Poller

pub fn Poller(comptime StreamEnum: type) type {
    return struct {
        const enum_fields = @typeInfo(StreamEnum).@"enum".fields;
        const PollFd = if (is_windows) void else posix.pollfd;

        gpa: Allocator,
        readers: [enum_fields.len]Reader,
        poll_fds: [enum_fields.len]PollFd,
        windows: if (is_windows) struct {
            first_read_done: bool,
            overlapped: [enum_fields.len]windows.OVERLAPPED,
            small_bufs: [enum_fields.len][128]u8,
            active: struct {
                count: math.IntFittingRange(0, enum_fields.len),
                handles_buf: [enum_fields.len]windows.HANDLE,
                stream_map: [enum_fields.len]StreamEnum,

                pub fn removeAt(self: *@This(), index: u32) void {
                    assert(index < self.count);
                    for (index + 1..self.count) |i| {
                        self.handles_buf[i - 1] = self.handles_buf[i];
                        self.stream_map[i - 1] = self.stream_map[i];
                    }
                    self.count -= 1;
                }
            },
        } else void,

        const Self = @This();

        pub fn deinit(self: *Self) void {
            const gpa = self.gpa;
            if (is_windows) {
                // cancel any pending IO to prevent clobbering OVERLAPPED value
                for (self.windows.active.handles_buf[0..self.windows.active.count]) |h| {
                    _ = windows.kernel32.CancelIo(h);
                }
            }
            inline for (&self.readers) |*r| gpa.free(r.buffer);
            self.* = undefined;
        }

        pub fn poll(self: *Self) !bool {
            if (is_windows) {
                return pollWindows(self, null);
            } else {
                return pollPosix(self, null);
            }
        }

        pub fn pollTimeout(self: *Self, nanoseconds: u64) !bool {
            if (is_windows) {
                return pollWindows(self, nanoseconds);
            } else {
                return pollPosix(self, nanoseconds);
            }
        }

        pub fn reader(self: *Self, which: StreamEnum) *Reader {
            return &self.readers[@intFromEnum(which)];
        }

        pub fn toOwnedSlice(self: *Self, which: StreamEnum) error{OutOfMemory}![]u8 {
            const gpa = self.gpa;
            const r = reader(self, which);
            if (r.seek == 0) {
                const new = try gpa.realloc(r.buffer, r.end);
                r.buffer = &.{};
                r.end = 0;
                return new;
            }
            const new = try gpa.dupe(u8, r.buffered());
            gpa.free(r.buffer);
            r.buffer = &.{};
            r.seek = 0;
            r.end = 0;
            return new;
        }

        fn pollWindows(self: *Self, nanoseconds: ?u64) !bool {
            const bump_amt = 512;
            const gpa = self.gpa;

            if (!self.windows.first_read_done) {
                var already_read_data = false;
                for (0..enum_fields.len) |i| {
                    const handle = self.windows.active.handles_buf[i];
                    switch (try windowsAsyncReadToFifoAndQueueSmallRead(
                        gpa,
                        handle,
                        &self.windows.overlapped[i],
                        &self.readers[i],
                        &self.windows.small_bufs[i],
                        bump_amt,
                    )) {
                        .populated, .empty => |state| {
                            if (state == .populated) already_read_data = true;
                            self.windows.active.handles_buf[self.windows.active.count] = handle;
                            self.windows.active.stream_map[self.windows.active.count] = @as(StreamEnum, @enumFromInt(i));
                            self.windows.active.count += 1;
                        },
                        .closed => {}, // don't add to the wait_objects list
                        .closed_populated => {
                            // don't add to the wait_objects list, but we did already get data
                            already_read_data = true;
                        },
                    }
                }
                self.windows.first_read_done = true;
                if (already_read_data) return true;
            }

            while (true) {
                if (self.windows.active.count == 0) return false;

                const status = windows.kernel32.WaitForMultipleObjects(
                    self.windows.active.count,
                    &self.windows.active.handles_buf,
                    0,
                    if (nanoseconds) |ns|
                        @min(std.math.cast(u32, ns / std.time.ns_per_ms) orelse (windows.INFINITE - 1), windows.INFINITE - 1)
                    else
                        windows.INFINITE,
                );
                if (status == windows.WAIT_FAILED)
                    return windows.unexpectedError(windows.GetLastError());
                if (status == windows.WAIT_TIMEOUT)
                    return true;

                if (status < windows.WAIT_OBJECT_0 or status > windows.WAIT_OBJECT_0 + enum_fields.len - 1)
                    unreachable;

                const active_idx = status - windows.WAIT_OBJECT_0;

                const stream_idx = @intFromEnum(self.windows.active.stream_map[active_idx]);
                const handle = self.windows.active.handles_buf[active_idx];

                const overlapped = &self.windows.overlapped[stream_idx];
                const stream_reader = &self.readers[stream_idx];
                const small_buf = &self.windows.small_bufs[stream_idx];

