base: add Sys.detectwsl()

[inkydragon]

Replace #36425, fix #36354

How to detect WSL?

There are a number of ways that can be used to detect WSL environments,
but each can have false positives.

We finally chose to use the same method as Snapd to detect WSL.
Because Windows installs Ubuntu LTS as WSL by default.
So we assume that Snapd's detection method will work for most users.

• Ubuntu/Snapd: https://github.com/canonical/snapd/blob/03a578a5dff26467dcc80580fcd4720a486185a5/release/release.go#L151-L172
• Provide a way to positively detect WSL from an app compiled on Linux. microsoft/WSL#423
• How to detect if running under WSL? microsoft/WSL#4071
• https://superuser.com/q/1749781/1460597

Known limitations

• this is a runtime test, and thus cannot meaningfully be used in @static if constructs.

• Linux users can create their own

  • /proc/sys/fs/binfmt_misc/WSLInterop file
  • or /run/WSL/ folder

  to pretend to be a WSL environment.

---

• I've tested this under: Ubuntu 22.04.5 LTS (default/Offical) and alpine-release-3.17.0 (win store)
• Add compat, NEWS, tests
• Take a look at different detect methods,
  figure out which one is more robust

[inkydragon]

Seems all Linux platforms failed.

error during bootstrap:
LoadError("sysimg.jl", 5, LoadError("Base.jl", 127, LoadError("sysinfo.jl", 3, UndefVarError(:isfile, Base.Sys))))
ijl_undefined_var_error at [buildroot]/src/rtutils.c:152
detectwsl at ./sysinfo.jl:550

Maybe I need to use Base.Filesystem.isfile.
But why this line works?

julia/base/sysinfo.jl

Line 619 in 316f0fe

if isfile(program_path) && isexecutable(program_path)

[ararslan]

Perhaps Sys.iswsl() for consistency with the other platform detection functions?

[inkydragon]

This is actually on purpose, as detectwsl detects at runtime, so names of the form is_platform are not used.
See discuss in #36354

[davidanthoff]

There is already a WSL detection in base at

julia/base/path.jl

Line 658 in 316f0fe

host = if ispath("/proc/sys/fs/binfmt_misc/WSLInterop") # WSL sigil

and it uses a slightly different approach. Would be good to figure out which one is more robust and then use that one consistently everywhere.

[ararslan]

As a data point, Ubuntu's Snap daemon looks for /proc/sys/fs/binfmt_misc/WSLInterop to detect whether it's running on WSL, but it's apparently also possible to override the name to something other than WSLInterop. Looks like the approach implemented here is also fairly common.

Some references:

• Provide a way to positively detect WSL from an app compiled on Linux. microsoft/WSL#423
• How to detect if running under WSL? microsoft/WSL#4071
• https://superuser.com/a/1749811

[davidanthoff]

I read through the links that @ararslan posted, and to me the ispath("/proc/sys/fs/binfmt_misc/WSLInterop") methods seems more robust and simplest. My understanding is that one can only get false positives or false negatives there if a user really went out of their way to reconfigure things, essentially on purpose trying to prevent detection.

The superuser post also mentions on the other hand that the method implemented here in this PR might give a false positive maybe even on Azure, where MS might be running a custom kernel, and so greping for just "Microsoft" might get fooled.

I also think that if Ubuntu uses a given approach for snap, then it is probably a good idea to just follow that. Ubuntu and MS have worked really closely together on this whole WSL stuff, so just following their lead strikes me as a good idea.

[ararslan]

I should note that my claim about Snap comes from the linked answer on SuperUser, which is from 2022; it's possible that Ubuntu has changed the implementation since then.

[inkydragon]

it's possible that Ubuntu has changed the implementation since then.

They added testing for the existence of “/run/WSL/” as a fallback.
We can use the same way to detect WSL.

// We detect WSL via the existence of /proc/sys/fs/binfmt_misc/WSLInterop
// Under some undocumented circumstances this file may be missing. We have /run/WSL as a backup.
//
// We detect WSL1 via the root filesystem type:
// - wslfs or lxfs mean WSL1
// - Anything else means WSL2
// After knowing we're in WSL, if any error occurs we assume WSL2 as it is the more flexible version
func getWSLVersion() int {
	if !fileExists("/proc/sys/fs/binfmt_misc/WSLInterop") && !fileExists("/run/WSL") {
		return 0
	}
	fstype, err := filesystemRootType()
	if err != nil {
		// TODO log error here once logger can be imported without circular imports
		return 2
	}

	if fstype == "wslfs" || fstype == "lxfs" {
		return 1
	}
	return 2
}

https://github.com/canonical/snapd/blob/03a578a5dff26467dcc80580fcd4720a486185a5/release/release.go#L151-L172

---

Some test under Ubuntu and alpine:

Ubuntu 22.04.5 LTS (default/Offical)

~$ lsb_release
No LSB modules are available.
~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 22.04.5 LTS
Release:        22.04
Codename:       jammy
~$ uname -a
Linux A309-Y9000P 5.15.137.3-microsoft-standard-WSL2+ #4 SMP Wed Jan 10 11:18:39 CST 2024 x86_64 x86_64 x86_64 GNU/Linux
~$ cat /proc/sys/fs/binfmt_misc/WSLInterop
enabled
interpreter /init
flags: PF
offset 0
magic 4d5a
~$ ls /run/WSL/
1_interop  2_interop  3217_interop  518_interop
~$ cat /proc/sys/kernel/osrelease
5.15.137.3-microsoft-standard-WSL2+
~$

alpine-release-3.17.0 (win store):

~$ cat /etc/alpine-release
3.17.0
~$ cat /proc/sys/fs/binfmt_misc/WSLInterop
enabled
interpreter /init
flags: PF
offset 0
magic 4d5a
~$ ls /run/WSL/
14_interop  1_interop
~$ cat /proc/sys/kernel/osrelease
5.15.137.3-microsoft-standard-WSL2+
~$