[Contributing Guidelines] Add script to check that versions are present

CONTRIBUTING.md

@@ -68,7 +68,8 @@ You can review such a PR by checking that the old URL redirects to the new one.
 * If it does not, you can ask the author why. This should be handled on a case-by-case basis. Be sure to check that:
     1. The package is not being hijacked; check for example that the person making the PR has registered a version of the package before, indicating they are authorized to do so.
     2. All the registered revisions of the package are accessible in the new repository.
-       Specifically, this means checking that all the git-tree-shas can be found in the new repository ([example](https://github.com/JuliaRegistries/General/pull/35965#issuecomment-832721704)).
+       Specifically, this means checking that all the git-tree-shas can be found in the new repository.
+       See [the appendix](#appendix-checking-if-a-repository-contains-all-registered-versions-of-a-package) below for a script to automate this checking.
 
 ### Other ways to help
 
@@ -89,6 +90,67 @@ You generally should not merge your own registrations (though you can make reque
 * [write] You can merge improvements to the README, these guidelines, or our workflows.
 * [admin] You can give other contributors triage-level access so they can apply labels to PRs, or write-level permissions to merge PRs.
 
+## Appendix: Checking if a repository contains all registered versions of a package
+
+When someone wishes to move a package from one repo to another, it is important that the new repo contains all of the tree hashes corresponding to registered versions of a package. That way these old versions of the package can continue to be installed from the new repository. In order to check if a given repository contains all of the registered versions of a package, the following script can be used:
+
+```julia
+using RegistryInstances, UUIDs, Git
+
+const GENERAL_UUID = UUID("23338594-aafe-5451-b93e-139f81909106")
+
+pretty_print_row(row) = println(row.pkg_name, ": v", row.version, " ", row.found ? "found" : "is missing")
+pretty_print_table(table) = foreach(pretty_print_row, table)
+
+function check_all_found(table)
+    idx = findfirst(row -> !row.found, table)
+    idx === nothing && return nothing
+    row = table[idx]
+    error(string("Repository missing v", row.version, " of package $(row.pkg_name)"))
+end
+
+function check_packages_versions(pkg_names, repo_url; registry_uuid=GENERAL_UUID, verbose=true, throw=true)
+    dir = mktempdir()
+    run(`$(git()) clone $(repo_url) $dir`)
+
+    registry = only(filter!(r -> r.uuid == registry_uuid, reachable_registries()))
+
+    table = @NamedTuple{pkg_name::String, version::VersionNumber, found::Bool}[]
+
+    for pkg_name in pkg_names
+        pkg = registry.pkgs[only(uuids_from_name(registry, pkg_name))]
+        versions = registry_info(pkg).version_info
+        for version in sort(collect(keys(versions)))
+            tree_sha = versions[version].git_tree_sha1
+            found = success(`$(git()) -C $dir rev-parse -q --verify "$(tree_sha)^{tree}"`)
+
+            push!(table, (; pkg_name, version, found))
+        end
+    end
+    verbose && pretty_print_table(table)
+    throw && check_all_found(table)
+    return table
+end
+
+check_package_versions(pkg_name, repo_url; kw...) = check_packages_versions([pkg_name], repo_url; kw...)
+```
+
+For example, in [General#75319](https://github.com/JuliaRegistries/General/pull/75319), a package author wanted to update the URL associated
+to their package "FastParzenWindows". At the time, the package had 1 registered version. We can check that it is present in the new repository via:
+```julia
+julia> check_package_versions("FastParzenWindows", "https://github.com/ngiann/FastParzenWindows.jl.git");
+Cloning into '/var/folders/jb/plyyfc_d2bz195_0rc0n_zcw0000gp/T/jl_ke9E8C'...
+...text omitted...
+FastParzenWindows: v0.1.2 found
+```
+We see that this version was found in the new repository. This script was based on [this comment from General#35965](https://github.com/JuliaRegistries/General/pull/35965#issuecomment-832721704),
+which involved checking if 4 packages in the same repository all had their versions present in the new repository. That example can be handled as follows:
+
+```julia
+pkg_names = ["ReinforcementLearningBase", "ReinforcementLearningCore",
+             "ReinforcementLearningEnvironments", "ReinforcementLearningZoo"]
+check_packages_versions(pkg_names, "https://github.com/JuliaReinforcementLearning/ReinforcementLearning.jl.git")
+```
 
 [FAQ]: https://github.com/JuliaRegistries/General#faq]
 [naming-guidelines]: https://pkgdocs.julialang.org/dev/creating-packages/#Package-naming-guidelines-1