Merge branch 'security-reverse-polarity-of-branch-compare' into 'master'

Make cross-repository comparisons happen in the source repository

Closes #29

See merge request gitlab-org/security/gitlab!84

app/services/compare_service.rb

@@ -18,7 +18,7 @@ class CompareService
     return unless raw_compare && raw_compare.base && raw_compare.head
 
     Compare.new(raw_compare,
-                target_project,
+                start_project,
                 base_sha: base_sha,
                 straight: straight)
   end

changelogs/unreleased/security-reverse-polarity-of-branch-compare.yml

+---
+title: Make cross-repository comparisons happen in the source repository
+merge_request:
+author:
+type: security

lib/gitlab/git/cross_repo_comparer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class CrossRepoComparer
+      attr_reader :source_repo, :target_repo
+
+      def initialize(source_repo, target_repo)
+        @source_repo = source_repo
+        @target_repo = target_repo
+      end
+
+      def compare(source_ref, target_ref, straight:)
+        ensuring_ref_in_source(target_ref) do |target_commit_id|
+          Gitlab::Git::Compare.new(
+            source_repo,
+            target_commit_id,
+            source_ref,
+            straight: straight
+          )
+        end
+      end
+
+      private
+
+      def ensuring_ref_in_source(ref, &blk)
+        return yield ref if source_repo == target_repo
+
+        # If the commit doesn't exist in the target, there's nothing we can do
+        commit_id = target_repo.commit(ref)&.sha
+        return unless commit_id
+
+        # The commit pointed to by ref may exist in the source even when they
+        # are different repositories. This is particularly true of close forks,
+        # but may also be the case if a temporary ref for this comparison has
+        # already been created in the past, and the result hasn't been GC'd yet.
+        return yield commit_id if source_repo.commit(commit_id)
+
+        # Worst case: the commit is not in the source repo so we need to fetch
+        # it. Use a temporary ref and clean up afterwards
+        with_commit_in_source_tmp(commit_id, &blk)
+      end
+
+      # Fetch the ref into source_repo from target_repo, using a temporary ref
+      # name that will be deleted once the method completes. This is a no-op if
+      # fetching the source branch fails
+      def with_commit_in_source_tmp(commit_id, &blk)
+        tmp_ref = "refs/tmp/#{SecureRandom.hex}"
+
+        yield commit_id if source_repo.fetch_source_branch!(target_repo, commit_id, tmp_ref)
+      ensure
+        source_repo.delete_refs(tmp_ref) # best-effort
+      end
+    end
+  end
+end

lib/gitlab/git/repository.rb

@@ -746,29 +746,9 @@ module Gitlab
       end
 
       def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
-        reachable_ref =
-          if source_repository == self
-            source_branch_name
-          else
-            # If a tmp ref was created before for a separate repo comparison (forks),
-            # we're able to short-circuit the tmp ref re-creation:
-            # 1. Take the SHA from the source repo
-            # 2. Read that in the current "target" repo
-            # 3. If that SHA is still known (readable), it means GC hasn't
-            # cleaned it up yet, so we can use it instead re-writing the tmp ref.
-            source_commit_id = source_repository.commit(source_branch_name)&.sha
-            commit(source_commit_id)&.sha if source_commit_id
-          end
-
-        return compare(target_branch_name, reachable_ref, straight: straight) if reachable_ref
-
-        tmp_ref = "refs/tmp/#{SecureRandom.hex}"
-
-        return unless fetch_source_branch!(source_repository, source_branch_name, tmp_ref)
-
-        compare(target_branch_name, tmp_ref, straight: straight)
-      ensure
-        delete_refs(tmp_ref) if tmp_ref
+        CrossRepoComparer
+          .new(source_repository, self)
+          .compare(source_branch_name, target_branch_name, straight: straight)
       end
 
       def write_ref(ref_path, ref, old_ref: nil)
@@ -1046,13 +1026,6 @@ module Gitlab
 
       private
 
-      def compare(base_ref, head_ref, straight:)
-        Gitlab::Git::Compare.new(self,
-                                 base_ref,
-                                 head_ref,
-                                 straight: straight)
-      end
-
       def empty_diff_stats
         Gitlab::Git::DiffStatsCollection.new([])
       end

