add entry.getMode()

[thejoshwolfe]

This issue was raised in #101.

We should implement a function to read externalFileAttributes and return the mode aka permission bits. it might be as simple as:

Entry.prototype.getMode = function() {
  return entry.externalFileAttributes >>> 16;
}

but this may only be correct when the entry is made by a unix zipfile creator. We should investigate how various zipfile creators encode permissions, especially windows clients.

[melroy89]

Never implemented?

[clarkttfu]

+1

[thejoshwolfe]

Here's what we need to test before we can implement this:

On Windows, create zip archives with each of the following tools:

1. Compressed folder (builtin to Explorer)
2. a .NET program using System.IO.Compression.ZipFile
3. 7-Zip? Other "popular" zipping programs? I'm trying to enumerate the archive creators that have been problematic in the past, but i've been doing this for a long time, so perhaps these tools are obsolete at this point in modern history.

The archive should contain:

• a .txt file
• an .exe file
• a file whose permission bits have been modified in WSL with chmod +x
• symlinks created in WSL to each of the above three files using ln -s. (I don't think target permissions will matter, but let's test.)
• Windows shortcuts to the .exe and .txt files
• Windows native symlinks to the .exe and .txt files created using mlink or something. I don't know much about this. see here: https://blogs.windows.com/windowsdeveloper/2016/12/02/symlinks-windows-10/
• bonus round: in addition to WSL, also perform the above WSL steps using cygwin. and/or also using Linux on a removable media formatted in NTFS, and then transfer the media to a Windows machine to build the archives.

With all that data, I would be confident enough to implement and document a .getMode() function. I can support Linux just fine, but Windows is inaccessible to me at this time in my life, and I don't want to exclude Windows support.

I'm less enthusiastic about supporting Mac due to the atrocious Archive Utility implementation. If it's buggy, let it be buggy.

[ayushmanchhabra]

To unzip MacOS and Linux archives, I use this logic:

function modeFromEntry(entry) {
  const attr = entry.externalFileAttributes >> 16 || 33188;

  return [448, 56, 7
    .map(mask => attr & mask)
    .reduce((a, b) => a + b, attr & 61440);
}

const isSymlink = const isSymlink = ((modeFromEntry(entry) & 0o170000) === 0o120000);

Been a while since I've looked at this code so can't remember the exact implementation details. Will try to dig up the references I used to put this together.