Invalidate cache when using import

[jonathantneal]

How do I invalidate the cache of import?

I have a function that installs missing modules when an import fails, but the import statement seems to preserve the failure while the script is still running.

import('some-module').catch(
  // this catch will only be reached the first time the script is run because resolveMissingModule will successfully install the module
  () => resolveMissingModule('some-module').then(
    // again, this will only be reached once, but it will fail, because the import seems to have cached the previous failure
    () => import('some-module')
  )
)

The only information I found in regards to import caching was this documentation, which does not tell me where the “separate cache” used by import can be found.

No require.cache

require.cache is not used by import. It has a separate cache.
— https://nodejs.org/api/esm.html#esm_no_require_cache

[devsnek]

the import cache is purposely unexposed. adding a query has been the generally accepted ecosystem practice to re-import something.

however, a failure to import something will not fill the cache.

this trivial program works fine for me (assuming nope.mjs does not exist):

import fs from 'fs';

import('./nope.mjs')
  .catch(() => fs.writeFileSync('./nope.mjs'))
  .then(() => import('./nope.mjs'))
  .then(console.log);

[jonathantneal]

@devsnek, hmm, might this be limited to imports that use node_modules? This similarly trivial program fails for me the first time, but not the second.

import child_process from 'child_process';

import('color-names')
  .catch(() => child_process.execSync('npm install --no-save color-names'))
  .then(() => import('color-names'))
  .then(console.log);

[bmeck]

Note that the JS spec requires imports to be deterministic/idempotent on a source text. Exposure of a cache would not allow you to fix the code above.

…

On Fri, Apr 5, 2019, 12:01 PM Gus Caplan ***@***.***> wrote: the import cache is purposely unexposed. adding a query has been the generally accepted ecosystem practice to re-import something. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#49442>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAOUo5N5tioTq2s_t431OrihH_wH8Qagks5vd4FSgaJpZM4cfV4g> .

[devsnek]

if its just happening with node_modules it could be #26926

[MylesBorins]

can this be closd?

[jkrems]

I think a use case like this would hopefully be implemented as a loader. Do we already track this as a use case in that context?

[bmeck]

@jkrems we have old documents with that as a feature, but no success criteria examples.

[giltayar]

FYI, I'm implementing ESM support in Mocha (mochajs/mocha#4038), and cannot currently implement "watch mode", whereby Mocha watches the test files, and reruns them when they change. So "watch mode" in Mocha, in the first iteration, will probably not support ESM, which is a bummer.

While we could use cache busting query parameters, that would mean that we are always increasing memory usage, and old and never-to-be-used versions of the file will continue staying in memory due to the cache holding on to them.

And I'm not sure a loader would help here, as the loader also has no access to the cache.

[guybedford]

An API for unloading modules certainly makes sense. Usually with a direct registry API there is the tracing issue. An API that handles dependency removal can be useful. A simple API might be something like - import { unload } from ‘module’; unload(import.meta.url); // returns true Where the unload function would remove that module including all its dependencies from the registry. If in a cycle the whole cycle would be removed. A subsequent module load would refresh all the loads anew. Other problems to ensure work out is what if modules in the tree are still in-progress. I’d be tempted to say it should fail for that case and only work when all modules have either errored or completed. We still have memory leak concerns as v8 doesn’t lend itself easily to module GC still. But Node.js can lead the way here as it should. It will be an ongoing process to get there, but the API can come first. The main questions then seem to be: * are we ok exposing this as a module or should it be tied to loaders? If tied to loaders how would userland code request this? Or don’t we want it to - as in Mocha should run a new context with a loader? * should it be a direct registry API (get/set) with tracing, or should it be a deep API like the example above * finally, ironing out the partially loaded tree edge cases

…

On Tue, Nov 26, 2019 at 00:09 Gil Tayar ***@***.***> wrote: FYI, I'm implementing ESM support in Mocha (mochajs/mocha#4038 <mochajs/mocha#4038>), and cannot currently implement "watch mode", whereby Mocha watches the test files, and reruns them when they change. So "watch mode" in Mocha, in the first iteration, will probably not support ESM, which is a bummer. While we *could* use cache busting query parameters, that would mean that we are always increasing memory usage, and old and never-to-be-used versions of the file will continue staying in memory due to the cache holding on to them. And I'm not sure a loader would help here, as the loader also has no access to the cache. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#49442?email_source=notifications&email_token=AAESFSTBSWVLXPTWDYZOHSDQVSVQRA5CNFSM4HD5LYQKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEFEXNDI#issuecomment-558462605>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAESFSWTJXGB4J6WWNMOLRDQVSVQRANCNFSM4HD5LYQA> .

[devsnek]

I'm really not a fan of the idea of our module cache being anything except insert-only. CJS cache modification is bad already, and CJS modules don't even form graphs.

Additionally, other runtimes (like browsers) will never expose this functionality, so some alternative system will have to be used for them regardless of what node does, in which case it seems like that system could just be used for node.

[bmeck]

@giltayar have you looked into using Workers or other solutions to have a module cache that you can destroy (such as by killing the Worker)?

[giltayar]

@bmeck - interesting. That would mean that the tests themselves run in Workers. While I am theoretically familiar with workers, I haven't yet had any experience with them: is any code that runs in the main process compatible with worker inside a worker? In other words, compatibility-wise, would all test code that works today in the "main process" work inside workers?

I wouldn't want Mocha to have a version (even a semver-major breaking one) where developers will need to tweak their code because now it's running inside a worker. I'm guessing that there's a vast amount of that code running inside Mocha, and any incompatibility would be a deal breaker.

[devsnek]

there are differences between workers and the main thread, mostly surrounding the functions on process, like process.exit() in a worker doesn't end the process, just the thread. There's a good list here: https://nodejs.org/api/worker_threads.html#worker_threads_class_worker