nxdns/src/filter/fetcher.zig

//! Blocklist download over HTTP/1.1.
//!
//! One `Fetcher` wraps a caller-owned `std.http.Client`, which owns the
//! connection pool and the CA bundle, exactly as `upstream/doh_client.zig`
//! does. This file knows nothing about parsing, files or the database: it GETs
//! a URL and streams the bytes into a writer the caller supplies.
//!
//! The body is never held whole. A blocklist can reach `max_body_bytes`, and
//! the caller writes into a temporary file anyway, so nothing here allocates.
//!
//! There is no timeout parameter and no sleep. `std.http.Client` has no
//! per-request deadline, so the caller runs `fetch` under `io.concurrent` and
//! cancels the future. A cancel during the body copy surfaces as
//! `error.ReceiveFailed` with `Canceled` in `last_failure`; elsewhere it
//! propagates as `error.Canceled`.

const std = @import("std");
const transport = @import("../upstream/transport.zig");

pub const max_body_bytes: usize = 64 * 1024 * 1024;

/// RFC 9110 recommends at least 8000 bytes for the redirect buffer
/// (`std.http.Client` doc comment, Client.zig:1128).
pub const redirect_buffer_len: usize = 8192;

pub const min_transfer_buf: usize = 16 * 1024;

pub const Error = error{
    BadUrl,
    ConnectFailed,
    TlsFailed,
    SendFailed,
    ReceiveFailed,
    HttpStatus,
    BodyTooLarge,
    Timeout,
    Canceled,
    OutOfMemory,
    SystemResources,
    Unexpected,
};

pub const Result = struct {
    bytes_read: u64,
    status: std.http.Status,
};

/// The concrete fault behind an `Error`. `cause` is unwrapped from whatever
/// `std.http.Client` stashed on the connection or the response, so it names a
/// record-layer TLS fault or an HTTP framing fault rather than the collapsed
/// `error.ReadFailed`.
pub const Failure = struct {
    phase: Phase,
    cause: anyerror,
    /// The response status if a head arrived before the failure.
    status: ?std.http.Status,
    /// Body bytes delivered to the caller's writer before the failure, and a
    /// lower bound rather than an exact count: at least this many reached `w`.
    /// A single `Reader.stream` call can hand bytes to `w` and then fail, and
    /// that partial delivery is not counted. Zero on every phase before
    /// `receive_body`.
    bytes_read: u64,
};

/// Which call failed. The phase is what decides the classification, and only
/// the call site knows it — guessing it from an error name would be wrong the
/// first time two phases shared an error. `receive_body` is distinct from
/// `receive`: the head arrived, so the fault is in the transfer, not the
/// response.
pub const Phase = enum { connect, send, receive, receive_body };

pub const Fetcher = struct {
    /// Caller-owned; shared across sources, pools connections.
    http: *std.http.Client,
    /// Caller-owned HTTP body transfer buffer, at least `min_transfer_buf`.
    transfer_buf: []u8,
    /// Caller-owned. `receiveHead` follows redirects itself and needs this to
    /// outlive `Request.uri`. At least `redirect_buffer_len`.
    redirect_buf: []u8,
    /// The status of the most recent response head, or null before the first
    /// one. `error.HttpStatus` carries no `Result`, and the operator's message
    /// needs the number, so it is readable here after a failed `fetch`.
    last_status: ?std.http.Status = null,
    /// What the most recent `fetch` failed on, or null if it succeeded or has
    /// not run. The taxonomy `Error` a caller receives names six outcomes; this
    /// names the one concrete fault behind the outcome, so an operator reading
    /// the log can tell a TLS alert from a reset connection from a truncated
    /// chunk.
    last_failure: ?Failure = null,

