Replace Molinillo with PubGrub for dependency resolution

[mlarraz]

What was the end-user or developer problem that led to this PR?

My attempt at fixing #9365.

What is your fix for the problem, implemented in this PR?

• Vendor PubGrub from Bundler's existing copy
• Re-namespace it under Gem::PubGrub
• Rewrite Gem::Resolver to implement PubGrub's source interface
• Delete Molinillo

We should probably to de-duplicate the copies at some point but I figured that can wait.

Gem::Resolver now implements PubGrub's source interface (all_versions_for, versions_for, incompatibilities_for, no_versions_incompatibility_for) instead of Molinillo's interfaces.

Some notes:

• Version preference: all_versions_for returns highest-first, with already-installed versions preferred to avoid unnecessary upgrades. The skip_gems mechanism continues to work for conservative updates.
• Prereleases: Excluded from the preference index by default, but still available to the solver when a constraint requires them (e.g., = 2.a).
• Platform specificity: spec_for prefers InstalledSpecification when available, then selects by
  Gem::Platform.platform_specificity_match
• Error handling: A pre-check still raises UnsatisfiableDependencyError for root deps with zero matches. Everything else flows through PubGrub's SolveFailure -> DependencyResolutionError with its full explanation chain.
• Compatibility fix for InstallerSet: PubGrub needs all available versions upfront, but Source::Local#find_gem only returns the highest matching version. Added Source::Local#find_all_gems (returns all matches) and updated InstallerSet#find_all to use it.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[colby-swandale]

I appreciate the effort here, but I should flag that I was already actively working on this. I opened the issue, I'm assigned to it, and I've had a WIP implementation going for several weeks now.

That said, I don't want to be a gatekeeper, but I'm not exactly sure what the path forward here is.

[mlarraz]

I appreciate the effort here, but I should flag that I was already actively working on this. I opened the issue, I'm assigned to it, and I've had a WIP implementation going for several weeks now.

That said, I don't want to be a gatekeeper, but I'm not exactly sure what the path forward here is.

Makes sense. Not sure what your version looks like but feel free to use any ideas from mine if it helps. I would just like the change to get done.

[hsbt]

@colby-swandale Could you open a pull request with your current WIP branch? Even if it's still in progress, having it visible will help us coordinate.

Once your PR is up, we can compare both implementations and incorporate any useful differences from this PR into yours. The goal is to combine the best of both approaches into the final PubGrub integration.

[colby-swandale]

Lots of progress over the weekend. Performance is noticeably much much improved, which is encouraging.

That said, there's still a fair amount of resolver behaviour to verify and work through before this is ready. Will block out some more time this week before RubyKaigi to get this ready for review.

[Copilot]

Pull request overview

This PR migrates RubyGems’ dependency resolution from Molinillo to PubGrub to align with Bundler’s solver and support the ongoing unification work discussed in #9365.

Changes:

• Vendors PubGrub under Gem::PubGrub, wires Gem::Resolver to PubGrub’s source interface, and removes vendored Molinillo.
• Updates local gem sourcing to support returning all matching local gems (find_all_gems) so PubGrub can see the full version universe.
• Adjusts RubyGems error handling/tests to reflect PubGrub’s failure explanations (SolveFailure → DependencyResolutionError).

