Disallow ESM-CJS-ESM-ESM cycles when using `require(esm)`

[nicolo-ribaudo]

Version

Unreleased (5f7fad2)

Platform

n/a

Subsystem

esm

What steps will reproduce the bug?

// c.mjs
import { createRequire } from "module";
console.log("Start c");
createRequire(import.meta.url)("./d.mjs");
throw new Error("Error from c");

// d.mjs
import "./c.mjs";
console.log("Execute d");

// b.mjs
import "./c.mjs";
console.log("Execute b");

// a.mjs
try {
  await import("./b.mjs");
  console.log("Dynamic 1 didn't fail")
} catch (err) {
  console.log(err);
}

try {
  await import("./d.mjs");
  console.log("Dynamic 2 didn't fail")
} catch (err) {
  console.log(err);
}

How often does it reproduce? Is there a required condition?

Whenever there is an ES modules that triggers a require() of a separate ES module that imports the original ES module.

What is the expected behavior? Why is that the expected behavior?

• d.mjs contains some code and then a throw statement at the top level, so import("./d.mjs") must always reject (either because of the "some code", of because of the throw)
• c.mjs imports d.mjs, so import("./c.mjs") must also always throw
• b.mjs imports c.mjs, so import("./b.mjs") must also always reject

Indeed, out/Release/node --experimental-require-module ./d.mjs throws

Start c
node:internal/modules/esm/loader:274
      return job.module.getNamespaceSync();
                        ^

Error: require() cannot be used on an ESM graph with top-level await. Use import() instead. To see where the top-level await comes from, use --experimental-print-required-tla.

(which is a wrong error, but at least it is an error 😛) and out/Release/node --experimental-require-module ./c.mjs and out/Release/node --experimental-require-module ./c.mjs throw

Start c
Execute d
file:///.../c.mjs:5
throw new Error("Error from c");
      ^

Error: Error from c

What do you see instead?

However, if you import() first b.mjs and then d.mjs, the second import() does not reject! out/Release/node --experimental-require-module ./d.mjs outputs

(node:928766) ExperimentalWarning: Support for loading ES Module in require() is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
Start c
Execute d
Error: Error from c
    at file://.../c.mjs:5:7
    at ModuleJob.run (node:internal/modules/esm/module_job:235:25)
    at async ModuleLoader.import (node:internal/modules/esm/loader:436:24)
    at async file://.../a.mjs:3:3
Dynamic 2 didn't fail

Additional information

I think the reason the second import() does not properly reject is because running two module evaluation algorithms at the same time messes up with the way V8 tracks cycles. Specifically, it violates the assumption made at step 11.c.ii of https://tc39.es/ecma262/#sec-innermoduleevaluation, causing them problems at step 16.

A solution to this would be, when doing require(esm), to check if the module or any of its transitive dependencies are currently being evaluated. Right now Node.js just checks if the module itself is being evaluated (and throws that top-level await error).

cc @joyeecheung (who is already aware of this, I'm just opening the issue so that it doesn't get forgotten)

[nicolo-ribaudo]

Oh, I just noticed that, when building Node.js in debug mode, V8 crashes when running c.mjs:

out/Debug/node --experimental-require-module ./c.mjs
(node:930777) ExperimentalWarning: Support for loading ES Module in require() is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
Start c


#
# Fatal error in ../../deps/v8/src/objects/module.cc, line 205
# Debug check failed: module->status() != kEvaluating (4 vs. 4).
#
#
#
#FailureMessage Object: 0x7ffef98cbe30
----- Native stack trace -----

 1: 0x55cfdded5ad8 node::DumpNativeBacktrace(_IO_FILE*) [out/Debug/node]
 2: 0x55cfde1252f9  [out/Debug/node]
 3: 0x55cfde12531d  [out/Debug/node]
 4: 0x55cfe0660f4b V8_Fatal(char const*, int, char const*, ...) [out/Debug/node]
 5: 0x55cfe0660f7b  [out/Debug/node]
 6: 0x55cfded97342 v8::internal::Module::PrepareInstantiate(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Module>, v8::Local<v8::Context>, v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::FixedArray>, v8::Local<v8::Module>), v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::Module>)) [out/Debug/node]
 7: 0x55cfdee31a78 v8::internal::SourceTextModule::PrepareInstantiate(v8::internal::Isolate*, v8::internal::Handle<v8::internal::SourceTextModule>, v8::Local<v8::Context>, v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::FixedArray>, v8::Local<v8::Module>), v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::Module>)) [out/Debug/node]
 8: 0x55cfded9824b v8::internal::Module::Instantiate(v8::internal::Isolate*, v8::internal::Handle<v8::internal::Module>, v8::Local<v8::Context>, v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::FixedArray>, v8::Local<v8::Module>), v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::Module>)) [out/Debug/node]
 9: 0x55cfde446021 v8::Module::InstantiateModule(v8::Local<v8::Context>, v8::MaybeLocal<v8::Module> (*)(v8::Local<v8::Context>, v8::Local<v8::String>, v8::Local<v8::FixedArray>, v8::Local<v8::Module>)) [out/Debug/node]
10: 0x55cfddf77132 node::loader::ModuleWrap::InstantiateSync(v8::FunctionCallbackInfo<v8::Value> const&) [out/Debug/node]
11: 0x55cf7f8d199d 
[1]    930777 trace trap (core dumped)  out/Debug/node --experimental-require-module

[joyeecheung]

I think we have two options:

1. Only return the module if it's already evaluated. But ESM-CJS-ESM-ESM can still be allowed if, by the time CJS -> ESM happens, the cycle is already fully evaluated (unless that graph involves TLA)
2. Distinguish between "modules (directly or indirectly) required() by CJS" and "modules (directly or indirectly) imported by ESM". And disallow cycles when the last ESM-ESM edge is the latter.