    /// GETs `url` and streams the body into `w`.
    pub fn fetch(
        self: *Fetcher,
        io: std.Io,
        url: []const u8,
        w: *std.Io.Writer,
    ) Error!Result {
        // `std.http.Client` carries the `std.Io` it was constructed with and
        // takes none per request. The parameter stays in the signature because
        // the manager drives every fetch through one `std.Io`.
        _ = io;

        std.debug.assert(self.transfer_buf.len >= min_transfer_buf);
        std.debug.assert(self.redirect_buf.len >= redirect_buffer_len);

        self.last_status = null;
        self.last_failure = null;

        const uri = parseUrl(url) catch |err| return self.record(err, .connect, err, null, 0);

        var req = self.http.request(.GET, uri, .{
            .keep_alive = true,
            .headers = .{
                // Identity only: a compressed transfer encoding would need
                // `Response.readerDecompressing`, a decompression buffer and a
                // second failure surface, for a download that runs once a day.
                .accept_encoding = .{ .override = "identity" },
            },
        }) catch |err| return self.record(mapError(err, .connect), .connect, err, null, 0);
        defer req.deinit();

        req.sendBodiless() catch |err|
            return self.record(mapError(err, .send), .send, transport.sendCause(&req, err), null, 0);

        var resp = req.receiveHead(self.redirect_buf) catch |err|
            return self.record(mapError(err, .receive), .receive, transport.headCause(&req, err), null, 0);

        self.last_status = resp.head.status;
        if (resp.head.status != .ok) {
            return self.record(error.HttpStatus, .receive, error.HttpStatus, resp.head.status, 0);
        }

        // `content-type` is deliberately not checked: blocklists are served as
        // text/plain, application/octet-stream and text/html alike, and the
        // compiler's invalid-line counters are the honest signal about content.
        if (resp.head.content_length) |declared| {
            if (declared > max_body_bytes) {
                return self.record(
                    error.BodyTooLarge,
                    .receive,
                    error.BodyTooLarge,
                    resp.head.status,
                    0,
                );
            }
        }

        const body = resp.reader(self.transfer_buf);
        var pump: Pump = .{};
        const bytes_read = pumpBody(body, w, max_body_bytes, &pump) catch |err| return self.record(
            err,
            .receive_body,
            transport.bodyCause(&resp, pump.cause),
            resp.head.status,
            pump.delivered,
        );
        return .{ .bytes_read = bytes_read, .status = resp.head.status };
    }

    /// Stores the failure and hands back `mapped` unchanged.
    ///
    /// `mapped` is the taxonomy error the call site already decided, never one
    /// this function derives. `mapError` answers for a raw std error, and the
    /// call sites that raise a taxonomy member directly — a non-200 head, a
    /// declared length over the cap — pass that member: feeding either back
    /// through `mapError` would find no case for it and fall to the phase
    /// default, turning `error.HttpStatus` into `error.ReceiveFailed`.
    ///
    /// `cause` never reaches the returned error. The unwrap is for the
    /// operator's line, and a caller that ignores `last_failure` sees exactly
    /// what it saw before the unwrap existed.
    fn record(
        self: *Fetcher,
        mapped: Error,
        phase: Phase,
        cause: anyerror,
        status: ?std.http.Status,
        bytes_read: u64,
    ) Error {
        self.last_failure = .{
            .phase = phase,
            .cause = cause,
            .status = status,
            .bytes_read = bytes_read,
        };
        return mapped;
    }
};

/// What `pumpBody` reports alongside its error: the reader error before
/// `mapError` collapses it, and the bytes already handed to `w`. `pumpBody`
/// knows nothing of `std.http`, so the unwrap of `cause` happens in `fetch`,
/// which holds the response.
const Pump = struct {
    cause: anyerror = error.Unexpected,
    delivered: u64 = 0,

    fn fail(self: *Pump, cause: anyerror, mapped: Error) Error {
        self.cause = cause;
        return mapped;
    }
};

/// Streams the body into `w`, refusing more than `cap` bytes. `fetch` always
/// passes `max_body_bytes`; the parameter exists so the refusal is testable
/// without a 64 MiB body.
///
/// `w` is the only destination: `Reader.stream` hands the reader's buffered
/// bytes straight to the writer, so the body is never copied twice and the
/// reader's own buffer is never a destination slice. Reading into that buffer
/// instead — `readSliceShort(self.transfer_buf)` on a reader constructed over
/// `self.transfer_buf` — makes `@memcpy` copy the buffer onto itself
/// (Reader.zig:677) and panics the process on the first read that finds bytes
/// already buffered.
fn pumpBody(body: *std.Io.Reader, w: *std.Io.Writer, cap: usize, out: *Pump) Error!u64 {
    var total: usize = 0;
    while (total < cap) {
        out.delivered = total;
        total += body.stream(w, .limited(cap - total)) catch |err| switch (err) {
            error.EndOfStream => return total,
            // The caller owns `w` and can read the concrete failure from its
            // own writer; this taxonomy has no member for a failing sink.
            error.WriteFailed => return out.fail(err, error.Unexpected),
            else => |e| return out.fail(e, mapError(e, .receive_body)),
        };
    }
    out.delivered = total;

