Install the General registry unpacked, so Julia 1.7 can read it

scripts/install_julia_versions.jl

@@ -1,5 +1,66 @@
+using Pkg
+
 for minor in 0:13
     version = "1.$minor"
     println("Installing Julia $version...")
     run(ignorestatus(`juliaup add $version`))
 end
+
+# Every version installed above shares this depot, and Julia 1.7's bundled 7z cannot
+# decompress zstd — 7z only learned that in v17.6. Pkg asks the package server for a zstd
+# compressed registry from Julia 1.13 on, everywhere except Windows, where it skips zstd
+# for exactly this reason (`Pkg/src/PlatformEngines.jl`). That carve-out is by platform
+# rather than by what else lives in the depot, so on Linux and macOS a `General.tar.zst`
+# lands here and `check_julia_version("1.7")` fails on it, while the same test passes on
+# Windows. An unpacked registry is a plain folder that every version reads.
+registries = joinpath(first(DEPOT_PATH), "registries")
+entries = isdir(registries) ? readdir(registries) : String[]
+packed = any(i -> startswith(i, "General.tar"), entries)
+unpacked = isdir(joinpath(registries, "General"))
+
+if !Sys.iswindows()
+    if packed || !unpacked
+        println("Installing the General registry unpacked, so that Julia 1.7 can read it...")
+
+        # Removing the packed registry has to happen outside the `withenv` below: with
+        # `JULIA_PKG_UNPACK_REGISTRY` set, Pkg looks for unpacked registries only and
+        # reports the packed one it is standing on as not installed.
+        packed && Pkg.Registry.rm("General")
+
+        withenv("JULIA_PKG_UNPACK_REGISTRY" => "true") do
+            Pkg.Registry.add("General")
+        end
+    end
+
+    # Converting what is here now is not enough: anything later in this job that installs
+    # or refreshes the registry — the run itself, or a test process resolving an
+    # environment — would put a packed one back. Setting the variable for the rest of the
+    # job keeps every such download unpacked too.
+    github_env = get(ENV, "GITHUB_ENV", nothing)
+
+    if github_env !== nothing
+        open(github_env, "a") do io
+            println(io, "JULIA_PKG_UNPACK_REGISTRY=true")
+        end
+    end
+elseif unpacked && !packed
+    # Windows never had the zstd problem — Pkg refuses zstd for registries there, for the
+    # same 7z reason — and an unpacked registry actively hurts: Pkg replaces it on the next
+    # registry operation, and its replace-on-write leaves `$XXXX.pid.deleted` entries that
+    # the concurrent test processes then trip over with `stat: permission denied`. The
+    # depot is cached between runs, so one unpacked registry would keep breaking every
+    # later Windows run. Put a packed one back, and sweep up what was left behind.
+    println("Restoring a packed General registry, which is what Windows wants...")
+
+    Pkg.Registry.rm("General")
+    Pkg.Registry.add("General")
+
+    for i in readdir(registries)
+        endswith(i, ".pid.deleted") || continue
+        try
+            rm(joinpath(registries, i); recursive=true, force=true)
+        catch err
+            @warn "Could not remove a leftover registry entry" entry = i err
+        end
+    end
+end