Implement User/Group from_name?/from_id? like Ruby

[wonderix]

Ruby handles ENOENT, ESRCH, EBADF and EPERM as not found when calling getgrgid_r, getgrnam_r, getpwnam_r or getpwuid_r.

I implemented the same behavior for crystal. Unfortunately, it's not possible to write unit tests for this.

Fixes #8069.

[straight-shoota]

Thank you for your recent contributions! It's very appreciated.

FYI: If you write Resolves #8069 or Fixes #8069 in the PR description, Github automatically links the issue to this PR and closes it on merge.

[j8r]

Note that your commit isn't attributed to your GitHub account. If you set locally a Git mail, which is also linked to your GH account, it will be.

[wonderix]

@j8r, I added my second email address to my github account. Hopefully, this does the job.

[asterite]

The code for all four methods looks very similar now, except for a few differences like what's the function to call and the name to use for from_errno. Could we extract them to a separate method that yields? That way all that logic will be in a single place. Or maybe not a method that yields, I'm sure there's a more DRY way to do this.

[wonderix]

I created a macro to wrap the functions.
I also tried putting it in a method that yields. But that was too complicated.

[asterite]

Please refactor to avoid duplicate code.

[straight-shoota]

I would suggest a yielding method similar to System.retry_wstr_buffer:

def from_username?(username : String)
  username.check_no_null_byte
  pwd = uninitialized LibC::Passwd
  pwd_pointer = pointerof(pwd)
  retry_with_buffer("getpwnam_r", GETPW_R_SIZE_MAX) do |buf|
    LibC.getpwnam_r(username, pwd_pointer, buf, buf.size, pointerof(pwd_pointer))
  end
 
  from_struct(pwd) if pwd_pointer
end

def retry_with_buffer(function_name, max_buffer)
  initial_buf = uninitialized UInt8[1024]
  buf = initial_buf

  while (ret = yield buf.to_slice) != 0
    case ret
    when LibC::ENOENT, LibC::ESRCH, LibC::EBADF, LibC::EPERM
      return nil
    when LibC::ERANGE
      raise RuntimeError.from_errno(function_name) if buf.size >= max_buffer
      buf = Bytes.new(buf.size * 2)
    else
      raise RuntimeError.from_errno(function_name)
    end
  end
end

[asterite]

Yes, a yielding method is better. Macros should be avoided unless absolutely necessary.

[wonderix]

Thanks for the hint. I implemented it now like suggested by @straight-shoota