add `os.isAdmin` to tell whether we are admin (windows) or root (posix)

[timotheecour]

refs https://github.com/nim-lang/Nim/pull/16859#issuecomment-769984578

on windows:

refs https://stackoverflow.com/a/16285248/1426932 running execShellCmd("net session") can tell you: if not admin you get:

System error 5 has occurred.

Access is denied.

else you might get an answer such as: There are no entries in the list.

on posix:

refs: https://stackoverflow.com/a/21622456/1426932 running getEnv("EUID") == "0" or similar will tell you if you're root (it's readonly, can't be set)

use cases

• giving more informative error messages, avoiding re-inventing the wheel with solutions that aren't robust, allow user to branch on this (eg symlinks on windows can be created by admin or, in developper mode, if some flag is set)
• writing tests (refs https://github.com/nim-lang/Nim/pull/16709#issuecomment-769258310 @rominf )

links

• https://security.stackexchange.com/questions/10194/why-do-you-have-to-be-an-admin-to-create-a-symlink-in-windows
• related but different: Add isWindowsDeveloperModeActive · Issue #343 · nim-lang/RFCs

[alaviss]

refs: https://stackoverflow.com/a/21622456/1426932 running getEnv("EUID") == "0" or similar will tell you if you're root (it's readonly, can't be set)

This is a shell feature, there is nothing preventing the caller from modifying this environment variable. The POSIX way is to use the geteuid() syscall.

refs https://stackoverflow.com/a/16285248/1426932 running execShellCmd("net session") can tell you: if not admin you get:

System error 5 has occurred.

Access is denied.

else you might get an answer such as: There are no entries in the list.

This is a terrible way to do it, if a malicious actor modify the PATH (or if there's a net.exe in the current directory) or Microsoft changes the output of net, it will fail. Here's how to do it in Win32: https://docs.microsoft.com/en-us/windows/win32/api/securitybaseapi/nf-securitybaseapi-checktokenmembership?redirectedfrom=MSDN

[juancarlospaco]

If the computer is already compromised nothing is totally secure but yeah... :shrug:

[rominf]

See https://github.com/nim-lang/Nim/pull/17012 for the implementation.