Send the moderator DM after responding, not before

The first real pre-block in production exposed this. Timeline from the logs:

  00:24:54.53  Mastodon calls (signup form render)
  00:24:55.33  ipapi.is answers (cold lookup, new range)
  00:24:55.89  ip_blocks POST 200  <- block installed, 1.35s in
  00:24:56.47  statuses POST 200   <- moderator DM, 582ms
  00:24:56.53  Mastodon gives up at its 2.0s budget

The block landed comfortably inside the budget and did its job -- the range is
blocked and no account was created from it. But the answer never got back,
because a moderator DM is a whole extra Mastodon round trip sitting on the
synchronous response path.

Nothing the caller acts on depends on that DM. It now goes out as a FastAPI
BackgroundTask, which runs after the response is sent, so a cold lookup
answers in ~1.35s instead of ~1.95s. Only classification and registering the
block have to happen before we answer; telling a human can wait.

reputation_check() takes a notify callable, defaulting to sending immediately
so any non-HTTP caller keeps the old behaviour; the endpoint passes one that
collects and defers. Tests assert the block is registered before the DM goes
out, and that a DM that raises cannot stop a block being installed.

app/main.py

@@ -1537,7 +1537,7 @@ def _external_lookup_allowed() -> bool:
         return True
 
 
-def _reputation_ip(ip: str, source: str) -> dict | None:
+def _reputation_ip(ip: str, source: str, notify) -> dict | None:
     """Classify one IP for the query API and install the block if it's flagged.
 
     Returns None when the IP side is switched off or the address doesn't parse.
@@ -1587,7 +1587,7 @@ def _reputation_ip(ip: str, source: str) -> dict | None:
             log.warning("reputation query: pre-blocked ip=%s cidr=%s classification=%s org=%s source=%s",
                         ip, cidr, classification, org, source or "-")
             if REPUTATION_API_DM_MODERATOR:
-                dm_moderator(
+                notify(
                     f"🛡️ Pre-blocked {cidr} before signup — {classification} "
                     f"({org or 'unknown org'})."
                 )
@@ -1599,7 +1599,7 @@ def _reputation_ip(ip: str, source: str) -> dict | None:
     }
 
 
-def _reputation_email(email: str, source: str) -> dict | None:
+def _reputation_email(email: str, source: str, notify) -> dict | None:
     """Classify one signup email's *domain* for the query API and install the
     domain block if it's flagged. The full address is never stored or logged —
     same rule as the webhook path (see classify_email_domain).
@@ -1640,7 +1640,7 @@ def _reputation_email(email: str, source: str) -> dict | None:
             log.warning("reputation query: pre-blocked domain=%s disposable=%s risk=%d source=%s",
                         domain, is_disposable, risk, source or "-")
             if REPUTATION_API_DM_MODERATOR:
-                dm_moderator(
+                notify(
                     f"🛡️ Pre-blocked email domain {domain} before signup "
                     f"(disposable={is_disposable}, risk={risk})."
                 )
@@ -1654,7 +1654,8 @@ def _reputation_email(email: str, source: str) -> dict | None:
     }
 
 
-def reputation_check(ip: str = "", email: str = "", source: str = "") -> dict:
+def reputation_check(ip: str = "", email: str = "", source: str = "",
+                     notify=None) -> dict:
     """Classify an IP and/or email domain, install any block they earn, and
     report what happened. The write half of the bot's pre-emptive path.
 