                const num_bytes_read = switch (try windowsGetReadResult(handle, overlapped, false)) {
                    .success => |n| n,
                    .closed => {
                        self.windows.active.removeAt(active_idx);
                        continue;
                    },
                    .aborted => unreachable,
                };
                const buf = small_buf[0..num_bytes_read];
                const dest = try writableSliceGreedyAlloc(stream_reader, gpa, buf.len);
                @memcpy(dest[0..buf.len], buf);
                advanceBufferEnd(stream_reader, buf.len);

                switch (try windowsAsyncReadToFifoAndQueueSmallRead(
                    gpa,
                    handle,
                    overlapped,
                    stream_reader,
                    small_buf,
                    bump_amt,
                )) {
                    .empty => {}, // irrelevant, we already got data from the small buffer
                    .populated => {},
                    .closed,
                    .closed_populated, // identical, since we already got data from the small buffer
                    => self.windows.active.removeAt(active_idx),
                }
                return true;
            }
        }

        fn pollPosix(self: *Self, nanoseconds: ?u64) !bool {
            const gpa = self.gpa;
            // We ask for ensureUnusedCapacity with this much extra space. This
            // has more of an effect on small reads because once the reads
            // start to get larger the amount of space an ArrayList will
            // allocate grows exponentially.
            const bump_amt = 512;

            const err_mask = posix.POLL.ERR | posix.POLL.NVAL | posix.POLL.HUP;

            const events_len = try posix.poll(&self.poll_fds, if (nanoseconds) |ns|
                std.math.cast(i32, ns / std.time.ns_per_ms) orelse std.math.maxInt(i32)
            else
                -1);
            if (events_len == 0) {
                for (self.poll_fds) |poll_fd| {
                    if (poll_fd.fd != -1) return true;
                } else return false;
            }

            var keep_polling = false;
            for (&self.poll_fds, &self.readers) |*poll_fd, *r| {
                // Try reading whatever is available before checking the error
                // conditions.
                // It's still possible to read after a POLL.HUP is received,
                // always check if there's some data waiting to be read first.
                if (poll_fd.revents & posix.POLL.IN != 0) {
                    const buf = try writableSliceGreedyAlloc(r, gpa, bump_amt);
                    const amt = posix.read(poll_fd.fd, buf) catch |err| switch (err) {
                        error.BrokenPipe => 0, // Handle the same as EOF.
                        else => |e| return e,
                    };
                    advanceBufferEnd(r, amt);
                    if (amt == 0) {
                        // Remove the fd when the EOF condition is met.
                        poll_fd.fd = -1;
                    } else {
                        keep_polling = true;
                    }
                } else if (poll_fd.revents & err_mask != 0) {
                    // Exclude the fds that signaled an error.
                    poll_fd.fd = -1;
                } else if (poll_fd.fd != -1) {
                    keep_polling = true;
                }
            }
            return keep_polling;
        }

        /// Returns a slice into the unused capacity of `buffer` with at least
        /// `min_len` bytes, extending `buffer` by resizing it with `gpa` as necessary.
        ///
        /// After calling this function, typically the caller will follow up with a
        /// call to `advanceBufferEnd` to report the actual number of bytes buffered.
        fn writableSliceGreedyAlloc(r: *Reader, allocator: Allocator, min_len: usize) Allocator.Error![]u8 {
            {
                const unused = r.buffer[r.end..];
                if (unused.len >= min_len) return unused;
            }
            if (r.seek > 0) {
                const data = r.buffer[r.seek..r.end];
                @memmove(r.buffer[0..data.len], data);
                r.seek = 0;
                r.end = data.len;
            }
            {
                var list: std.ArrayListUnmanaged(u8) = .{
                    .items = r.buffer[0..r.end],
                    .capacity = r.buffer.len,
                };
                defer r.buffer = list.allocatedSlice();
                try list.ensureUnusedCapacity(allocator, min_len);
            }
            const unused = r.buffer[r.end..];
            assert(unused.len >= min_len);
            return unused;
        }

        /// After writing directly into the unused capacity of `buffer`, this function
        /// updates `end` so that users of `Reader` can receive the data.
        fn advanceBufferEnd(r: *Reader, n: usize) void {
            assert(n <= r.buffer.len - r.end);
            r.end += n;
        }

        /// The `ReadFile` docuementation states that `lpNumberOfBytesRead` does not have a meaningful
        /// result when using overlapped I/O, but also that it cannot be `null` on Windows 7. For
        /// compatibility, we point it to this dummy variables, which we never otherwise access.
        /// See: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile
        var win_dummy_bytes_read: u32 = undefined;

