Improve legacy CJS resolve

[anonrig]

The following code calls multiple C++ calls for each new URL and fileExists function call. We can avoid these changes, make 1 C++ call, and handle the rest in the C++ layer.

function legacyMainResolve(packageJSONUrl, packageConfig, base) {
  let guess;
  if (packageConfig.main !== undefined) {
    // Note: fs check redundances will be handled by Descriptor cache here.
    if (fileExists(guess = new URL(`./${packageConfig.main}`,
                                   packageJSONUrl))) {
      return guess;
    } else if (fileExists(guess = new URL(`./${packageConfig.main}.js`,
                                          packageJSONUrl)));
    else if (fileExists(guess = new URL(`./${packageConfig.main}.json`,
                                        packageJSONUrl)));
    else if (fileExists(guess = new URL(`./${packageConfig.main}.node`,
                                        packageJSONUrl)));
    else if (fileExists(guess = new URL(`./${packageConfig.main}/index.js`,
                                        packageJSONUrl)));
    else if (fileExists(guess = new URL(`./${packageConfig.main}/index.json`,
                                        packageJSONUrl)));
    else if (fileExists(guess = new URL(`./${packageConfig.main}/index.node`,
                                        packageJSONUrl)));
    else guess = undefined;
    if (guess) {
      emitLegacyIndexDeprecation(guess, packageJSONUrl, base,
                                 packageConfig.main);
      return guess;
    }
    // Fallthrough.
  }
  if (fileExists(guess = new URL('./index.js', packageJSONUrl)));
  // So fs.
  else if (fileExists(guess = new URL('./index.json', packageJSONUrl)));
  else if (fileExists(guess = new URL('./index.node', packageJSONUrl)));
  else guess = undefined;
  if (guess) {
    emitLegacyIndexDeprecation(guess, packageJSONUrl, base, packageConfig.main);
    return guess;
  }
  // Not found.
  throw new ERR_MODULE_NOT_FOUND(
    fileURLToPath(new URL('.', packageJSONUrl)), fileURLToPath(base));
}

[aduh95]

Does it matter though? I'd expect that packages that do not define a "main" nor "exports" field in their package.json is rather rare – and maybe you could/should measure that before working on this.
EDIT: I misread the code, there might still be lots of cases where packages define a "main" but without an extension, in which case it might indeed be worth trying to optimize it.

[aduh95]

Not related to performance, but another problem with that function is that it seems to assume that "main" contains a URL-compatible string, when other part of the codebase (and the docs) treats it as a path.

[metcoder95]

Then we have 2 separate issues?
Standardize its usage, and make optimizations out of it, right?

Not sure the former relates to performance but we can work at both ends, should this issue reflect that?

[H4ad]

I started working on this issue, I will try more ideas and then share when the code is more mature.

[H4ad]

Hey, I created a PR for NodeJS with current work: nodejs/node#48325

I'm almost done, the performance improvements have been really nice and also bring the possibility to improve other parts of the system like fileURLToPath.