base: add Sys.detectwsl()

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