Reader

pub const Reader = struct {
    in: *std.Io.Reader,
    /// This is preallocated memory that might be used by `bodyReader`. That
    /// function might return a pointer to this field, or a different
    /// `*std.Io.Reader`. Advisable to not access this field directly.
    interface: std.Io.Reader,
    /// Keeps track of whether the stream is ready to accept a new request,
    /// making invalid API usage cause assertion failures rather than HTTP
    /// protocol violations.
    state: State,
    /// HTTP trailer bytes. These are at the end of a transfer-encoding:
    /// chunked message. This data is available only after calling one of the
    /// "end" functions and points to data inside the buffer of `in`, and is
    /// therefore invalidated on the next call to `receiveHead`, or any other
    /// read from `in`.
    trailers: []const u8 = &.{},
    body_err: ?BodyError = null,
    max_head_len: usize,

    pub const RemainingChunkLen = enum(u64) {
        head = 0,
        n = 1,
        rn = 2,
        _,

        pub fn init(integer: u64) RemainingChunkLen {
            return @fromBackingInt(@intCast(integer));
        }

        pub fn int(rcl: RemainingChunkLen) u64 {
            return @backingInt(rcl);
        }
    };

    pub const State = union(enum) {
        /// The stream is available to be used for the first time, or reused.
        ready,
        received_head,
        /// The stream goes until the connection is closed.
        body_none,
        body_remaining_content_length: u64,
        body_remaining_chunk_len: RemainingChunkLen,
        /// The stream would be eligible for another HTTP request, however the
        /// client and server did not negotiate a persistent connection.
        closing,
    };

    pub const BodyError = error{
        HttpChunkInvalid,
        HttpChunkTruncated,
        HttpHeadersOversize,
    };

    pub const HeadError = error{
        /// Too many bytes of HTTP headers.
        ///
        /// The HTTP specification suggests to respond with a 431 status code
        /// before closing the connection.
        HttpHeadersOversize,
        /// Partial HTTP request was received but the connection was closed
        /// before fully receiving the headers.
        HttpRequestTruncated,
        /// The client sent 0 bytes of headers before closing the stream. This
        /// happens when a keep-alive connection is finally closed.
        HttpConnectionClosing,
        /// Transitive error occurred reading from `in`.
        ReadFailed,
    };

    /// Buffers the entire head inside `in`.
    ///
    /// The resulting memory is invalidated by any subsequent consumption of
    /// the input stream.
    pub fn receiveHead(reader: *Reader) HeadError![]const u8 {
        reader.trailers = &.{};
        const in = reader.in;
        const max_head_len = reader.max_head_len;
        var hp: HeadParser = .{};
        var head_len: usize = 0;
        while (true) {
            if (head_len >= max_head_len) return error.HttpHeadersOversize;
            const remaining = in.buffered()[head_len..];
            if (remaining.len == 0) {
                in.fillMore() catch |err| switch (err) {
                    error.EndOfStream => switch (head_len) {
                        0 => return error.HttpConnectionClosing,
                        else => return error.HttpRequestTruncated,
                    },
                    error.ReadFailed => |e| return e,
                };
                continue;
            }
            head_len += hp.feed(remaining);
            if (hp.state == .finished) {
                reader.state = .received_head;
                const head_buffer = in.buffered()[0..head_len];
                in.toss(head_len);
                return head_buffer;
            }
        }
    }

    /// If compressed body has been negotiated this will return compressed bytes.
    ///
    /// Asserts only called once and after `receiveHead`.
    ///
    /// See also:
    /// * `interfaceDecompressing`
    pub fn bodyReader(
        reader: *Reader,
        transfer_buffer: []u8,
        transfer_encoding: TransferEncoding,
        content_length: ?u64,
    ) *std.Io.Reader {
        assert(reader.state == .received_head);
        switch (transfer_encoding) {
            .chunked => {
                reader.state = .{ .body_remaining_chunk_len = .head };
                reader.interface = .{
                    .buffer = transfer_buffer,
                    .seek = 0,
                    .end = 0,
                    .vtable = &.{
                        .stream = chunkedStream,
                        .discard = chunkedDiscard,
                    },
                };
                return &reader.interface;
            },
            .none => {
                if (content_length) |len| {
                    reader.state = if (len == 0) .ready else .{ .body_remaining_content_length = len };
                    reader.interface = .{
                        .buffer = transfer_buffer,
                        .seek = 0,
                        .end = 0,
                        .vtable = &.{
                            .stream = contentLengthStream,
                            .discard = contentLengthDiscard,
                        },
                    };
                    return &reader.interface;
                } else {
                    reader.state = .body_none;
                    return reader.in;
                }
            },
        }
    }

