 
 import pytest
 
@@ -82,6 +83,77 @@ def test_fetch_new_messages_parses_unseen_and_marks_seen(monkeypatch) -> None:
     assert items_again == []
 
 
+def test_fetch_new_messages_retries_once_when_imap_connection_goes_stale(monkeypatch) -> None:
+    raw = _make_raw_email(subject="Invoice", body="Please pay")
+    fail_once = {"pending": True}
+
+    class FlakyIMAP:
+        def __init__(self) -> None:
+            self.store_calls: list[tuple[bytes, str, str]] = []
+            self.search_calls = 0
+
+        def login(self, _user: str, _pw: str):
+            return "OK", [b"logged in"]
+
+        def select(self, _mailbox: str):
+            return "OK", [b"1"]
+
+        def search(self, *_args):
+            self.search_calls += 1
+            if fail_once["pending"]:
+                fail_once["pending"] = False
+                raise imaplib.IMAP4.abort("socket error")
+            return "OK", [b"1"]
+
+        def fetch(self, _imap_id: bytes, _parts: str):
+            return "OK", [(b"1 (UID 123 BODY[] {200})", raw), b")"]
+
+        def store(self, imap_id: bytes, op: str, flags: str):
+            self.store_calls.append((imap_id, op, flags))
+            return "OK", [b""]
+
+        def logout(self):
+            return "BYE", [b""]
+
+    fake_instances: list[FlakyIMAP] = []
+
+    def _factory(_host: str, _port: int):
+        instance = FlakyIMAP()
+        fake_instances.append(instance)
+        return instance
+
+    monkeypatch.setattr("nanobot.channels.email.imaplib.IMAP4_SSL", _factory)
+
+    channel = EmailChannel(_make_config(), MessageBus())
+    items = channel._fetch_new_messages()
+
+    assert len(items) == 1
+    assert len(fake_instances) == 2
+    assert fake_instances[0].search_calls == 1
+    assert fake_instances[1].search_calls == 1
+
+
+def test_fetch_new_messages_skips_missing_mailbox(monkeypatch) -> None:
+    class MissingMailboxIMAP:
+        def login(self, _user: str, _pw: str):
+            return "OK", [b"logged in"]
+
+        def select(self, _mailbox: str):
+            raise imaplib.IMAP4.error("Mailbox doesn't exist")
+
+        def logout(self):
+            return "BYE", [b""]
+
+    monkeypatch.setattr(
+        "nanobot.channels.email.imaplib.IMAP4_SSL",
+        lambda _h, _p: MissingMailboxIMAP(),
+    )
+
+    channel = EmailChannel(_make_config(), MessageBus())
+
+    assert channel._fetch_new_messages() == []
+
+
 def test_extract_text_body_falls_back_to_html() -> None:
     msg = EmailMessage()
     msg["From"] = "alice@example.com"