[issue37834] readlink on Windows cannot read app exec links

[Steve Dower]

Steve Dower <steve.dower at python.org> added the comment:

> I looked into this spawn problem. It's due to Cygwin's spawnve, which calls NtOpenFile to open the file, and then memory-maps it and reads the image header [1].

Great, that's roughly what I suspected. Unfortunately, I've been told that looking into Cygwin's (GPL'd) code myself is going to cost me two weeks of dev work ("cooling off" period), so someone else will have to help them fix it.

> It could parse out as much as possible and return a struct sequence

TBH, I feel like that's more work than is worth us doing for something that will be relatively rarely used, will live in the stdlib, and is obviously something that will become outdated as Microsoft adds new reparse points.

If we return the parsed data as opaque bytes then someone can write a simple PyPI package to extract the contents. (Presumably the reparse tag will have come from stat() already.)

> Maybe have two non-overlapping options, follow_symlinks and follow_nonlinks

I read this suggestion the first time and I think it would send the message that we are more capable than we really are :)

In theory, we can't follow any reparse point that isn't documented as being followable and provides the target name is a stable, documented manner. The appexec links don't do this (I just looked at the returned buffer), so we really should just not follow them. They exist solely so that CreateProcess internally returns a unique error code that can be handled without impacting regular process start, which means we *don't* want to follow them.

Again, if someone writes the PyPI package to parse all known reparse points, they can do that.

Now, directory junctions are far more interesting. My gut feel is that we should treat them the same as symlinks (with respect to stat vs. lstat) for consistency, but I'll gladly defer to you (Eryk) for the edge cases that may introduce.

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My plan for PR 15231 is:
* make os.stat only follow symlinks and junctions
* make os.readlink only read symlinks and junctions
* add st_reparse_tag field to os.stat

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<https://bugs.python.org/issue37834>
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