Cache a shared git source only once per command

lib/bundler/source/git.rb

@@ -30,6 +30,7 @@ def initialize(options)
 
         @copied     = false
         @local      = false
+        @cached_app_cache_path = nil
       end
 
       def remote!
@@ -277,6 +278,11 @@ def cache_to(custom_path, try_migrate: false)
 
         app_cache_path = app_cache_path(custom_path)
 
+        # When several gems share a single git source, this is called once per
+        # gem. Copying the repository is expensive for large repos, so skip it
+        # if we already populated this cache during the same command.
+        return if @cached_app_cache_path == app_cache_path
+
         migrate = try_migrate ? bare_repo?(app_cache_path) : false
 
         set_cache_path!(nil) if migrate
@@ -288,6 +294,8 @@ def cache_to(custom_path, try_migrate: false)
         git_proxy.checkout if migrate || requires_checkout?
         git_proxy.copy_to(app_cache_path, @submodules)
         serialize_gemspecs_in(app_cache_path)
+
+        @cached_app_cache_path = app_cache_path
       end
 
       def checkout

spec/bundler/source/git_spec.rb

@@ -120,4 +120,32 @@
       end
     end
   end
+
+  describe "#cache" do
+    let(:options) do
+      { "uri" => uri, "revision" => "123abc" }
+    end
+    let(:app_cache_path) { Pathname.new("vendor/cache/bar-123abc") }
+    let(:git_proxy_stub) do
+      instance_double(Bundler::Source::Git::GitProxy, revision: "123abc", copy_to: nil)
+    end
+
+    before do
+      allow(Bundler::Source::Git::GitProxy).to receive(:new).and_return(git_proxy_stub)
+      allow(Bundler.settings).to receive(:[]).and_call_original
+      allow(Bundler.settings).to receive(:[]).with(:cache_all).and_return(true)
+      allow(subject).to receive(:app_cache_path).and_return(app_cache_path)
+      allow(subject).to receive(:cache_path).and_return(Pathname.new("global/git/bar-123abc"))
+      allow(subject).to receive(:requires_checkout?).and_return(false)
+      allow(subject).to receive(:serialize_gemspecs_in)
+      allow(FileUtils).to receive(:rm_rf)
+    end
+
+    it "copies the repository only once when several gems share the same source" do
+      subject.cache(double("spec for gem a"))
+      subject.cache(double("spec for gem b"))
+
+      expect(git_proxy_stub).to have_received(:copy_to).once
+    end
+  end
 end

spec/cache/git_spec.rb

@@ -83,6 +83,29 @@
     expect(the_bundle).to include_gems "foo 1.0"
   end
 
+  it "caches a repository shared by multiple gems only once" do
+    build_lib "foo", path: lib_path("shared/foo")
+    build_lib "bar", path: lib_path("shared/bar")
+    build_git "foo", path: lib_path("shared")
+    git = build_git "bar", path: lib_path("shared")
+    ref = git.ref_for("main", 11)
+
+    install_gemfile <<-G
+      source "https://gem.repo1"
+      git "#{lib_path("shared")}" do
+        gem "foo"
+        gem "bar"
+      end
+    G
+
+    bundle :cache
+
+    expect(bundled_app("vendor/cache/shared-#{ref}")).to exist
+
+    FileUtils.rm_r lib_path("shared")
+    expect(the_bundle).to include_gems "foo 1.0", "bar 1.0"
+  end
+
   it "tracks updates" do
     git = build_git "foo"
     old_ref = git.ref_for("main", 11)