Harden email IMAP polling retries

nanobot/channels/email.py

@@ -80,6 +80,21 @@ class EmailChannel(BaseChannel):
         "Nov",
         "Dec",
     )
+    _IMAP_RECONNECT_MARKERS = (
+        "disconnected for inactivity",
+        "eof occurred in violation of protocol",
+        "socket error",
+        "connection reset",
+        "broken pipe",
+        "bye",
+    )
+    _IMAP_MISSING_MAILBOX_MARKERS = (
+        "mailbox doesn't exist",
+        "select failed",
+        "no such mailbox",
+        "can't open mailbox",
+        "does not exist",
+    )
 
     @classmethod
     def default_config(cls) -> dict[str, Any]:
@@ -266,6 +281,21 @@ class EmailChannel(BaseChannel):
         mark_seen: bool,
         dedupe: bool,
         limit: int,
+    ) -> list[dict[str, Any]]:
+        try:
+            return self._fetch_messages_once(search_criteria, mark_seen, dedupe, limit)
+        except Exception as exc:
+            if not self._is_stale_imap_error(exc):
+                raise
+            logger.warning("Email IMAP connection went stale, retrying once: {}", exc)
+            return self._fetch_messages_once(search_criteria, mark_seen, dedupe, limit)
+
+    def _fetch_messages_once(
+        self,
+        search_criteria: tuple[str, ...],
+        mark_seen: bool,
+        dedupe: bool,
+        limit: int,
     ) -> list[dict[str, Any]]:
         """Fetch messages by arbitrary IMAP search criteria."""
         messages: list[dict[str, Any]] = []
@@ -278,8 +308,15 @@ class EmailChannel(BaseChannel):
 
         try:
             client.login(self.config.imap_username, self.config.imap_password)
+            try:
                 status, _ = client.select(mailbox)
+            except Exception as exc:
+                if self._is_missing_mailbox_error(exc):
+                    logger.warning("Email mailbox unavailable, skipping poll for {}: {}", mailbox, exc)
+                    return messages
+                raise
             if status != "OK":
+                logger.warning("Email mailbox select returned {}, skipping poll for {}", status, mailbox)
                 return messages
 
             status, data = client.search(None, *search_criteria)
@@ -358,6 +395,16 @@ class EmailChannel(BaseChannel):
 
         return messages
 
+    @classmethod
+    def _is_stale_imap_error(cls, exc: Exception) -> bool:
+        message = str(exc).lower()
+        return any(marker in message for marker in cls._IMAP_RECONNECT_MARKERS)
+
+    @classmethod
+    def _is_missing_mailbox_error(cls, exc: Exception) -> bool:
+        message = str(exc).lower()
+        return any(marker in message for marker in cls._IMAP_MISSING_MAILBOX_MARKERS)
+
     @classmethod
     def _format_imap_date(cls, value: date) -> str:
         """Format date for IMAP search (always English month abbreviations)."""

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"