don't evaluate the default lockfile at all in certain cases (#20)

if an alternate lockfile is active, and while that's active for some reason
the default lockfile is not evaluatable, just don't evaluate it (but
without having to know outside of ruby which one will be active)

README.md

@@ -88,6 +88,33 @@ BUNDLE_LOCKFILE=rails-7.0 bundle exec rspec
 You can also dynamically select it in your Gemfile, and pass `current: true`
 to (exactly one!) `lockfile` method.
 
+In some cases, you may want to essentially disable bundler-multilock's
+syncing behavior, while still allowing the Gemfile to select the active
+lockfile. For example, if you have gems in the default lockfile that cannot
+be installed under a certain Ruby version, but still want to be able to CI
+against that Ruby version, you may set it up like this:
+
+```ruby
+lockfile active: RUBY_VERSION >= "2.7" do
+  gem "debug", "~> 1.9"
+end
+
+lockfile "ruby-2.6", active: RUBY_VERSION < "2.7" do
+  gem "debug", "~> 1.8.0"
+end
+```
+
+However, you cannot run a regular `bundle install` while running Ruby 2.6 in
+this situation, since simply evaluating the default lockfile's block, even
+for checking that the lockfiles are valid, will raise an exception that debug
+1.9.0 require Ruby 2.7. You could set `BUNDLE_LOCKFILE=ruby-2.6` to have
+bundler-multilock only consider the one lockfile, but then your CI may need
+additional logic to _not_ set it for other Ruby versions. Instead, you can
+set `BUNDLE_LOCKFILE=active`, which will not override the Gemfile's selection
+of which lockfile is active, but still behave as if `BUNDLE_LOCKFILE` is set,
+bypassing any other syncing logic and not evaluating the default lockfile in
+our example.
+
 ## Comparison to Appraisal
 
 [Appraisal](https://github.com/thoughtbot/appraisal) is a gem that might serve

lib/bundler/multilock.rb

@@ -60,7 +60,8 @@ def add_lockfile(lockfile = nil,
           raise ArgumentError, "Lockfile #{lockfile} is already defined"
         end
 
-        env_lockfile = ENV["BUNDLE_LOCKFILE"]&.then { |l| expand_lockfile(l) }
+        env_lockfile = lockfile if active && ENV["BUNDLE_LOCKFILE"] == "active"
+        env_lockfile ||= ENV["BUNDLE_LOCKFILE"]&.then { |l| expand_lockfile(l) }
         active = env_lockfile == lockfile if env_lockfile
 
         if active && (old_active = lockfile_definitions.find { |definition| definition[:active] })

spec/bundler/multilock_spec.rb

@@ -736,6 +736,25 @@
     end
   end
 
+  it "does not evaluate the default lockfile at all if an alternate is active, " \
+     "without specifying that lockfile explicitly" do
+    with_gemfile(<<~RUBY) do
+      gem "inst-jobs", "3.1.13"
+
+      lockfile active: ENV["ALTERNATE"] != "1" do
+        raise "evaluated!" if ENV["ALTERNATE"] == "1"
+      end
+
+      lockfile "alt", active: ENV["ALTERNATE"] == "1" do
+        gem "activerecord-pg-extensions"
+      end
+    RUBY
+      invoke_bundler("install")
+
+      invoke_bundler("install", env: { "ALTERNATE" => "1", "BUNDLE_LOCKFILE" => "active" })
+    end
+  end
+
   private
 
   def create_local_gem(name, content = "", subdirectory: true)