sync-v2 step 6: memory + connection bounds (§7, §4)

- concurrency.loop_semaphore: per-event-loop semaphores (safe across pytest loops
  and process restarts via a WeakKeyDictionary keyed by the running loop).
- provider: holds a per-account (≤4) and per-host (PER_HOST_CONN_CAP) slot for the
  whole lifetime of an IMAP socket, released on close; SMTP send takes a per-host
  slot for its duration. A worker can no longer open hundreds of sockets to one
  host/account.
- indexer: parsing goes through a PARSE_CONCURRENCY semaphore, and a message over
  STREAM_PARSE_MB is parsed from its blob file (parse_message_file) instead of an
  in-memory tree, so peak RAM ~= PARSE_CONCURRENCY x STREAM_PARSE_MB regardless of
  mailbox sizes. Backfill parallelism stays bounded by the bulk queue only.
- config: PER_HOST_CONN_CAP, PARSE_CONCURRENCY, STREAM_PARSE_MB.
- tests: per-account cap blocks the 5th connection / distinct accounts independent;
  file-vs-bytes parse equivalence + large-message streaming routing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/concurrency.py

@@ -0,0 +1,32 @@
+"""Per-event-loop semaphores for bounded concurrency (refactor §4, §7).
+
+Connection and parse caps are process-local but must be bound to the running
+event loop: an :class:`asyncio.Semaphore` created on one loop cannot be awaited
+on another (a real hazard under pytest, which uses a fresh loop per test). These
+helpers hand back a semaphore keyed by ``(group, key)`` for the current loop,
+creating it on first use and dropping the whole set when the loop is collected.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import weakref
+
+# group -> {loop: {key: Semaphore}}. WeakKeyDictionary drops a loop's semaphores
+# once the loop is gone, so no state leaks across tests or restarts.
+_registry: dict[
+    str, weakref.WeakKeyDictionary[asyncio.AbstractEventLoop, dict[str, asyncio.Semaphore]]
+] = {}
+
+
+def loop_semaphore(group: str, key: str, cap: int) -> asyncio.Semaphore:
+    """Return the current loop's semaphore for ``(group, key)`` (cap on first use)."""
+    loop = asyncio.get_running_loop()
+    per_group = _registry.setdefault(group, weakref.WeakKeyDictionary())
+    per_loop = per_group.get(loop)
+    if per_loop is None:
+        per_loop = per_group[loop] = {}
+    sem = per_loop.get(key)
+    if sem is None:
+        sem = per_loop[key] = asyncio.Semaphore(cap)
+    return sem

backend/app/config.py

@@ -50,6 +50,10 @@ class Settings(BaseSettings):
     tier_session_grace_min: int = Field(default=30, alias="TIER_SESSION_GRACE_MIN")
     tier_cold_after_d: int = Field(default=3, alias="TIER_COLD_AFTER_D")
     hot_budget_per_worker: int = Field(default=250, alias="HOT_BUDGET_PER_WORKER")
+    # Memory/connection bounds (§7, §4): peak RAM ~= PARSE_CONCURRENCY x STREAM_PARSE_MB.
+    per_host_conn_cap: int = Field(default=50, alias="PER_HOST_CONN_CAP")
+    parse_concurrency: int = Field(default=4, alias="PARSE_CONCURRENCY")
+    stream_parse_mb: int = Field(default=8, alias="STREAM_PARSE_MB")
 
     oauth_google_client_id: str = Field(default="", alias="OAUTH_GOOGLE_CLIENT_ID")
     oauth_google_client_secret: str = Field(default="", alias="OAUTH_GOOGLE_CLIENT_SECRET")

backend/app/indexer/indexer.py

@@ -17,8 +17,10 @@ from sqlalchemy import select
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.concurrency import loop_semaphore
+from app.config import get_settings
 from app.indexer import blobstore
-from app.indexer.mime import ParsedEmail, parse_message
+from app.indexer.mime import ParsedEmail, parse_message, parse_message_file
 from app.indexer.sanitize import sanitize_html
 from app.models import Attachment, BodyPart, KnownRecipient, Message
 from app.providers.base import RemoteMessage
@@ -115,6 +117,21 @@ async def _resolve_message_id(
     return parsed_id
 