spec/lib/gitlab/git/cross_repo_comparer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Git::CrossRepoComparer do
+  let(:source_project) { create(:project, :repository) }
+  let(:target_project) { create(:project, :repository) }
+
+  let(:source_repo) { source_project.repository.raw_repository }
+  let(:target_repo) { target_project.repository.raw_repository }
+
+  let(:source_branch) { 'feature' }
+  let(:target_branch) { 'master' }
+  let(:straight) { false }
+
+  let(:source_commit) { source_repo.commit(source_branch) }
+  let(:target_commit) { source_repo.commit(target_branch) }
+
+  subject(:result) { described_class.new(source_repo, target_repo).compare(source_branch, target_branch, straight: straight) }
+
+  describe '#compare' do
+    context 'within a single repository' do
+      let(:target_project) { source_project }
+
+      context 'a non-straight comparison' do
+        it 'compares without fetching from another repo' do
+          expect(source_repo).not_to receive(:fetch_source_branch!)
+
+          expect_compare(result, from: source_commit, to: target_commit)
+          expect(result.straight).to eq(false)
+        end
+      end
+
+      context 'a straight comparison' do
+        let(:straight) { true }
+
+        it 'compares without fetching from another repo' do
+          expect(source_repo).not_to receive(:fetch_source_branch!)
+
+          expect_compare(result, from: source_commit, to: target_commit)
+          expect(result.straight).to eq(true)
+        end
+      end
+    end
+
+    context 'across two repositories' do
+      context 'target ref exists in source repo' do
+        it 'compares without fetching from another repo' do
+          expect(source_repo).not_to receive(:fetch_source_branch!)
+          expect(source_repo).not_to receive(:delete_refs)
+
+          expect_compare(result, from: source_commit, to: target_commit)
+        end
+      end
+
+      context 'target ref does not exist in source repo' do
+        it 'compares in the source repo by fetching from the target to a temporary ref' do
+          new_commit_id = create_commit(target_project.owner, target_repo, target_branch)
+          new_commit = target_repo.commit(new_commit_id)
+
+          # This is how the temporary ref is generated
+          expect(SecureRandom).to receive(:hex).at_least(:once).and_return('foo')
+
+          expect(source_repo)
+            .to receive(:fetch_source_branch!)
+            .with(target_repo, new_commit_id, 'refs/tmp/foo')
+            .and_call_original
+
+          expect(source_repo).to receive(:delete_refs).with('refs/tmp/foo').and_call_original
+
+          expect_compare(result, from: source_commit, to: new_commit)
+        end
+      end
+
+      context 'source ref does not exist in source repo' do
+        let(:source_branch) { 'does-not-exist' }
+
+        it 'returns an empty comparison' do
+          expect(source_repo).not_to receive(:fetch_source_branch!)
+          expect(source_repo).not_to receive(:delete_refs)
+
+          expect(result).to be_a(::Gitlab::Git::Compare)
+          expect(result.commits.size).to eq(0)
+        end
+      end
+
+      context 'target ref does not exist in target repo' do
+        let(:target_branch) { 'does-not-exist' }
+
+        it 'returns nil' do
+          expect(source_repo).not_to receive(:fetch_source_branch!)
+          expect(source_repo).not_to receive(:delete_refs)
+
+          is_expected.to be_nil
+        end
+      end
+    end
+  end
+
+  def expect_compare(of, from:, to:)
+    expect(of).to be_a(::Gitlab::Git::Compare)
+    expect(from).to be_a(::Gitlab::Git::Commit)
+    expect(to).to be_a(::Gitlab::Git::Commit)
+
+    expect(of.commits).not_to be_empty
+    expect(of.head).to eq(from)
+    expect(of.base).to eq(to)
+  end
+
+  def create_commit(user, repo, branch)
+    action = { action: :create, file_path: '/FILE', content: 'content' }
+
+    result = repo.multi_action(user, branch_name: branch, message: 'Commit', actions: [action])
+
+    result.newrev
+  end
+end

spec/lib/gitlab/git/repository_spec.rb

@@ -1962,66 +1962,15 @@ describe Gitlab::Git::Repository, :seed_helper do
   end
 
   describe '#compare_source_branch' do
-    let(:repository) { Gitlab::Git::Repository.new('default', TEST_GITATTRIBUTES_REPO_PATH, '', 'group/project') }
-
-    context 'within same repository' do
-      it 'does not create a temp ref' do
-        expect(repository).not_to receive(:fetch_source_branch!)
-        expect(repository).not_to receive(:delete_refs)
-
-        compare = repository.compare_source_branch('master', repository, 'feature', straight: false)
-        expect(compare).to be_a(Gitlab::Git::Compare)
-        expect(compare.commits.count).to be > 0
-      end
-
-      it 'returns empty commits when source ref does not exist' do
-        compare = repository.compare_source_branch('master', repository, 'non-existent-branch', straight: false)
+    it 'delegates to Gitlab::Git::CrossRepoComparer' do
+      expect_next_instance_of(::Gitlab::Git::CrossRepoComparer) do |instance|
+        expect(instance.source_repo).to eq(:source_repository)
+        expect(instance.target_repo).to eq(repository)
 
-        expect(compare.commits).to be_empty
+        expect(instance).to receive(:compare).with('feature', 'master', straight: :straight)
       end
-    end
 
-    context 'with different repositories' do
-      context 'when ref is known by source repo, but not by target' do
-        before do
-          mutable_repository.write_ref('another-branch', 'feature')
-        end
-
-        it 'creates temp ref' do
-          expect(repository).not_to receive(:fetch_source_branch!)
-          expect(repository).not_to receive(:delete_refs)
-
-          compare = repository.compare_source_branch('master', mutable_repository, 'another-branch', straight: false)
-          expect(compare).to be_a(Gitlab::Git::Compare)
-          expect(compare.commits.count).to be > 0
-        end
-      end
-
-      context 'when ref is known by source and target repos' do
-        before do
-          mutable_repository.write_ref('another-branch', 'feature')
-          repository.write_ref('another-branch', 'feature')
-        end
-
-        it 'does not create a temp ref' do
-          expect(repository).not_to receive(:fetch_source_branch!)
-          expect(repository).not_to receive(:delete_refs)
-
-          compare = repository.compare_source_branch('master', mutable_repository, 'another-branch', straight: false)
-          expect(compare).to be_a(Gitlab::Git::Compare)
-          expect(compare.commits.count).to be > 0
-        end
-      end
-
-      context 'when ref is unknown by source repo' do
-        it 'returns nil when source ref does not exist' do
-          expect(repository).to receive(:fetch_source_branch!).and_call_original
-          expect(repository).to receive(:delete_refs).and_call_original
-
-          compare = repository.compare_source_branch('master', mutable_repository, 'non-existent-branch', straight: false)
-          expect(compare).to be_nil
-        end
-      end
+      repository.compare_source_branch('master', :source_repository, 'feature', straight: :straight)
     end
   end