[rubygems/rubygems] Fix unnecessary downgrade of top level dependency when unlocking

Bundler is very conservative by default, trying to preserve versions
from the lockfile as possible, and never downgrading them. However, when
it runs into a resolution error, it still tries to find a valid
resolution.

This fallback behavior was too "brute-force" though, completely
unrestricting any gem found in the resolution conflict, and that could
lead to direct dependencies being downgraded in some edge cases.

Instead, unlock things a bit more carefully:

* First try unlocking fully pinned indirect dependencies, but leave a
  lower bound requirement in place to prevent downgrades.
* Then try unlocking any fully pinned dependency, also leaving a lower
  bound requirement in place.
* Finally completely unrestrict dependencies if nothing else worked.

https://github.com/rubygems/rubygems/commit/7f55ed8302

1 files changed, 83 insertions, 0 deletions

diff --git a/spec/bundler/commands/lock_spec.rb b/spec/bundler/commands/lock_spec.rb
index a148d6dd19..126853e35f 100644
--- a/spec/bundler/commands/lock_spec.rb
+++ b/spec/bundler/commands/lock_spec.rb
@@ -1161,4 +1161,87 @@ RSpec.describe "bundle lock" do
       L
     end
   end
+
+  context "when adding a new gem that requires unlocking other transitive deps" do
+    before do
+      build_repo4 do
+        build_gem "govuk_app_config", "0.1.0"
+
+        build_gem "govuk_app_config", "4.13.0" do |s|
+          s.add_dependency "railties", ">= 5.0"
+        end
+
+        %w[7.0.4.1 7.0.4.3].each do |v|
+          build_gem "railties", v do |s|
+            s.add_dependency "actionpack", v
+            s.add_dependency "activesupport", v
+          end
+
+          build_gem "activesupport", v
+          build_gem "actionpack", v
+        end
+      end
+
+      gemfile <<~G
+        source "#{file_uri_for(gem_repo4)}"
+
+        gem "govuk_app_config"
+        gem "activesupport", "7.0.4.3"
+      G
+
+      # Simulate out of sync lockfile because top level dependency on
+      # activesuport has just been added to the Gemfile, and locked to a higher
+      # version
+      lockfile <<~L
+        GEM
+          remote: #{file_uri_for(gem_repo4)}/
+          specs:
+            actionpack (7.0.4.1)
+            activesupport (7.0.4.1)
+            govuk_app_config (4.13.0)
+              railties (>= 5.0)
+            railties (7.0.4.1)
+              actionpack (= 7.0.4.1)
+              activesupport (= 7.0.4.1)
+
+        PLATFORMS
+          arm64-darwin-22
+
+        DEPENDENCIES
+          govuk_app_config
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+    end
+
+    it "does not downgrade top level dependencies" do
+      simulate_platform "arm64-darwin-22" do
+        bundle "lock"
+      end
+
+      expect(lockfile).to eq <<~L
+        GEM
+          remote: #{file_uri_for(gem_repo4)}/
+          specs:
+            actionpack (7.0.4.3)
+            activesupport (7.0.4.3)
+            govuk_app_config (4.13.0)
+              railties (>= 5.0)
+            railties (7.0.4.3)
+              actionpack (= 7.0.4.3)
+              activesupport (= 7.0.4.3)
+
+        PLATFORMS
+          arm64-darwin-22
+
+        DEPENDENCIES
+          activesupport (= 7.0.4.3)
+          govuk_app_config
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+    end
+  end
 end