 
+async def _parse_bounded(user_id: uuid.UUID, sha: str, raw: bytes) -> ParsedEmail:
+    """Parse under the PARSE_CONCURRENCY cap; stream large messages from disk (§7).
+
+    Peak RAM stays ~PARSE_CONCURRENCY x STREAM_PARSE_MB: only a few parses run at
+    once, and a message above the threshold is parsed from its (already written)
+    blob file instead of being held as an in-memory tree beside its raw bytes.
+    """
+    settings = get_settings()
+    threshold = settings.stream_parse_mb * 1024 * 1024
+    async with loop_semaphore("mime_parse", "parse", settings.parse_concurrency):
+        if len(raw) > threshold:
+            return parse_message_file(blobstore.blob_path(user_id, sha))
+        return parse_message(raw)
+
+
 async def persist_message(
     session: AsyncSession,
     *,
@@ -132,7 +149,7 @@ async def persist_message(
         return None
 
     sha = blobstore.store(user_id, remote.raw)
-    parsed = parse_message(remote.raw)
+    parsed = await _parse_bounded(user_id, sha, remote.raw)
 
     message_id_hdr = await _resolve_message_id(
         session, user_id=user_id, account_id=account_id, parsed_id=parsed.message_id, raw=remote.raw

backend/app/indexer/mime.py

@@ -7,6 +7,7 @@ Header RFC2047 decoding is automatic under ``policy.default``.
 from __future__ import annotations
 
 import email
+import os
 from dataclasses import dataclass, field
 from datetime import datetime
 from email.headerregistry import AddressHeader
@@ -104,10 +105,21 @@ def _extract_attachments(msg: EmailMessage) -> list[ParsedAttachment]:
 
 
 def parse_message(raw: bytes) -> ParsedEmail:
-    """Parse a raw RFC822 message into a :class:`ParsedEmail`."""
+    """Parse a raw RFC822 message (in memory) into a :class:`ParsedEmail`."""
     msg = email.message_from_bytes(raw, policy=default_policy)
     assert isinstance(msg, EmailMessage)
+    return _parse_email(msg)
 
+
+def parse_message_file(path: str | os.PathLike[str]) -> ParsedEmail:
+    """Parse a message straight from a blob file handle (large-message path, §7)."""
+    with open(path, "rb") as handle:
+        msg = email.message_from_binary_file(handle, policy=default_policy)
+    assert isinstance(msg, EmailMessage)
+    return _parse_email(msg)
+
+
+def _parse_email(msg: EmailMessage) -> ParsedEmail:
     message_id = normalize_message_id(msg.get("Message-ID"))
     in_reply_to_ids = normalize_reference_list(msg.get("In-Reply-To"))
     in_reply_to = in_reply_to_ids[0] if in_reply_to_ids else None

backend/app/providers/imap_smtp/provider.py

@@ -17,6 +17,8 @@ from dataclasses import dataclass
 import aiosmtplib
 from aioimaplib import aioimaplib
 
+from app.concurrency import loop_semaphore
+from app.config import get_settings
 from app.providers.base import (
     FlagChange,
     MailboxDelta,
@@ -37,6 +39,7 @@ from app.providers.imap_smtp.parsing import (
 _FETCH_ITEMS = "(UID FLAGS INTERNALDATE RFC822.SIZE BODY.PEEK[])"
 _IDLE_TIMEOUT = 25 * 60  # SPEC §5.1: re-issue IDLE at least every 25 min
 _CONNECT_TIMEOUT = 20
+_PER_ACCOUNT_CONN_CAP = 4  # SPEC §6.3 / §4: IDLE + transient + short-lived jobs
 _APPENDUID_RE = re.compile(rb"APPENDUID\s+\d+\s+(\d+)")
 _COPYUID_RE = re.compile(rb"COPYUID\s+\d+\s+\S+\s+(\d+)")
 
@@ -59,34 +62,65 @@ class ImapSmtpProvider(MailProvider):
         self._imap: aioimaplib.IMAP4 | None = None
         self._selected: str | None = None
         self._uidvalidity_seen: dict[str, int] = {}
+        self._conn_slots: list[asyncio.Semaphore] = []
 
