getLocked

    fn getLocked(q: *TypeErasedQueue, io: Io, buffer: []u8, min: usize, uncancelable: bool) (QueueClosedError || Cancelable)!usize {
        // The ring buffer gets first priority, then data should come from any
        // queued putters, then finally the ring buffer should be filled with
        // data from putters so they can be resumed.

        // The number of elements we received immediately, before possibly blocking.
        var n: usize = 0;

        while (q.gettableSlice()) |slice| {
            const copy_len = @min(slice.len, buffer.len - n);
            assert(copy_len > 0);
            @memcpy(buffer[n..][0..copy_len], slice[0..copy_len]);
            q.start += copy_len;
            if (q.buffer.len - q.start == 0) q.start = 0;
            q.len -= copy_len;
            n += copy_len;
            if (n == buffer.len) {
                q.fillRingBufferFromPutters(io);
                return buffer.len;
            }
        }

        // Copy directly from putters into buffer.
        while (q.putters.popFirst()) |putter_node| {
            const putter: *Put = @alignCast(@fieldParentPtr("node", putter_node));
            const copy_len = @min(putter.remaining.len, buffer.len - n);
            assert(copy_len > 0);
            @memcpy(buffer[n..][0..copy_len], putter.remaining[0..copy_len]);
            putter.remaining = putter.remaining[copy_len..];
            putter.needed -|= copy_len;
            n += copy_len;
            if (putter.needed == 0) {
                putter.condition.signal(io);
            } else {
                assert(n == buffer.len); // we didn't have enough space for the putter
                q.putters.prepend(putter_node);
            }
            if (n == buffer.len) {
                q.fillRingBufferFromPutters(io);
                return buffer.len;
            }
        }

        // No need to call `fillRingBufferFromPutters` from this point onwards,
        // because we emptied the ring buffer *and* the putter queue!

        // Don't block if we hit the min or if the queue is closed. Return how
        // many elements we could get immediately, unless the queue was closed and
        // empty, in which case report `error.Closed`.
        if (n == 0 and q.closed) return error.Closed;
        if (n >= min or q.closed) return n;

        var pending: Get = .{
            .remaining = buffer[n..],
            .needed = min - n,
            .condition = .init,
            .node = .{},
        };
        q.getters.append(&pending.node);
        defer if (pending.needed > 0) q.getters.remove(&pending.node);

        while (pending.needed > 0 and !q.closed) {
            if (uncancelable) {
                pending.condition.waitUncancelable(io, &q.mutex);
                continue;
            }
            pending.condition.wait(io, &q.mutex) catch |err| switch (err) {
                error.Canceled => if (pending.remaining.len == buffer.len) {
                    // Canceled while waiting, and received no elements.
                    return error.Canceled;
                } else {
                    // Canceled while waiting, but received some elements, so report those first.
                    io.recancel();
                    return buffer.len - pending.remaining.len;
                },
            };
        }
        if (pending.remaining.len == buffer.len) {
            // The queue was closed while we were waiting. We received no elements.
            assert(q.closed);
            return error.Closed;
        }
        return buffer.len - pending.remaining.len;
    }