    /// If compressed body has been negotiated this will return decompressed bytes.
    ///
    /// Asserts only called once and after `receiveHead`.
    ///
    /// See also:
    /// * `interface`
    pub fn bodyReaderDecompressing(
        reader: *Reader,
        transfer_buffer: []u8,
        transfer_encoding: TransferEncoding,
        content_length: ?u64,
        content_encoding: ContentEncoding,
        decompress: *Decompress,
        decompress_buffer: []u8,
    ) *std.Io.Reader {
        if (transfer_encoding == .none and content_length == null) {
            assert(reader.state == .received_head);
            reader.state = .body_none;
            switch (content_encoding) {
                .identity => {
                    return reader.in;
                },
                .deflate => {
                    decompress.* = .{ .flate = .init(reader.in, .zlib, decompress_buffer) };
                    return &decompress.flate.reader;
                },
                .gzip => {
                    decompress.* = .{ .flate = .init(reader.in, .gzip, decompress_buffer) };
                    return &decompress.flate.reader;
                },
                .zstd => {
                    decompress.* = .{ .zstd = .init(reader.in, decompress_buffer, .{ .verify_checksum = false }) };
                    return &decompress.zstd.reader;
                },
                .compress => unreachable,
            }
        }
        const transfer_reader = bodyReader(reader, transfer_buffer, transfer_encoding, content_length);
        return decompress.init(transfer_reader, decompress_buffer, content_encoding);
    }

    fn contentLengthStream(
        io_r: *std.Io.Reader,
        w: *Writer,
        limit: std.Io.Limit,
    ) std.Io.Reader.StreamError!usize {
        const reader: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
        if (reader.state == .ready) return error.EndOfStream;
        const remaining_content_length = &reader.state.body_remaining_content_length;
        const remaining = remaining_content_length.*;
        const n = try reader.in.stream(w, limit.min(.limited64(remaining)));
        if (n == remaining) {
            reader.state = .ready;
        } else {
            remaining_content_length.* = remaining - n;
        }
        return n;
    }

    fn contentLengthDiscard(io_r: *std.Io.Reader, limit: std.Io.Limit) std.Io.Reader.Error!usize {
        const reader: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
        if (reader.state == .ready) return error.EndOfStream;
        const remaining_content_length = &reader.state.body_remaining_content_length;
        const remaining = remaining_content_length.*;
        const n = try reader.in.discard(limit.min(.limited64(remaining)));
        if (n == remaining) {
            reader.state = .ready;
        } else {
            remaining_content_length.* = remaining - n;
        }
        return n;
    }

    fn chunkedStream(io_r: *std.Io.Reader, w: *Writer, limit: std.Io.Limit) std.Io.Reader.StreamError!usize {
        const reader: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
        const chunk_len_ptr = switch (reader.state) {
            .ready => return error.EndOfStream,
            .body_remaining_chunk_len => |*x| x,
            else => unreachable,
        };
        return chunkedReadEndless(reader, w, limit, chunk_len_ptr) catch |err| switch (err) {
            error.ReadFailed, error.WriteFailed => |e| return e,
            error.EndOfStream => {
                reader.body_err = error.HttpChunkTruncated;
                return error.ReadFailed;
            },
            else => |e| {
                reader.body_err = e;
                return error.ReadFailed;
            },
        };
    }

