Regression: path.parentPath.get(path.listKey) can clobber a traversal's visitors with those from a separate traversal in the global path cache

Bug Report

• I would like to work on a fix!

Input Code

const {parse} = require('@babel/core')
const generate = require('@babel/generator').default
const traverse = require('@babel/traverse').default

const code = `
import { Foo } from './Foo'
import { Bar } from './Bar'
`

const presets = [
  ['@babel/preset-env', { targets: { node: 'current' } }],
]

const ast = parse(code, {plugins, presets})

traverse(ast, {
  Program(path) {
    path.traverse({
      ImportDeclaration: {
        enter(path) {
          console.log('ENTER', generate(path.node).code)
          if (path.node.source.value === './Bar')
            path.parentPath.get(path.listKey)
        },
        exit(path) {
          console.log('EXIT ', generate(path.node).code)
        }
      }
    })
  }
})

Expected behavior

I haven’t bisected to find the version that introduced the regression yet, but with @babel/core, @babel/traverse, and @babel/preset-env 7.4.5, the exit visitor is called for both import statements:

ENTER import { Foo } from './Foo';
EXIT  import { Foo } from './Foo';
ENTER import { Bar } from './Bar';
EXIT  import { Bar } from './Bar';

Current behavior

With these versions: ├─ @babel/core@7.12.10 ├─ @babel/preset-env@7.12.11 └─ @babel/traverse@7.12.12

The exit visitor for the second import statement isn’t called because path.parentPath.get(path.listKey) has the side effect of replacing path.opts with those of the parent traversal (which only has a Program visitor)

ENTER import { Foo } from './Foo';
EXIT  import { Foo } from './Foo';
ENTER import { Bar } from './Bar';

(missing EXIT import { Bar } from './Bar')

Additional context

babel-plugin-flow-runtime is no longer working after I upgraded @babel/*, and I uncovered this issue debugging it.

[jedwards1211]

Huh I see. FWIW, I wonder if a different API design would be possible where the context information is passed around instead of being a property on the paths that gets mutated. This issue certainly wasn’t a showstopper for me but I did burn a lot of time in a painful debugging session on it.

[jedwards1211]

@JLHwung oh interesting.

I noticed that visitor is already an argument to path.traverse etc. And this issue involved traversal getting the visitors from path.opts in some cases, and that getting blown away by context changes. So aside from changes in that PR, I wonder if it would be possible for traversal to always use the visitor passed around via function arguments instead of anything related to context?