signal-bot/signal_bot/analytics/probe.py

# Copyright (c) 2026 Krzysztof Rudnicki. MIT licence.
"""What one running command did, collected without changing any signature.

Timing a command means knowing things the handler learns and the dispatcher
records: how long the upstream fetch took, how long the send took, which
item was served. Threading that back would mean a return value or an extra
argument on every handler, every fetcher method and every reply helper.

A ``ContextVar`` avoids all of it. The dispatcher opens a probe around the
handler call; anything running underneath finds it and notes what it did.
``contextvars`` is stdlib, so the runtime stays at exactly six dependencies.

``Bot.listen`` awaits ``dispatcher.handle`` directly rather than spawning a
task per frame, so set/reset in a ``try/finally`` cannot interleave. A
handler that does spawn a task gets a *copy* of the context, which is the
behaviour wanted: the copy's notes land in the probe, and the reset in the
parent cannot be undone by the child.

Every ``note_*`` is a no-op when nothing is probing. That branch lives here,
once, so no caller needs to ask whether it is being measured -- which is
what keeps ``http_client`` and ``context`` free of analytics conditionals.
"""

from __future__ import annotations

import contextlib
import time
from contextvars import ContextVar
from dataclasses import dataclass, field
from typing import TYPE_CHECKING, Final

if TYPE_CHECKING:
    from collections.abc import AsyncIterator, Iterator

# No default sentinel object: None is the whole vocabulary, and the helpers
# below are the only readers.
CURRENT: ContextVar[Probe | None] = ContextVar("command_probe", default=None)

# A request that never got an answer at all -- DNS, connect, timeout. Kept
# distinct from a 5xx, which is upstream answering and answering badly; the
# two mean different things when a command starts failing.
NO_RESPONSE: Final = 0

MS_PER_SECOND: Final = 1000


@dataclass(frozen=True, slots=True)
class UpstreamCall:
    """One outbound HTTP request made while serving a command."""

    host: str
    status: int
    ms: int


@dataclass(slots=True)
class Probe:
    """The mutable notes one invocation accumulates.

    Upstream calls are *buffered* rather than written as they happen: they
    belong to a ``command_events`` row that does not exist yet, because the
    outcome and the duration are only known once the handler has returned.
    """

    send_ms: int = 0
    item_id: str | None = None
    calls: list[UpstreamCall] = field(default_factory=list)


@contextlib.contextmanager
def probing() -> Iterator[Probe]:
    """Open a probe for the duration of one command, and always close it."""
    active = Probe()
    token = CURRENT.set(active)
    try:
        yield active
    finally:
        CURRENT.reset(token)


def note_send(ms: int) -> None:
    """Add the cost of one outbound Signal send.

    Accumulated rather than assigned: a handler may send twice, and
    ``!pułapka`` does -- a picture and then its description.
    """
    active = CURRENT.get()
    if active is not None:
        active.send_ms += ms


@contextlib.asynccontextmanager
async def timed_send() -> AsyncIterator[None]:
    """Time one outbound send and note it, whether or not it succeeds.

    ``DOCS-latency.md`` measured ``POST /v2/send`` at 2.65 s of a 3.7 s
    reply -- 72% of the wall clock, none of it this repository's code -- by
    reading three log lines by hand, once. This makes that measurement
    continuous, which is what the document's own rule asks for: an upstream
    latency claim gets measured before it gets designed around.
    """
    started = time.perf_counter()
    try:
        yield
    finally:
        note_send(round((time.perf_counter() - started) * MS_PER_SECOND))


def note_upstream(host: str, status: int, ms: int) -> None:
    """Add one upstream request. Host only -- never the URL.

    A URL carries identifiers, and some of them are the ones ``item_id``
    exists to govern. A hostname cannot carry anything.
    """
    active = CURRENT.get()
    if active is not None:
        active.calls.append(UpstreamCall(host=host, status=status, ms=ms))


def note_item(item_id: str) -> None:
    """Record which item was served, as a public identifier.

    A Wikidata QID, a Commons page id, a Wolne Lektury slug, a pseudonym
    key, a content-addressed filename. Never a title, a caption or a name:
    this is what makes "the pope pool feels repetitive" a number instead of
    an impression, and an identifier cannot carry a person.
    """
    active = CURRENT.get()
    if active is not None:
        active.item_id = item_id