        /// Read as much data as possible from `handle` with `overlapped`, and write it to the FIFO. Before
        /// returning, queue a read into `small_buf` so that `WaitForMultipleObjects` returns when more data
        /// is available. `handle` must have no pending asynchronous operation.
        fn windowsAsyncReadToFifoAndQueueSmallRead(
            gpa: Allocator,
            handle: windows.HANDLE,
            overlapped: *windows.OVERLAPPED,
            r: *Reader,
            small_buf: *[128]u8,
            bump_amt: usize,
        ) !enum { empty, populated, closed_populated, closed } {
            var read_any_data = false;
            while (true) {
                const fifo_read_pending = while (true) {
                    const buf = try writableSliceGreedyAlloc(r, gpa, bump_amt);
                    const buf_len = math.cast(u32, buf.len) orelse math.maxInt(u32);

                    if (0 == windows.kernel32.ReadFile(
                        handle,
                        buf.ptr,
                        buf_len,
                        &win_dummy_bytes_read,
                        overlapped,
                    )) switch (windows.GetLastError()) {
                        .IO_PENDING => break true,
                        .BROKEN_PIPE => return if (read_any_data) .closed_populated else .closed,
                        else => |err| return windows.unexpectedError(err),
                    };

                    const num_bytes_read = switch (try windowsGetReadResult(handle, overlapped, false)) {
                        .success => |n| n,
                        .closed => return if (read_any_data) .closed_populated else .closed,
                        .aborted => unreachable,
                    };

                    read_any_data = true;
                    advanceBufferEnd(r, num_bytes_read);

                    if (num_bytes_read == buf_len) {
                        // We filled the buffer, so there's probably more data available.
                        continue;
                    } else {
                        // We didn't fill the buffer, so assume we're out of data.
                        // There is no pending read.
                        break false;
                    }
                };

                if (fifo_read_pending) cancel_read: {
                    // Cancel the pending read into the FIFO.
                    _ = windows.kernel32.CancelIo(handle);

                    // We have to wait for the handle to be signalled, i.e. for the cancellation to complete.
                    switch (windows.kernel32.WaitForSingleObject(handle, windows.INFINITE)) {
                        windows.WAIT_OBJECT_0 => {},
                        windows.WAIT_FAILED => return windows.unexpectedError(windows.GetLastError()),
                        else => unreachable,
                    }

                    // If it completed before we canceled, make sure to tell the FIFO!
                    const num_bytes_read = switch (try windowsGetReadResult(handle, overlapped, true)) {
                        .success => |n| n,
                        .closed => return if (read_any_data) .closed_populated else .closed,
                        .aborted => break :cancel_read,
                    };
                    read_any_data = true;
                    advanceBufferEnd(r, num_bytes_read);
                }

                // Try to queue the 1-byte read.
                if (0 == windows.kernel32.ReadFile(
                    handle,
                    small_buf,
                    small_buf.len,
                    &win_dummy_bytes_read,
                    overlapped,
                )) switch (windows.GetLastError()) {
                    .IO_PENDING => {
                        // 1-byte read pending as intended
                        return if (read_any_data) .populated else .empty;
                    },
                    .BROKEN_PIPE => return if (read_any_data) .closed_populated else .closed,
                    else => |err| return windows.unexpectedError(err),
                };

                // We got data back this time. Write it to the FIFO and run the main loop again.
                const num_bytes_read = switch (try windowsGetReadResult(handle, overlapped, false)) {
                    .success => |n| n,
                    .closed => return if (read_any_data) .closed_populated else .closed,
                    .aborted => unreachable,
                };
                const buf = small_buf[0..num_bytes_read];
                const dest = try writableSliceGreedyAlloc(r, gpa, buf.len);
                @memcpy(dest[0..buf.len], buf);
                advanceBufferEnd(r, buf.len);
                read_any_data = true;
            }
        }

        /// Simple wrapper around `GetOverlappedResult` to determine the result of a `ReadFile` operation.
        /// If `!allow_aborted`, then `aborted` is never returned (`OPERATION_ABORTED` is considered unexpected).
        ///
        /// The `ReadFile` documentation states that the number of bytes read by an overlapped `ReadFile` must be determined using `GetOverlappedResult`, even if the
        /// operation immediately returns data:
        /// "Use NULL for [lpNumberOfBytesRead] if this is an asynchronous operation to avoid potentially
        /// erroneous results."
        /// "If `hFile` was opened with `FILE_FLAG_OVERLAPPED`, the following conditions are in effect: [...]
        /// The lpNumberOfBytesRead parameter should be set to NULL. Use the GetOverlappedResult function to
        /// get the actual number of bytes read."
        /// See: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile
        fn windowsGetReadResult(
            handle: windows.HANDLE,
            overlapped: *windows.OVERLAPPED,
            allow_aborted: bool,
        ) !union(enum) {
            success: u32,
            closed,
            aborted,
        } {
            var num_bytes_read: u32 = undefined;
            if (0 == windows.kernel32.GetOverlappedResult(
                handle,
                overlapped,
                &num_bytes_read,
                0,
            )) switch (windows.GetLastError()) {
                .BROKEN_PIPE => return .closed,
                .OPERATION_ABORTED => |err| if (allow_aborted) {
                    return .aborted;
                } else {
                    return windows.unexpectedError(err);
                },
                else => |err| return windows.unexpectedError(err),
            };
            return .{ .success = num_bytes_read };
        }
    };
}