    // `cap` written exactly. One more byte separates a body that fits from one
    // that was cut off, and it must not reach `w`.
    var probe_buf: [1]u8 = undefined;
    var probe: std.Io.Writer.Discarding = .init(&probe_buf);
    const extra = body.stream(&probe.writer, .limited(1)) catch |err| switch (err) {
        error.EndOfStream => return total,
        error.WriteFailed => return out.fail(err, error.Unexpected),
        else => |e| return out.fail(e, mapError(e, .receive_body)),
    };
    return if (extra == 0) total else out.fail(error.BodyTooLarge, error.BodyTooLarge);
}

/// A scheme other than `http`/`https`, an unparseable URL and a URL with no
/// host are one fault to the operator: the source row is unusable.
fn parseUrl(url: []const u8) Error!std.Uri {
    const uri = std.Uri.parse(url) catch return error.BadUrl;
    if (!std.mem.eql(u8, uri.scheme, "http") and
        !std.mem.eql(u8, uri.scheme, "https")) return error.BadUrl;
    const host = uri.host orelse return error.BadUrl;
    if (host.isEmpty()) return error.BadUrl;
    return uri;
}

// `error.X` in an expression names a member into existence rather than
// referring to one, so the switch in `mapError` would keep compiling — and
// silently stop matching — if std renamed either of these. This is what fails
// the build instead.
comptime {
    for ([_][]const u8{ "TlsInitializationFailed", "CertificateBundleLoadFailure" }) |name| {
        if (!errorSetHas(std.http.Client.RequestError, name)) {
            @compileError("std.http.Client.RequestError no longer names " ++ name);
        }
    }
}

fn errorSetHas(comptime Set: type, comptime name: []const u8) bool {
    for (@typeInfo(Set).error_set.?) |member| {
        if (std.mem.eql(u8, member.name, name)) return true;
    }
    return false;
}

/// The two names are the whole TLS surface this file can reach.
/// `std.http.Client` collapses every handshake fault into
/// `error.TlsInitializationFailed` (Client.zig:1470) and every bundle fault into
/// `error.CertificateBundleLoadFailure`, and this file never unwraps a
/// connection's stashed read cause *for classification* — it maps the collapsed
/// `error.ReadFailed` by phase — so the record-layer members of
/// `std.crypto.tls.Client.ReadError` cannot arrive here. `fetch` does unwrap
/// them, into `Failure.cause`, which no classification reads.
fn mapError(err: anyerror, phase: Phase) Error {
    if (transport.mapLocal(err)) |local| return narrowLocal(local);
    switch (err) {
        error.Timeout => return error.Timeout,
        // Both are ruled out by `parseUrl` before the client is touched.
        error.UnsupportedUriScheme, error.UriMissingHost => return error.BadUrl,
        error.TooManyHttpRedirects => return error.HttpStatus,
        else => {},
    }
    switch (err) {
        error.TlsInitializationFailed, error.CertificateBundleLoadFailure => return error.TlsFailed,
        else => {},
    }
    return switch (phase) {
        .connect => error.ConnectFailed,
        .send => error.SendFailed,
        .receive, .receive_body => error.ReceiveFailed,
    };
}

/// `transport.mapLocal` answers in `transport.ExchangeError`, which is wider
/// than this file's taxonomy. Both file-descriptor quotas are the same
/// exhaustion to a downloader, and `error.BufferTooSmall` cannot occur because
/// this file hands the client no undersized buffer.
fn narrowLocal(local: transport.ExchangeError) Error {
    return switch (local) {
        error.OutOfMemory => error.OutOfMemory,
        error.SystemResources,
        error.ProcessFdQuotaExceeded,
        error.SystemFdQuotaExceeded,
        => error.SystemResources,
        error.Canceled => error.Canceled,
        else => error.Unexpected,
    };
}

const testing = std.testing;