Reviewed changes

Copilot reviewed 27 out of 67 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tool/bundler/vendor_gems.rb.lock	Removes Molinillo from the vendoring lockfile.
tool/bundler/vendor_gems.rb	Removes Molinillo from Bundler’s vendoring Gemfile.
tool/automatiek/vendor.rb	Switches automatiek vendoring config from Molinillo to PubGrub.
tool/automatiek/pub_grub-rubygems.patch	Applies RubyGems-specific PubGrub tweaks (logger + strategy nil-index handling).
tool/automatiek/molinillo-master.patch	Removes now-obsolete Molinillo patch.
test/rubygems/test_gem_source_local.rb	Adds tests for Source::Local#find_all_gems and prerelease behavior.
test/rubygems/test_gem_resolver_strategy.rb	Adds focused tests for strategy selection + caching behavior.
test/rubygems/test_gem_resolver_conflict.rb	Removes Molinillo-era conflict explanation tests.
test/rubygems/test_gem_resolver.rb	Updates resolver tests for new error shapes/messages; adds PubGrub-related scenarios.
test/rubygems/test_gem_impossible_dependencies_error.rb	Removes tests for removed ImpossibleDependenciesError.
test/rubygems/test_gem_dependency_resolution_error.rb	Updates DependencyResolutionError expectations for PubGrub explanation-based errors.
test/rubygems/test_gem_dependency_installer.rb	Updates installer error expectations and adds --force/unsatisfiable dep test.
test/rubygems/test_gem_commands_install_command.rb	Loosens error matching to account for new error text.
test/rubygems/test_gem_commands_exec_command.rb	Adds exec-command test for dependency resolution failures.
test/rubygems/test_gem.rb	Updates activation test to reflect PubGrub backtracking behavior.
lib/rubygems/vendored_pub_grub.rb	Adds a new vendored entrypoint for PubGrub.
lib/rubygems/vendored_molinillo.rb	Removes Molinillo vendored entrypoint.
lib/rubygems/vendor/pub_grub/lib/pub_grub.rb	Adds vendored PubGrub entrypoint (namespaced as Gem::PubGrub).
lib/rubygems/vendor/pub_grub/lib/pub_grub/assignment.rb	Vendored PubGrub solver support code.
lib/rubygems/vendor/pub_grub/lib/pub_grub/basic_package_source.rb	Vendored PubGrub package source base implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/failure_writer.rb	Vendored PubGrub failure explanation writer.
lib/rubygems/vendor/pub_grub/lib/pub_grub/incompatibility.rb	Vendored PubGrub incompatibility representation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/package.rb	Vendored PubGrub package abstraction.
lib/rubygems/vendor/pub_grub/lib/pub_grub/partial_solution.rb	Vendored PubGrub partial solution state.
lib/rubygems/vendor/pub_grub/lib/pub_grub/rubygems.rb	RubyGems requirement → PubGrub range/constraint conversion helpers.
lib/rubygems/vendor/pub_grub/lib/pub_grub/solve_failure.rb	Vendored PubGrub SolveFailure error type.
lib/rubygems/vendor/pub_grub/lib/pub_grub/static_package_source.rb	Vendored PubGrub static source (mostly for tests/examples).
lib/rubygems/vendor/pub_grub/lib/pub_grub/strategy.rb	Vendored PubGrub default strategy implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/term.rb	Vendored PubGrub term implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/version.rb	Vendored PubGrub version constant.
lib/rubygems/vendor/pub_grub/lib/pub_grub/version_constraint.rb	Vendored PubGrub version constraint implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/version_range.rb	Vendored PubGrub version range implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/version_solver.rb	Vendored PubGrub version solver implementation.
lib/rubygems/vendor/pub_grub/lib/pub_grub/version_union.rb	Vendored PubGrub union-of-ranges implementation.
lib/rubygems/vendor/pub_grub/LICENSE.txt	Adds PubGrub license.
lib/rubygems/source/local.rb	Adds find_all_gems; rewires find_gem to use it.
lib/rubygems/resolver/strategy.rb	Adds a RubyGems-specific PubGrub strategy with caching.
lib/rubygems/resolver/stats.rb	Removes Molinillo-era resolver stats.
lib/rubygems/resolver/installer_set.rb	Switches local lookup to find_all_gems so all versions are visible.
lib/rubygems/resolver/incompatibility.rb	Adds RubyGems wrapper for PubGrub incompatibilities with extended explanation.
lib/rubygems/resolver/conflict.rb	Removes Molinillo-era conflict type.
lib/rubygems/resolver.rb	Replaces Molinillo resolver with PubGrub integration and error translation.
lib/rubygems/exceptions.rb	Reworks DependencyResolutionError; removes ImpossibleDependenciesError.
lib/rubygems/commands/install_command.rb	Adds handling for DependencyResolutionError exit code behavior.
lib/rubygems/commands/exec_command.rb	Adds handling for DependencyResolutionError exit code behavior.
Manifest.txt	Updates shipped file list: removes Molinillo, adds PubGrub + new strategy/incompatibility files.

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[colby-swandale]

I ran the top 1000 gems through both Molinillo & PubGrub. 996 of 1000 resolved identically.

The 3 edge cases

Both resolvers produce resolutions that satisfy the declared constraints; they just pick different valid solutions in their graphs.

Gem	Molinillo picks	PubGrub picks
redis-rails	Rails 7.0	Rails 7.2
webdrivers	selenium-webdriver 4.10.0, rubyzip 2.4.1	selenium-webdriver 4.1.0, rubyzip 3.2.2
ohai	chef-config 19.2.12	chef-config 18.9.4

The one that errors

ruby_dep fails to resolve on both parsers due to ruby ~> 2.x requirement. This is essentially a broken gem.

Performance

	Molinillo	PubGrub	Ratio
p50	1.14s	1.10s	0.97x
p95	5.15s	6.86s	1.33x
Total	1772s	2372s	1.34x

PubGrub is at parity at the median and ~30% slower at the tail and in aggregate.