Accept environment variables from the `pre-receive` script.

1. Starting version 2.11, git changed the way the pre-receive flow works.

  - Previously, the new potential objects would be added to the main repo. If the
    pre-receive passes, the new objects stay in the repo but are linked up. If
    the pre-receive fails, the new objects stay orphaned in the repo, and are
    cleaned up during the next `git gc`.

  - In 2.11, the new potential objects are added to a temporary "alternate object
    directory", that git creates for this purpose. If the pre-receive passes, the
    objects from the alternate object directory are migrated to the main repo. If
    the pre-receive fails the alternate object directory is simply deleted.

2. In our workflow, the pre-recieve script (in `gitlab-shell) calls the
   `/allowed` endpoint, which calls out directly to git to perform
   various checks. These direct calls to git do _not_ have the necessary
   environment variables set which allow access to the "alternate object
   directory" (explained above). Therefore these calls to git are not able to
   access any of the new potential objects to be added during this push.

3. We fix this by accepting the relevant environment variables
   (GIT_ALTERNATE_OBJECT_DIRECTORIES, GIT_OBJECT_DIRECTORY) on the
   `/allowed` endpoint, and then include these environment variables while
   calling out to git.

4. This commit includes (whitelisted) these environment variables while making
   the "force push" check. A `Gitlab::Git::RevList` module is extracted to
   prevent `ForcePush` from being littered with these checks.

lib/api/helpers/internal_helpers.rb

@@ -52,6 +52,14 @@ module API
           :push_code
         ]
       end
+
+      def parse_allowed_environment_variables
+        return if params[:env].blank?
+
+        JSON.parse(params[:env])
+
+      rescue JSON::ParserError
+      end
     end
   end
 end

lib/api/internal.rb

@@ -32,7 +32,11 @@ module API
           if wiki?
             Gitlab::GitAccessWiki.new(actor, project, protocol, authentication_abilities: ssh_authentication_abilities)
           else
-            Gitlab::GitAccess.new(actor, project, protocol, authentication_abilities: ssh_authentication_abilities)
+            Gitlab::GitAccess.new(actor,
+                                  project,
+                                  protocol,
+                                  authentication_abilities: ssh_authentication_abilities,
+                                  env: parse_allowed_environment_variables)
           end
 
         access_status = access.check(params[:action], params[:changes])

lib/gitlab/checks/change_access.rb

@@ -3,11 +3,12 @@ module Gitlab
     class ChangeAccess
       attr_reader :user_access, :project
 
-      def initialize(change, user_access:, project:)
+      def initialize(change, user_access:, project:, env: {})
         @oldrev, @newrev, @ref = change.values_at(:oldrev, :newrev, :ref)
         @branch_name = Gitlab::Git.branch_name(@ref)
         @user_access = user_access
         @project = project
+        @env = env
       end
 
       def exec
@@ -68,7 +69,7 @@ module Gitlab
       end
 
       def forced_push?
-        Gitlab::Checks::ForcePush.force_push?(@project, @oldrev, @newrev)
+        Gitlab::Checks::ForcePush.force_push?(@project, @oldrev, @newrev, env: @env)
       end
 
       def matching_merge_request?

lib/gitlab/checks/force_push.rb

 module Gitlab
   module Checks
     class ForcePush
-      def self.force_push?(project, oldrev, newrev)
+      def self.force_push?(project, oldrev, newrev, env: {})
         return false if project.empty_repo?
 
         # Created or deleted branch
         if Gitlab::Git.blank_ref?(oldrev) || Gitlab::Git.blank_ref?(newrev)
           false
         else
-          missed_ref, _ = Gitlab::Popen.popen(%W(#{Gitlab.config.git.bin_path} --git-dir=#{project.repository.path_to_repo} rev-list --max-count=1 #{oldrev} ^#{newrev}))
+          missed_ref, _ = Gitlab::Git::RevList.new(oldrev, newrev, project: project, env: env).execute
           missed_ref.present?
         end
       end

lib/gitlab/git/rev_list.rb

+# Call out to the `git rev-list` command
+
+module Gitlab
+  module Git
+    class RevList
+      def initialize(oldrev, newrev, project:, env: nil)
+        @args = [Gitlab.config.git.bin_path,
+                 "--git-dir=#{project.repository.path_to_repo}",
+                 "rev-list",
+                 "--max-count=1",
+                 oldrev,
+                 "^#{newrev}"]
+
+        @env = env.slice(*allowed_environment_variables)
+      end
+
+      def execute
+        Gitlab::Popen.popen(@args, nil, @env.slice(*allowed_environment_variables))
+      end
+
+      private
+
+      def allowed_environment_variables
+        %w(GIT_ALTERNATE_OBJECT_DIRECTORIES GIT_OBJECT_DIRECTORY)
+      end
+    end
+  end
+end

lib/gitlab/git_access.rb

@@ -17,12 +17,13 @@ module Gitlab
 
     attr_reader :actor, :project, :protocol, :user_access, :authentication_abilities
 
-    def initialize(actor, project, protocol, authentication_abilities:)
+    def initialize(actor, project, protocol, authentication_abilities:, env: {})
       @actor    = actor
       @project  = project
       @protocol = protocol
       @authentication_abilities = authentication_abilities
       @user_access = UserAccess.new(user, project: project)
+      @env = env
     end
 
     def check(cmd, changes)
@@ -103,7 +104,7 @@ module Gitlab
     end
 
     def change_access_check(change)
-      Checks::ChangeAccess.new(change, user_access: user_access, project: project).exec
+      Checks::ChangeAccess.new(change, user_access: user_access, project: project, env: @env).exec
     end
 
     def protocol_allowed?

lib/gitlab/popen.rb

@@ -5,13 +5,13 @@ module Gitlab
   module Popen
     extend self
 
-    def popen(cmd, path = nil)
+    def popen(cmd, path = nil, vars = {})
       unless cmd.is_a?(Array)
         raise "System commands must be given as an array of strings"
       end
 
       path ||= Dir.pwd
-      vars = { "PWD" => path }
+      vars['PWD'] = path
       options = { chdir: path }
 
       unless File.directory?(path)