// `Reader.fixed` hands back a reader whose whole payload is already buffered
// (seek 0, end len) — the state that made the previous implementation copy the
// reader's buffer onto itself and abort the process mid-download.

test "pumpBody streams a fully buffered body without aliasing its source" {
    const payload = "0.0.0.0 ads.example.com\n0.0.0.0 tracker.example\n";
    var body: std.Io.Reader = .fixed(payload);
    var out: [payload.len]u8 = undefined;
    var sink: std.Io.Writer = .fixed(&out);

    var pump: Pump = .{};
    const n = try pumpBody(&body, &sink, max_body_bytes, &pump);
    try testing.expectEqual(@as(u64, payload.len), n);
    try testing.expectEqualStrings(payload, out[0..payload.len]);
}

test "pumpBody accepts a body of exactly the cap" {
    const payload = "abcdefgh";
    var body: std.Io.Reader = .fixed(payload);
    var out: [payload.len]u8 = undefined;
    var sink: std.Io.Writer = .fixed(&out);

    var pump: Pump = .{};
    const n = try pumpBody(&body, &sink, payload.len, &pump);
    try testing.expectEqual(@as(u64, payload.len), n);
    try testing.expectEqualStrings(payload, out[0..payload.len]);
}

test "pumpBody refuses a body one byte over the cap" {
    const payload = "abcdefgh";
    var body: std.Io.Reader = .fixed(payload);
    var out: [payload.len]u8 = undefined;
    var sink: std.Io.Writer = .fixed(&out);

    var pump: Pump = .{};
    try testing.expectError(
        error.BodyTooLarge,
        pumpBody(&body, &sink, payload.len - 1, &pump),
    );
    try testing.expectEqual(error.BodyTooLarge, pump.cause);
}

test "pumpBody reports an empty body as zero bytes" {
    var body: std.Io.Reader = .fixed("");
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);

    var pump: Pump = .{};
    try testing.expectEqual(@as(u64, 0), try pumpBody(&body, &discarding.writer, max_body_bytes, &pump));
}

fn undefinedFetcher(transfer_buf: []u8, redirect_buf: []u8) Fetcher {
    // `http` is never driven: every test below asserts a rejection that
    // happens before the first client call.
    const http: *std.http.Client = undefined;
    return .{ .http = http, .transfer_buf = transfer_buf, .redirect_buf = redirect_buf };
}

test "fetch rejects a non-http scheme before any client use" {
    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);
    try testing.expectError(
        error.BadUrl,
        f.fetch(undefined, "ftp://example.com/list.txt", &discarding.writer),
    );
}

test "fetch rejects a url with no scheme before any client use" {
    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);
    try testing.expectError(error.BadUrl, f.fetch(undefined, "x", &discarding.writer));
}

test "fetch rejects a url with no host before any client use" {
    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);
    try testing.expectError(error.BadUrl, f.fetch(undefined, "https://", &discarding.writer));
}

test "parseUrl accepts http and https urls" {
    const plain = try parseUrl("http://example.com/hosts.txt");
    try testing.expectEqualStrings("http", plain.scheme);
    const secure = try parseUrl("https://example.com:8443/hosts.txt");
    try testing.expectEqualStrings("https", secure.scheme);
    try testing.expectEqual(@as(?u16, 8443), secure.port);
}

test "parseUrl rejects the url forms the source table can hold" {
    try testing.expectError(error.BadUrl, parseUrl("ftp://example.com/list"));
    try testing.expectError(error.BadUrl, parseUrl("file:///etc/hosts"));
    try testing.expectError(error.BadUrl, parseUrl("x"));
    try testing.expectError(error.BadUrl, parseUrl(""));
    try testing.expectError(error.BadUrl, parseUrl("https://"));
}

test "mapError maps local errors before phase errors" {
    try testing.expectEqual(error.OutOfMemory, mapError(error.OutOfMemory, .connect));
    try testing.expectEqual(error.Canceled, mapError(error.Canceled, .receive));
    try testing.expectEqual(error.Unexpected, mapError(error.Unexpected, .send));
    try testing.expectEqual(error.SystemResources, mapError(error.SystemResources, .connect));
    try testing.expectEqual(
        error.SystemResources,
        mapError(error.ProcessFdQuotaExceeded, .connect),
    );
    try testing.expectEqual(
        error.SystemResources,
        mapError(error.SystemFdQuotaExceeded, .connect),
    );
}