@@ -1668,10 +1669,19 @@ def reputation_check(ip: str = "", email: str = "", source: str = "") -> dict:
     as a result of this call", which is what the caller acts on: it is true
     both for a block installed just now and for one we installed earlier that
     is still in force.
+
+    `notify` receives the moderator-alert text for each newly-installed block.
+    It defaults to sending immediately, but the HTTP endpoint passes one that
+    defers until after the response — a moderator DM is a whole extra Mastodon
+    round trip (measured at 582ms), and the caller is a live signup waiting on
+    a timeout. Only the classification and the block registration have to
+    happen before we answer; telling a human can wait.
     """
+    if notify is None:
+        notify = dm_moderator
     result = {
-        "ip": _reputation_ip(ip.strip(), source),
-        "email": _reputation_email(email.strip(), source),
+        "ip": _reputation_ip(ip.strip(), source, notify),
+        "email": _reputation_email(email.strip(), source, notify),
         "checked_at": _now_iso(),
     }
     sides = [side for side in (result["ip"], result["email"]) if side]
@@ -2031,6 +2041,7 @@ def _reputation_guard(key: str | None) -> None:
 @app.post("/api/reputation")
 def api_reputation(
     body: ReputationRequest,
+    background: BackgroundTasks,
     x_reputation_key: str | None = Header(default=None),
 ):
     """Look up an IP and/or email domain, record the answer, install whatever
@@ -2040,11 +2051,24 @@ def api_reputation(
     Declared as a plain `def` on purpose: FastAPI runs those in a threadpool,
     so the blocking provider calls underneath never tie up the event loop and
     stall webhook deliveries.
+
+    Moderator DMs are deferred to after the response. The caller is a live
+    signup on a ~2s budget, and a cold lookup already spends most of it on the
+    provider call plus registering the block; adding a Mastodon status POST on
+    top blew the budget in production the first time a real bad range showed
+    up (the block landed at 1.35s, the DM pushed the answer past 2.0s and the
+    caller timed out). Nothing the caller acts on depends on the DM.
     """
     _reputation_guard(x_reputation_key)
     if not body.ip and not body.email:
         raise HTTPException(status_code=400, detail="ip or email is required")
-    return reputation_check(ip=body.ip, email=body.email, source=body.source)
+
+    alerts: list[str] = []
+    result = reputation_check(ip=body.ip, email=body.email, source=body.source,
+                              notify=alerts.append)
+    for message in alerts:
+        background.add_task(dm_moderator, message)
+    return result
 
 
 @app.post("/webhook")

test_local.py

@@ -1036,6 +1036,8 @@ def reputation_api_tests():
     main.register_ip_block = (
         lambda ip, acct, org, cidr="", comment="":
         ipblocks.append((ip, cidr, comment)) or True)
+    # TestClient runs BackgroundTasks to completion before returning, so the
+    # deferred moderator DMs are still observable in `dms` after a request.
     main.register_email_domain_block = (
         lambda domain, acct: domain_blocks.append((domain, acct)) or True)
 
@@ -1251,6 +1253,38 @@ def reputation_api_tests():
     active = {e["cidr"] for e in client.get("/api/blocks/ip-ranges").json()["items"]}
     assert "203.0.113.0/24" not in active, "an expired block must not be published"
 
+    # N. The moderator DM is deferred until after the response. It costs a
+    #    whole Mastodon round trip (582ms measured), and the caller is a live
+    #    signup on a ~2s budget -- keeping it inline blew that budget the
+    #    first time a real bad range showed up in production.
+    sent_order = []
+    main.register_ip_block = (
+        lambda ip, acct, org, cidr="", comment="":
+        sent_order.append("block") or ipblocks.append((ip, cidr, comment)) or True)
+    main.dm_moderator = lambda message: sent_order.append("dm") or dms.append(message)
+    classifications["198.51.103.7"] = ("datacenter", "Late DM Inc", True, "198.51.103.0/24")
+
+    dms.clear()
+    r = ask({"ip": "198.51.103.7", "source": "defer-check"})
+    assert r.json()["blocked"] is True, r.json()
+    assert sent_order == ["block", "dm"], \
+        f"the block must be registered before the DM is sent, got {sent_order}"
+    assert any("198.51.103.0/24" in d for d in dms), "the DM still goes out, just later"
+
+    #    ...and the response body never depends on the DM having been sent.
+    main.dm_moderator = lambda message: (_ for _ in ()).throw(
+        RuntimeError("moderator DM must not be able to break a lookup"))
+    classifications["198.51.104.7"] = ("datacenter", "Broken DM Inc", True, "198.51.104.0/24")
+    try:
+        r = ask({"ip": "198.51.104.7", "source": "defer-check"})
+    except RuntimeError:
+        # TestClient surfaces background-task errors; the important part is
+        # that the block was still installed and the answer computed first.
+        pass
+    assert main.reputation_block_installed("ip", "198.51.104.0/24"), \
+        "a failing DM must not prevent the block being registered"
+    main.dm_moderator = lambda message: dms.append(message)
+
     #    Email domain blocks don't expire in Mastodon, so they carry none here
     #    either: recorded with no expiry, they stay installed indefinitely.
     main.record_reputation_block("email", "rep-forever.test", "", "")