I think the first is probably easier to implement - just throw when the status is not evaluated. In practice, that's probably suits the "best effort" approach of CJS/ESM interop better - we give users whatever we can when it's safe, otherwise we throw. The latter would be a bit trickier to implement - we need to keep track of where the modules come from and search for cycles. Not impossible but I wonder if that's really worth the effort. We could just implement 1 first, and, if it turns out to be problematic, we do 2.

[nicolo-ribaudo]

Only return the module if it's already evaluated. But ESM-CJS-ESM-ESM can still be allowed if, by the time CJS -> ESM happens, the cycle is already fully evaluated (unless that graph involves TLA)

I'm not sure I understand what you are proposing here. The cycle never fully evaluated when require() returns, because the module that contains require() is part of the cycle and thus it will not be finished evaluating yet.

[joyeecheung]

Hmm, I guess I was too jet-legged when I posted that, and was thinking about the other cycle we talked about (import cjs -> require(esm) cycle).

re. a.mjs, it seems in this case, d.mjs gets cached in the first case, so you wouldn't see an error later. I am not sure what really should be the expected behavior. Funny enough if you switch createRequire(import.meta.url)("./d.mjs"); to await import("./d.mjs") in c.mjs it gets even more bizzare (only logs Start c. On main with the recent changes I added in #51999 it will show you that await import("./b.mjs") in a.mjs was never resolved). I suspect we need to fix the caching strategy for this, not just for require(esm).

re. the DCHECK, we should add a status check before instantiation. I'll dig into it.

[joyeecheung]

Working on a branch to just ban ESM -> CJS and CJS -> ESM edges when they participate in a cycle. For the example in the OP, b-d.mjs fails, the two cases in a.mjs fail individually but not together, still need to properly fix up the cache when the evaluation fails.

[joyeecheung]

Opened #52264

[joyeecheung]

On a side note, #52145 (comment) is still there (i.e. if this is await import(), the code below TLA never runs), though I am not sure what the spec says about that. Looks like a footgun to have silently never-executed code below await import() that trigger a cycle...

[Jamesernator]

Wouldn't it be better here to just check if the module is evaluating and simply return the namespace if it's already evaluating? This would more easily allow people to migrate file-at-a-time between CJS and ESM (and it mirrors how CJS already works so wouldn't be a big divergence).

i.e. The problem here basically reduces down to this:

globalThis.mod = new vm.SourceTextModule(`
    function requireImpl(specifier) {
        // specifier gets resolved to the currently to the currently evaluating module
        const mod = globalThis.mod;
        mod.evaluate();
        return mod.namespace;
    }
    
    console.log("Before require");
    const getMod = requireImpl("specifier-that-results-in-evaluate-of-the-current-module");
    console.log("After require");
`);

my suggested fix is just to check if evaluation is already happening:

globalThis.mod = new vm.SourceTextModule(`
    function requireImpl(specifier) {
        // specifier gets resolved to the currently to the currently evaluating module
        const mod = globalThis.mod;

        // In a cycle so just return the namespace
        if (mod.status === "evaluating") {
            return mod.namespace;
        }
        mod.evaluate();
        return mod.namespace;
    }
    
    console.log("Before require");
    const getMod = requireImpl("specifier-that-results-in-evaluate-of-the-current-module");
    console.log("After require");
`);

[nicolo-ribaudo]

What if the require()d module itself is not evaluating, but one of its dependencies is?

[Jamesernator]

What if the require()d module itself is not evaluating, but one of its dependencies is?

True, it might be worth seeing if we can get a spec change here as this behaviour is very similar to the change proposed for deferred module evaluation.

In fact I wonder if, with that proposal and the fix, implementing require("esm") like this would just work:

const mod = new SourceTextModule(...);
mod.linkSync(); // Whatever the internal method here is for linking sync
// Accessing any member of the namespace triggers EvaluateSync
mod.namespace.ANY_PROP_TO_TRIGGER_EVAL;

[joyeecheung]

This would more easily allow people to migrate file-at-a-time between CJS and ESM (and it mirrors how CJS already works so wouldn't be a big divergence).

Technically with CJS, it's not simply returning circular exports that are being loaded, but a proxy (pretty slow) that emits a warning if you are accessing a property before it's added to the exports object (which is still more of a guess - one can still create a race condition by modifying the exports later, so in the cycle then you can't be certain what version of the export you are getting from a consumer point of view).