test "mapError maps the reachable tls errors regardless of phase" {
    try testing.expectEqual(error.TlsFailed, mapError(error.TlsInitializationFailed, .connect));
    try testing.expectEqual(error.TlsFailed, mapError(error.TlsInitializationFailed, .receive));
    try testing.expectEqual(error.TlsFailed, mapError(error.CertificateBundleLoadFailure, .connect));
}

test "mapError maps a redirect overrun to HttpStatus" {
    try testing.expectEqual(error.HttpStatus, mapError(error.TooManyHttpRedirects, .receive));
}

test "mapError maps a connect timeout to Timeout" {
    try testing.expectEqual(error.Timeout, mapError(error.Timeout, .connect));
}

test "mapError maps remaining errors by phase" {
    try testing.expectEqual(error.ConnectFailed, mapError(error.ConnectionRefused, .connect));
    try testing.expectEqual(error.SendFailed, mapError(error.WriteFailed, .send));
    try testing.expectEqual(error.ReceiveFailed, mapError(error.ReadFailed, .receive));
    try testing.expectEqual(error.ReceiveFailed, mapError(error.HttpHeadersInvalid, .receive));
}

test "caps are the values the memory budget was sized against" {
    try testing.expectEqual(@as(usize, 64 * 1024 * 1024), max_body_bytes);
    try testing.expectEqual(@as(usize, 8192), redirect_buffer_len);
}

/// Only the fields the unwrap helpers read are set; the rest of a `Connection`
/// is two buffered streams, a host name and a pool node. `.plain` for the
/// reason `doh_client.zig` gives: `getReadError` reaches a TLS connection's
/// cause through `@fieldParentPtr`, which on a stub would read memory that was
/// never a `Tls`.
fn stubConnection(read_err: ?std.Io.net.Stream.Reader.Error) std.http.Client.Connection {
    var connection: std.http.Client.Connection = undefined;
    connection.protocol = .plain;
    connection.stream_reader.err = read_err;
    connection.stream_writer.err = null;
    return connection;
}

test "a head failure records the stashed cause, not the collapsed read error" {
    var connection = stubConnection(error.ConnectionResetByPeer);
    var req: std.http.Client.Request = undefined;
    req.connection = &connection;

    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);

    const err = error.ReadFailed;
    const mapped = f.record(mapError(err, .receive), .receive, transport.headCause(&req, err), null, 0);

    // The taxonomy the caller sees is what it was before the unwrap existed.
    try testing.expectEqual(error.ReceiveFailed, mapped);
    const failure = f.last_failure.?;
    try testing.expectEqual(Phase.receive, failure.phase);
    try testing.expectEqual(error.ConnectionResetByPeer, failure.cause);
    try testing.expectEqual(@as(?std.http.Status, null), failure.status);
    try testing.expectEqual(@as(u64, 0), failure.bytes_read);
}

test "a body failure records the bytes delivered before it and the body phase" {
    const payload = "0.0.0.0 ads.example.com\n";
    var buffer: [payload.len]u8 = (payload ++ "").*;
    // Buffered bytes reach `w` first; the vtable is only asked for more once
    // the buffer is drained, which is the shape of a transfer cut off mid-body.
    var body: std.Io.Reader = .{
        .vtable = std.Io.Reader.failing.vtable,
        .buffer = &buffer,
        .seek = 0,
        .end = buffer.len,
    };
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);

    var pump: Pump = .{};
    try testing.expectError(
        error.ReceiveFailed,
        pumpBody(&body, &discarding.writer, max_body_bytes, &pump),
    );
    try testing.expectEqual(@as(u64, payload.len), pump.delivered);

    var connection = stubConnection(null);
    var req: std.http.Client.Request = undefined;
    req.connection = &connection;
    req.reader.body_err = error.HttpChunkTruncated;
    const resp: std.http.Client.Response = .{ .request = &req, .head = undefined };

