Fix pkgdir for extensions (#55720)

Fixes #55719

---------

Co-authored-by: Max Horn <241512+fingolfin@users.noreply.github.com>

Author: IanButterworth

base/loading.jl

@@ -508,6 +508,8 @@ package root.
 To get the root directory of the package that implements the current module
 the form `pkgdir(@__MODULE__)` can be used.
 
+If an extension module is given, the root of the parent package is returned.
+
 ```julia-repl
 julia> pkgdir(Foo)
 "/path/to/Foo.jl"
@@ -525,7 +527,19 @@ function pkgdir(m::Module, paths::String...)
     rootmodule = moduleroot(m)
     path = pathof(rootmodule)
     path === nothing && return nothing
-    return joinpath(dirname(dirname(path)), paths...)
+    original = path
+    path, base = splitdir(dirname(path))
+    if base == "src"
+        # package source in `../src/Foo.jl`
+    elseif base == "ext"
+        # extension source in `../ext/FooExt.jl`
+    elseif basename(path) == "ext"
+        # extension source in `../ext/FooExt/FooExt.jl`
+        path = dirname(path)
+    else
+        error("Unexpected path structure for module source: $original")
+    end
+    return joinpath(path, paths...)
 end
 
 function get_pkgversion_from_path(path)

test/loading.jl

@@ -1034,6 +1034,16 @@ end
 end
 
 @testset "Extensions" begin
+    test_ext = """
+    function test_ext(parent::Module, ext::Symbol)
+        _ext = Base.get_extension(parent, ext)
+        _ext isa Module || error("expected extension \$ext to be loaded")
+        _pkgdir = pkgdir(_ext)
+        _pkgdir == pkgdir(parent) != nothing || error("unexpected extension \$ext pkgdir path: \$_pkgdir")
+        _pkgversion = pkgversion(_ext)
+        _pkgversion == pkgversion(parent) || error("unexpected extension \$ext version: \$_pkgversion")
+    end
+    """
     depot_path = mktempdir()
     try
         proj = joinpath(@__DIR__, "project", "Extensions", "HasDepWithExtensions.jl")
@@ -1044,13 +1054,15 @@ end
             cmd = """
             $load_distr
             begin
+                $ew $test_ext
                 $ew push!(empty!(DEPOT_PATH), $(repr(depot_path)))
                 using HasExtensions
                 $ew using HasExtensions
                 $ew Base.get_extension(HasExtensions, :Extension) === nothing || error("unexpectedly got an extension")
                 $ew HasExtensions.ext_loaded && error("ext_loaded set")
                 using HasDepWithExtensions
                 $ew using HasDepWithExtensions
+                $ew test_ext(HasExtensions, :Extension)
                 $ew Base.get_extension(HasExtensions, :Extension).extvar == 1 || error("extvar in Extension not set")
                 $ew HasExtensions.ext_loaded || error("ext_loaded not set")
                 $ew HasExtensions.ext_folder_loaded && error("ext_folder_loaded set")
@@ -1102,13 +1114,14 @@ end
 
         test_ext_proj = """
         begin
+            $test_ext
             using HasExtensions
             using ExtDep
-            Base.get_extension(HasExtensions, :Extension) isa Module || error("expected extension to load")
+            test_ext(HasExtensions, :Extension)
             using ExtDep2
-            Base.get_extension(HasExtensions, :ExtensionFolder) isa Module || error("expected extension to load")
+            test_ext(HasExtensions, :ExtensionFolder)
             using ExtDep3
-            Base.get_extension(HasExtensions, :ExtensionDep) isa Module || error("expected extension to load")
+            test_ext(HasExtensions, :ExtensionDep)
         end
         """
         for compile in (`--compiled-modules=no`, ``)