     # --- connection lifecycle ------------------------------------------------
 
+    async def _acquire_conn_slots(self) -> None:
+        """Bound simultaneous open sockets: ≤4 per account and ≤cap per host (§4).
+
+        Held for the connection's whole lifetime and released in :meth:`close`.
+        """
+        cfg = self._cfg
+        account = loop_semaphore(
+            "imap_account", f"{cfg.username}@{cfg.imap_host}", _PER_ACCOUNT_CONN_CAP
+        )
+        host = loop_semaphore("imap_host", cfg.imap_host, get_settings().per_host_conn_cap)
+        await account.acquire()  # consistent order (account, then host) avoids deadlock
+        try:
+            await host.acquire()
+        except BaseException:
+            account.release()
+            raise
+        self._conn_slots = [host, account]
+
+    def _release_conn_slots(self) -> None:
+        for sem in self._conn_slots:
+            sem.release()
+        self._conn_slots = []
+
     async def _connect(self) -> aioimaplib.IMAP4:
         if self._imap is not None:
             return self._imap
         cfg = self._cfg
+        await self._acquire_conn_slots()
         try:
-            if cfg.imap_tls == "ssl":
-                imap = aioimaplib.IMAP4_SSL(
-                    host=cfg.imap_host, port=cfg.imap_port, timeout=_CONNECT_TIMEOUT
-                )
-            else:
-                imap = aioimaplib.IMAP4(
-                    host=cfg.imap_host, port=cfg.imap_port, timeout=_CONNECT_TIMEOUT
-                )
-            await imap.wait_hello_from_server()
-            if cfg.imap_tls == "starttls":
-                await imap.starttls()
-        except (OSError, TimeoutError) as exc:
-            raise ProviderConnectError(f"cannot connect to {cfg.imap_host}: {exc}") from exc
+            try:
+                if cfg.imap_tls == "ssl":
+                    imap = aioimaplib.IMAP4_SSL(
+                        host=cfg.imap_host, port=cfg.imap_port, timeout=_CONNECT_TIMEOUT
+                    )
+                else:
+                    imap = aioimaplib.IMAP4(
+                        host=cfg.imap_host, port=cfg.imap_port, timeout=_CONNECT_TIMEOUT
+                    )
+                await imap.wait_hello_from_server()
+                if cfg.imap_tls == "starttls":
+                    await imap.starttls()
+            except (OSError, TimeoutError) as exc:
+                raise ProviderConnectError(f"cannot connect to {cfg.imap_host}: {exc}") from exc
 
-        resp = await imap.login(cfg.username, cfg.password)
-        if resp.result != "OK":
-            detail = (
-                b" ".join(resp.lines).decode("utf-8", "ignore") if resp.lines else "login failed"
-            )
-            raise ProviderAuthError(detail)
+            resp = await imap.login(cfg.username, cfg.password)
+            if resp.result != "OK":
+                detail = (
+                    b" ".join(resp.lines).decode("utf-8", "ignore")
+                    if resp.lines
+                    else "login failed"
+                )
+                raise ProviderAuthError(detail)
+        except BaseException:
+            self._release_conn_slots()  # never hold a slot without an open socket
+            raise
         self._imap = imap
         return imap
 
@@ -104,6 +138,7 @@ class ImapSmtpProvider(MailProvider):
                 await self._imap.logout()
             self._imap = None
             self._selected = None
+            self._release_conn_slots()  # free the per-account / per-host slots
 
     # --- MailProvider protocol ----------------------------------------------
 
@@ -293,19 +328,21 @@ class ImapSmtpProvider(MailProvider):
         cfg = self._cfg
         use_tls = cfg.smtp_tls == "ssl"
         start_tls = cfg.smtp_tls == "starttls"
+        host_slot = loop_semaphore("smtp_host", cfg.smtp_host, get_settings().per_host_conn_cap)
         try:
-            await aiosmtplib.send(
-                raw,
-                sender=envelope_from,
-                recipients=envelope_to,
-                hostname=cfg.smtp_host,
-                port=cfg.smtp_port,
-                username=cfg.username,
-                password=cfg.password,
-                use_tls=use_tls,
-                start_tls=start_tls,
-                timeout=_CONNECT_TIMEOUT,
-            )
+            async with host_slot:  # bound simultaneous sockets to the SMTP host (§4)
+                await aiosmtplib.send(
+                    raw,
+                    sender=envelope_from,
+                    recipients=envelope_to,
+                    hostname=cfg.smtp_host,
+                    port=cfg.smtp_port,
+                    username=cfg.username,
+                    password=cfg.password,
+                    use_tls=use_tls,
+                    start_tls=start_tls,
+                    timeout=_CONNECT_TIMEOUT,
+                )
         except aiosmtplib.SMTPAuthenticationError as exc:
             raise ProviderAuthError(f"SMTP auth failed: {exc}") from exc
         except aiosmtplib.SMTPException as exc: