check symlinks work on windows on artifact hash mismatch

src/Artifacts.jl

@@ -265,6 +265,22 @@ function unbind_artifact!(artifacts_toml::String, name::String;
     return
 end
 
+if Sys.iswindows()
+    # check whether symlinks can be made
+    function check_symlinks()
+        PlatformEngines.copy_symlinks() === true && return false # for testing
+        tmpdir = mktempdir()
+        fpath = joinpath(tmpdir, "f")
+        touch(fpath)
+        try
+            symlink(fpath, joinpath(tmpdir, "s"))
+            return true
+        catch
+            return false
+        end
+    end
+end
+
 """
     download_artifact(tree_hash::SHA1, tarball_url::String, tarball_hash::String;
                       verbose::Bool = false, io::IO=stderr)
@@ -314,6 +330,18 @@ function download_artifact(
         msg  = "Tree Hash Mismatch!\n"
         msg *= "  Expected git-tree-sha1:   $(bytes2hex(tree_hash.bytes))\n"
         msg *= "  Calculated git-tree-sha1: $(bytes2hex(calc_hash.bytes))"
+        @static if Sys.iswindows()
+            if !check_symlinks()
+                msg *= """
+
+                Note: Julia cannot create symlinks, which may be the reason for the hash mismatch.
+                One solution is to activate Developer Mode in Windows, for instructions on how to do that, see:
+                    https://learn.microsoft.com/en-us/gaming/game-bar/guide/developer-mode
+                An alternative path is to temporarily set `ENV["JULIA_PKG_IGNORE_HASHES"]=true`
+                to ignore artifact hash mismatches in this session, however this is generally not recommended.
+                """
+            end
+        end
         # Since tree hash calculation is still broken on some systems, e.g. Pkg.jl#1860,
         # and Pkg.jl#2317, we allow setting JULIA_PKG_IGNORE_HASHES=1 to ignore the
         # error and move the artifact to the expected location and return true

test/artifacts.jl

@@ -796,4 +796,26 @@ end
     end
 end
 
+if Sys.iswindows()
+    @testset "installing artifacts when symlinks are copied" begin
+        # copy symlinks to simulate the typical Microsoft Windows user experience where
+        # developer mode is not enabled (no admin rights)
+        withenv("BINARYPROVIDER_COPYDEREF" => "true") do
+            temp_pkg_dir() do tmpdir
+                artifacts_toml = joinpath(tmpdir, "Artifacts.toml")
+                cp(joinpath(@__DIR__, "test_packages", "ArtifactInstallation", "Artifacts.toml"), artifacts_toml)
+                Pkg.activate(tmpdir)
+                cts_hash = artifact_hash("collapse_the_symlink", artifacts_toml)
+                @test !artifact_exists(cts_hash)
+                @test_throws "Julia cannot create symlinks" Pkg.instantiate()
+                @test !artifact_exists(cts_hash)
+                withenv("JULIA_PKG_IGNORE_HASHES"=>1) do
+                    Pkg.instantiate()
+                    @test artifact_exists(cts_hash)
+                end
+            end
+        end
+    end
+end
+
 end # module