Transform readFile/writeFile methods are not called #3022

I have:

• Searched in the repository for existing result
• Developed a POC to fix this issue at ye-yu/assemblyscript@23cdb69

I have a sample code fix on readFile in my fork that I would like to contribute to this repository as a fix to this issue. I would like to obtain suggestions and discussions on how to fix this issue before submitting a PR.

Bug description

Custom compiler transformers extending Transform do not reliably receive readFile / writeFile calls during compilation. This prevents transformer implementations from intercepting file I/O and breaks transform-based file resolution.

Actual behavior

• Transform.prototype.readFile / writeFile are not properly wired for derived transformer classes.
  super.readFile(filename, baseDir) can fail or be skipped.
• When multiple transformers are configured, the first transformer that returns a result should short-circuit, but current handling does not consistently enforce that.

Expected behavior

• A transformer subclass should be able to override readFile and writeFile and have those methods called during compilation.
• Calling super.readFile(...) inside an override should delegate to the next transform or the default loader.
  If multiple transformers are configured, the first non-null result should be used and remaining transformers should not override it.

My use case: another source to source on top of AS transpiler

I would like to develop a feature that will transform unsupported syntaxes to an intermediary source to work around some limitation. I'd figure that a transformer that was promised to me in this documentation Compiler - Transforms - readFile would work. However, it did not return the expected result :(

Steps to reproduce

1. In tests/transform/index.js, add a tracking variable readFileCalled and override readFile method to update this variable

   var constructorCalled = false;
   var afterParseCalled = false;
   var afterInitializeCalled = false;
   var afterCompileCalled = false;
   + var readFileCalled = false;
   
   console.log("Transform loaded");
   
   export default class MyTransform extends Transform {
     // ...
   
     readFile(filename, baseDir) {
   +     readFileCalled = true;
       return super.readFile(filename, baseDir);
     }
   
     // ...

2. Assert readFile is called on afterParse

3. Run npm run test:transformer

Expected result: readFileCalled is true
Actual result: readFileCalled is false

AssemblyScript version

v0.28.17