base: add Sys.detectwsl() (#57069)

Close #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
- microsoft/WSL#423
- 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)
- [x] Add compat, NEWS, tests
- [x] Take a look at different detect methods,
  figure out which one is more robust

---------

Co-authored-by: Gautam Mishra <[email protected]>
Co-authored-by: Max Horn <[email protected]>
Co-authored-by: Chengyu Han <[email protected]>
Co-authored-by: Alex Arslan <[email protected]>

NEWS.md

@@ -97,6 +97,7 @@ New library functions
 * `uuid7()` creates an RFC 9652 compliant UUID with version 7 ([#54834]).
 * `insertdims(array; dims)` allows to insert singleton dimensions into an array which is the inverse operation to `dropdims`. ([#45793])
 * The new `Fix` type is a generalization of `Fix1/Fix2` for fixing a single argument ([#54653]).
+* `Sys.detectwsl()` allows to testing if Julia is running inside WSL at runtime. ([#57069])
 
 New library features
 --------------------

base/sysinfo.jl

@@ -36,7 +36,8 @@ export BINDIR,
        isreadable,
        iswritable,
        username,
-       which
+       which,
+       detectwsl
 
 import ..Base: show
 
@@ -532,6 +533,27 @@ including e.g. a WebAssembly JavaScript embedding in a web browser.
 """
 isjsvm(os::Symbol) = (os === :Emscripten)
 
+"""
+    Sys.detectwsl()
+
+Runtime predicate for testing if Julia is running inside
+Windows Subsystem for Linux (WSL).
+
+!!! note
+    Unlike `Sys.iswindows`, `Sys.islinux` etc., this is a runtime test, and thus
+    cannot meaningfully be used in `@static if` constructs.
+
+!!! compat "Julia 1.12"
+    This function requires at least Julia 1.12.
+"""
+function detectwsl()
+    # We use the same approach as canonical/snapd do to detect WSL
+    islinux() && (
+        isfile("/proc/sys/fs/binfmt_misc/WSLInterop")
+        || isdir("/run/WSL")
+    )
+end
+
 for f in (:isunix, :islinux, :isbsd, :isapple, :iswindows, :isfreebsd, :isopenbsd, :isnetbsd, :isdragonfly, :isjsvm)
     @eval $f() = $(getfield(@__MODULE__, f)(KERNEL))
 end

test/osutils.jl

@@ -29,6 +29,11 @@ using Libdl
     else
         @test Sys.windows_version() >= v"1.0.0-"
     end
+
+    # TODO: When we have a WSL CI, add a new test here `@test detectwsl()`
+    if !Sys.islinux()
+        @test !Sys.detectwsl()
+    end
 end
 
 @testset "@static" begin

test/path.jl

@@ -314,7 +314,7 @@
     @testset "uripath" begin
         host = if Sys.iswindows()
             ""
-        elseif ispath("/proc/sys/fs/binfmt_misc/WSLInterop")
+        elseif Sys.detectwsl()
             distro = get(ENV, "WSL_DISTRO_NAME", "") # See <https://patrickwu.space/wslconf/>
             "wsl%24/$distro" # See <https://github.com/microsoft/terminal/pull/14993> and <https://learn.microsoft.com/en-us/windows/wsl/filesystems>
         else