Avoid clone same repo for Git gems

lib/gel/installer.rb

@@ -27,6 +27,7 @@ def initialize(store)
     @dependencies = Hash.new { |h, k| h[k] = [] }
     @weights = Hash.new(1)
     @pending = Hash.new(0)
+    @git_remotes = Hash.new
 
     @download_pool = Gel::WorkPool.new(DOWNLOAD_CONCURRENCY, monitor: self, name: "gel-download", collect_errors: true)
     @compile_pool = Gel::WorkPool.new(COMPILE_CONCURRENCY, monitor: self, name: "gel-compile", collect_errors: true)
@@ -76,15 +77,44 @@ def install_gem(catalogs, name, version)
   def load_git_gem(remote, revision, name)
     synchronize do
       @pending[name] += 1
-      @download_pool.queue(name) do
-        work_git(remote, revision, name)
+
+      checkout_key = "#{remote}@#{revision}"
+
+      # Has another gem already been pulled from this git repo?
+      if @git_remotes.key?(checkout_key)
+        if waiting_gems = @git_remotes[checkout_key]
+          # The checkout is currently queued or in progress; we'll wait
+          # for it too
+          waiting_gems << name
+        else
+          # The checkout has already finished, so this gem is already
+          # available
+          git_ready(name)
+        end
+      else
+        @git_remotes[checkout_key] = [name]
+
+        @download_pool.queue(name) do
+          work_git(remote, revision, checkout_key)
+        end
       end
     end
   end
 
-  def work_git(remote, revision, name)
+  def work_git(remote, revision, checkout_key)
     @git_depot.checkout(remote, revision)
 
+    synchronize do
+      gem_names = @git_remotes[checkout_key]
+      @git_remotes[checkout_key] = nil
+
+      gem_names.each do |name|
+        git_ready(name)
+      end
+    end
+  end
+
+  def git_ready(name)
     @messages << "Using #{name} (git)\n"
     @pending[name] -= 1
   end

lib/gel/lock_loader.rb

@@ -98,7 +98,7 @@ def activate(env, base_store, install: false, output: nil)
             revision = resolved_gem.body["revision"].first
 
             dir = git_depot.git_path(remote, revision)
-            if installer && !Dir.exist?(dir)
+            if installer
               installer.load_git_gem(remote, revision, name)
 
               locks[name] = -> { Gel::DirectGem.new(dir, name, resolved_gem.version) }

test/installer_test.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+require "gel/installer"
+
+class InstallerTest < Minitest::Test
+  def test_install_git_gems_only_checkout_once
+    Dir.mktmpdir do |dir|
+      remote = "https://github.com/rails/rails.git"
+      revision = "d56d2e740612717334840b0d0ea979e1ca7cf5b1"
+      names = ["actioncable", "actionmailbox"]
+      store = Gel::Store.new(dir)
+      installer = Gel::Installer.new(store)
+      fake_download_pool = Minitest::Mock.new
+      installer.instance_variable_set(
+        :@download_pool,
+        fake_download_pool
+      )
+      fake_download_pool.expect(:queue, true) do |argument|
+        argument == "actioncable"
+      end
+
+      names.each do |name|
+        installer.load_git_gem(remote, revision, name)
+      end
+    end
+  end
+end