Directory.resolveSymbolicLinksSync could throw more specific exception

[davidmorgan]

It throws FileSystemException even when there is a more specific type that would fit if it failed due to "not found" or "access denied".

Here is a code snippet that converts to a better exception type.

try {
  directory.resolveSymbolicLinksSync());
} on FileSystemException catch (e, s) {
  if (e.message.contains('Cannot resolve symbolic links')) {
    if (e.osError?.errorCode == 2) {
      throw Error.throwWithStackTrace(
        PathNotFoundException(directory.path, e.osError!), s);
    } else if (e.osError?.errorCode == 5) {
      throw Error.throwWithStackTrace(
        PathAccessException(directory.path, e.osError!), s);
    }
}

Discovered because of package:watcher code that tries to ignore specific subtypes of FileSystemException.

[biggs0125]

From @brianquinlan: There is a table available that maps unix errors to Dart exception types

sdk/sdk/lib/io/file.dart

Line 1075 in d4f2fec

factory FileSystemException._fromOSError(

It seems like this codepath isn't using that helper.

[brianquinlan]

Probably _throwIfError needs to change like so:

  static _throwIfError(Object result, String msg, [String? path]) {
    if (result is OSError) {
      throw FileSystemException._fromOSError(
        result,
        msg,
        path,
      );
    }...

[biggs0125]

The VM team discussed this a bit and this might be a breaking change as changing the type of the thrown exception could lead to different behavior. But the risk is fairly low so we might be okay doing this without the full breaking change process.