[Bug] recursive instantiation mutates original config object's interpolations

[jeffknaide]

🐛 Bug

Description

when instantiating a config object that includes interpolations, those interpolations are unexpectedly resolved in the original config object

Checklist

• I checked on the latest version of Hydra
• I created a minimal repro (See this for tips).

To reproduce

** Minimal Code/Config snippet to reproduce **

from omegaconf import OmegaConf
from hydra.utils import instantiate

def foo(**kwargs):
    pass


if __name__ == "__main__":
    cfg2 = OmegaConf.create(
        {
            'A': {
                '1': 'string'
            },
            'B': {
                '_target_': '__main__.foo',
                '2': '${A.1}',
            },
            'C': {
                '_target_': '__main__.foo',
                '3': '${B}',
                '4': '${A.1}'
            }
        }
    )
    print(OmegaConf.to_yaml(cfg2))
    instantiate(cfg2.C)
    print(OmegaConf.to_yaml(cfg2))

** Stack trace/error message **
when printing cfg2 after the instantiate, B.2 now reads string where we would expect it to still read ${A.1}

A:
  '1': string
B:
  _target_: __main__.foo
  '2': ${A.1}
C:
  _target_: __main__.foo
  '3': ${B}
  '4': ${A.1}

A:
  '1': string
B:
  _target_: __main__.foo
  '2': string
C:
  _target_: __main__.foo
  '3': ${B}
  '4': ${A.1}

Expected Behavior

we expect the second print to show that B.2 is ${A.1}

A:
  '1': string
B:
  _target_: __main__.foo
  '2': ${A.1}
C:
  _target_: __main__.foo
  '3': ${B}
  '4': ${A.1}

A:
  '1': string
B:
  _target_: __main__.foo
  '2': ${A.1}
C:
  _target_: __main__.foo
  '3': ${B}
  '4': ${A.1}

System information

• Hydra Version : 1.1.1
• Python version : 3.7.10
• Virtual environment type and version :
• Operating system :

[Jasha10]

Thanks for the bug report!
Yes, I agree with your sense that instantiate should not mutate the original config object...