    fn chunkedReadEndless(
        reader: *Reader,
        w: *Writer,
        limit: std.Io.Limit,
        chunk_len_ptr: *RemainingChunkLen,
    ) (BodyError || std.Io.Reader.StreamError)!usize {
        const in = reader.in;
        len: switch (chunk_len_ptr.*) {
            .head => {
                var cp: ChunkParser = .init;
                while (true) {
                    const i = cp.feed(in.buffered());
                    switch (cp.state) {
                        .invalid => return error.HttpChunkInvalid,
                        .data => {
                            in.toss(i);
                            break;
                        },
                        else => {
                            in.toss(i);
                            try in.fillMore();
                            continue;
                        },
                    }
                }
                if (cp.chunk_len == 0) return parseTrailers(reader, 0);
                const n = try in.stream(w, limit.min(.limited64(cp.chunk_len)));
                chunk_len_ptr.* = .init(cp.chunk_len + 2 - n);
                return n;
            },
            .n => {
                if ((try in.peekByte()) != '\n') return error.HttpChunkInvalid;
                in.toss(1);
                continue :len .head;
            },
            .rn => {
                const rn = try in.peekArray(2);
                if (rn[0] != '\r' or rn[1] != '\n') return error.HttpChunkInvalid;
                in.toss(2);
                continue :len .head;
            },
            else => |remaining_chunk_len| {
                const n = try in.stream(w, limit.min(.limited64(@backingInt(remaining_chunk_len) - 2)));
                chunk_len_ptr.* = .init(@backingInt(remaining_chunk_len) - n);
                return n;
            },
        }
    }

    fn chunkedDiscard(io_r: *std.Io.Reader, limit: std.Io.Limit) std.Io.Reader.Error!usize {
        const reader: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
        const chunk_len_ptr = switch (reader.state) {
            .ready => return error.EndOfStream,
            .body_remaining_chunk_len => |*x| x,
            else => unreachable,
        };
        return chunkedDiscardEndless(reader, limit, chunk_len_ptr) catch |err| switch (err) {
            error.ReadFailed => |e| return e,
            error.EndOfStream => {
                reader.body_err = error.HttpChunkTruncated;
                return error.ReadFailed;
            },
            else => |e| {
                reader.body_err = e;
                return error.ReadFailed;
            },
        };
    }

    fn chunkedDiscardEndless(
        reader: *Reader,
        limit: std.Io.Limit,
        chunk_len_ptr: *RemainingChunkLen,
    ) (BodyError || std.Io.Reader.Error)!usize {
        const in = reader.in;
        len: switch (chunk_len_ptr.*) {
            .head => {
                var cp: ChunkParser = .init;
                while (true) {
                    const i = cp.feed(in.buffered());
                    switch (cp.state) {
                        .invalid => return error.HttpChunkInvalid,
                        .data => {
                            in.toss(i);
                            break;
                        },
                        else => {
                            in.toss(i);
                            try in.fillMore();
                            continue;
                        },
                    }
                }
                if (cp.chunk_len == 0) return parseTrailers(reader, 0);
                const n = try in.discard(limit.min(.limited64(cp.chunk_len)));
                chunk_len_ptr.* = .init(cp.chunk_len + 2 - n);
                return n;
            },
            .n => {
                if ((try in.peekByte()) != '\n') return error.HttpChunkInvalid;
                in.toss(1);
                continue :len .head;
            },
            .rn => {
                const rn = try in.peekArray(2);
                if (rn[0] != '\r' or rn[1] != '\n') return error.HttpChunkInvalid;
                in.toss(2);
                continue :len .head;
            },
            else => |remaining_chunk_len| {
                const n = try in.discard(limit.min(.limited64(remaining_chunk_len.int() - 2)));
                chunk_len_ptr.* = .init(remaining_chunk_len.int() - n);
                return n;
            },
        }
    }

    /// Called when next bytes in the stream are trailers, or "\r\n" to indicate
    /// end of chunked body.
    fn parseTrailers(reader: *Reader, amt_read: usize) (BodyError || std.Io.Reader.Error)!usize {
        const in = reader.in;
        const rn = try in.peekArray(2);
        if (rn[0] == '\r' and rn[1] == '\n') {
            in.toss(2);
            reader.state = .ready;
            assert(reader.trailers.len == 0);
            return amt_read;
        }
        var hp: HeadParser = .{ .state = .seen_rn };
        var trailers_len: usize = 2;
        while (true) {
            if (in.buffer.len - trailers_len == 0) return error.HttpHeadersOversize;
            const remaining = in.buffered()[trailers_len..];
            if (remaining.len == 0) {
                try in.fillMore();
                continue;
            }
            trailers_len += hp.feed(remaining);
            if (hp.state == .finished) {
                reader.state = .ready;
                reader.trailers = in.buffered()[0..trailers_len];
                in.toss(trailers_len);
                return amt_read;
            }
        }
    }
}