input buffer of type Halide::Runtime::Buffer gets modified by generator

I have a generator that has input and output:

Input<Buffer<uint8_t>> input{"input", 2};
Output<Buffer<uint8_t>> output{"output", 2};

In generate method I defined following algorithm:

output(c,x) = Halide::cast<uint8_t> (input(mux(c, {1,0,2,3,0,2}), x));

The problem is that when I pass input and output buffer from main program I get the desired output but input buffer gets also modified which I would like to avoid. I tried to make function and then apply algorithm but I have same effect:

    Func decode;
    Func in;
    in(c,x) = input(c,x);
    decode(c,x) = Halide::cast<uint8_t> (in(mux(c, {1,0,2,3,0,2}), x));
    output(c,x) = decode(c,x);
    ...

I also tried to create copy of input buffer from Input<Buffer<uint8_t>> input{"input", 2} like:

in(c,x) = input(c,x);
Halide::Buffer<uint8_t> in_copy = in.realize({Halide::Internal::as_const_int(input.dim(0).extent()), Halide::Internal::as_const_int(input.dim(1).extent())});

but this results in Unhandled exception: Error: Buffer argument input is nullptr which is understandable. Do You have any suggestion how to avoid input buffer mutation? @Alex asked to post compile-able generator so here is a version with using the functions

#include "Halide.h"

using namespace Halide;

class Yuv422Decoder : public Halide::Generator<Yuv422Decoder> {
public:

    Input<Buffer<uint8_t>> input{"input", 2};
    Output<Buffer<uint8_t>> output{"output", 2};

    Var c,x,xo,xi,co,ci;

    void generate() {
        Func decode;
        Func in;
        in(c,x) = input(c,x);
        // define algorithm
        decode(c,x) = Halide::cast<uint8_t> (in(mux(c, {1,0,2,3,0,2}), x));
        output(c,x) = decode(c,x);       
    }

    void schedule() {
        output.bound_extent(c,6);
        output.split(x, xo, xi, input.dim(1).extent()/8);
        output.parallel(xo,2);
        output.parallel(xi,2);
        output.unroll(c);
        output.vectorize(xi,128);
       }
    };

// Use this macro to create function that you can call in your program

HALIDE_REGISTER_GENERATOR(Yuv422Decoder, yuv422decoder);

Problem was in size of the output buffer. I doubled it and now I don't have mutation of input buffer. I am not sure how exactly this happens when size of the output buffer is too small but anyhow it was programming mistake in the end.

uint8_t* buffer_out = new uint8_t [2*size];
Runtime::Buffer<uint8_t> out(buffer_out, {{0,6,1}, {0,size/4,6}});