repl: support --loader option in builtin REPL

[targos]

Fixes: #33435

I'll try to write a test if there are no objections to the change.

/cc @nodejs/modules-active-members

[targos]

With the example from the issue:

$ echo 'console.log("hooks executing")' > hooks.mjs
$ ./node --loader ./hooks.mjs
(node:1190852) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
hooks executing
Welcome to Node.js v15.0.0-pre.
Type ".help" for more information.

With a wrong path:

$ ./node --loader ./hooks.js
(node:1190903) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
internal/main/repl.js:70
    internalBinding('errors').triggerUncaughtException(
                              ^

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/home/mzasso/git/nodejs/node/hooks.js' imported from /home/mzasso/git/nodejs/node/
    at finalizeResolution (internal/modules/esm/resolve.js:284:11)
    at moduleResolve (internal/modules/esm/resolve.js:662:10)
    at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:752:11)
    at Loader.resolve (internal/modules/esm/loader.js:97:40)
    at Loader.getModuleJob (internal/modules/esm/loader.js:242:28)
    at Loader.import (internal/modules/esm/loader.js:177:28)
    at internal/process/esm_loader.js:55:31
    at Object.initializeLoader (internal/process/esm_loader.js:60:5)
    at internal/main/repl.js:37:13 {
  code: 'ERR_MODULE_NOT_FOUND'

With error in the loader code:

$ echo 'throw new Error("test")' > hooks.mjs
$ ./node --loader ./hooks.mjs
(node:1190987) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
file:///home/mzasso/git/nodejs/node/hooks.mjs:1
throw new Error("test")
      ^

Error: test
    at file:///home/mzasso/git/nodejs/node/hooks.mjs:1:7
    at ModuleJob.run (internal/modules/esm/module_job.js:138:23)
    at async Loader.import (internal/modules/esm/loader.js:178:24)
    at async internal/process/esm_loader.js:55:9

[BridgeAR]

I am fine with adding this feature. It would be good in general to support esm better in the REPL.

[devsnek]

Maybe this is too heavy for this pr, but refactoring internal/process/esm_loader to gate exposing the loader instance behind a function which also ensures the loader is properly initialized would be nice.

[BridgeAR]

Ping @targos

[targos]

Updated with a different approach. PTAL.

[devsnek]

should probably gate this on ESMLoader being undefined

[targos]

do you mean add something like assert(ESMLoader === undefined) ?

[devsnek]

yeah that works.

[targos]

I can't. There's a default loader defined above, and removing it breaks things.

[BridgeAR]

LGTM

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31669/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31748/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31883/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31884/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/32325/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/32326/

[targos]

Landed in 05539c1

[codebytere]

@targos could you please backport this to v12.x?