    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    const mapped = f.record(
        mapError(error.ReadFailed, .receive_body),
        .receive_body,
        transport.bodyCause(&resp, pump.cause),
        .ok,
        pump.delivered,
    );

    try testing.expectEqual(error.ReceiveFailed, mapped);
    const failure = f.last_failure.?;
    try testing.expectEqual(Phase.receive_body, failure.phase);
    try testing.expectEqual(error.HttpChunkTruncated, failure.cause);
    try testing.expectEqual(@as(?std.http.Status, .ok), failure.status);
    try testing.expectEqual(@as(u64, payload.len), failure.bytes_read);
}

test "a rejected url records the phase without claiming a connect failure" {
    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);

    try testing.expectError(
        error.BadUrl,
        f.fetch(undefined, "ftp://example.com/list.txt", &discarding.writer),
    );
    try testing.expectEqual(error.BadUrl, f.last_failure.?.cause);
}

test "record returns the taxonomy member the call site raised, not a remapped one" {
    // `error.HttpStatus` and `error.BodyTooLarge` are raised by `fetch` itself
    // rather than by std, so `mapError` has no case for either and would fall
    // to the phase default. `record` must not run them through it.
    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);

    try testing.expectEqual(
        error.HttpStatus,
        f.record(error.HttpStatus, .receive, error.HttpStatus, .not_found, 0),
    );
    try testing.expectEqual(@as(?std.http.Status, .not_found), f.last_failure.?.status);
    try testing.expectEqual(error.HttpStatus, f.last_failure.?.cause);

    try testing.expectEqual(
        error.BodyTooLarge,
        f.record(error.BodyTooLarge, .receive, error.BodyTooLarge, .ok, 0),
    );
    try testing.expectEqual(error.BodyTooLarge, f.last_failure.?.cause);

    // This is why: `mapError` is unchanged and does collapse both to the phase
    // default. `fetch` must never route these two through it.
    try testing.expectEqual(error.ReceiveFailed, mapError(error.HttpStatus, .receive));
    try testing.expectEqual(error.ReceiveFailed, mapError(error.BodyTooLarge, .receive));
}

test "a cancelled body read records the cancellation and the progress so far" {
    // This is the shape the expiry race leaves behind. `Select.cancelDiscard`
    // cancels the fetch, the cancellation surfaces from the socket as a
    // stashed `error.Canceled` under a collapsed `error.ReadFailed`, and the
    // fetch records it on its way out. The manager then reports its own
    // `error.Timeout` beside this record.
    const payload = "0.0.0.0 ads.example.com\n";
    var buffer: [payload.len]u8 = (payload ++ "").*;
    var body: std.Io.Reader = .{
        .vtable = std.Io.Reader.failing.vtable,
        .buffer = &buffer,
        .seek = 0,
        .end = buffer.len,
    };
    var sink_buf: [0]u8 = .{};
    var discarding: std.Io.Writer.Discarding = .init(&sink_buf);

    var pump: Pump = .{};
    try testing.expectError(
        error.ReceiveFailed,
        pumpBody(&body, &discarding.writer, max_body_bytes, &pump),
    );

    var connection = stubConnection(error.Canceled);
    var req: std.http.Client.Request = undefined;
    req.connection = &connection;
    req.reader.body_err = null;
    const resp: std.http.Client.Response = .{ .request = &req, .head = undefined };

    var transfer_buf: [min_transfer_buf]u8 = undefined;
    var redirect_buf: [redirect_buffer_len]u8 = undefined;
    var f = undefinedFetcher(&transfer_buf, &redirect_buf);
    // `pumpBody` maps the collapsed `error.ReadFailed` it was handed, not the
    // unwrapped cause, so the taxonomy stays `ReceiveFailed` here. That is what
    // this path returned before `Failure` existed, and the addendum keeps it:
    // only the recorded cause is new. The manager reports `error.Timeout` from
    // the expiry race anyway, so nothing reads this return value on this path.
    const cause = transport.bodyCause(&resp, error.ReadFailed);
    const mapped = f.record(error.ReceiveFailed, .receive_body, cause, .ok, pump.delivered);

    try testing.expectEqual(error.ReceiveFailed, mapped);
    try testing.expectEqual(Phase.receive_body, f.last_failure.?.phase);
    try testing.expectEqual(error.Canceled, f.last_failure.?.cause);
    try testing.expectEqual(@as(?std.http.Status, .ok), f.last_failure.?.status);
    try testing.expectEqual(@as(u64, payload.len), f.last_failure.?.bytes_read);
}