Merge branch 'osw-avoid-errors-due-to-concurrent-calls' into 'master'

Add exclusive lease to mergeability check process

See merge request gitlab-org/gitlab-ce!31082

app/models/merge_request.rb

@@ -752,7 +752,7 @@ class MergeRequest < ApplicationRecord
   end
 
   def check_mergeability
-    MergeRequests::MergeabilityCheckService.new(self).execute
+    MergeRequests::MergeabilityCheckService.new(self).execute(retry_lease: false)
   end
   # rubocop: enable CodeReuse/ServiceClass
 

app/services/merge_requests/mergeability_check_service.rb

@@ -3,6 +3,7 @@
 module MergeRequests
   class MergeabilityCheckService < ::BaseService
     include Gitlab::Utils::StrongMemoize
+    include Gitlab::ExclusiveLeaseHelpers
 
     delegate :project, to: :@merge_request
     delegate :repository, to: :project
@@ -21,13 +22,35 @@ module MergeRequests
     # where we need the current state of the merge ref in repository, the `recheck`
     # argument is required.
     #
+    # retry_lease - Concurrent calls wait for at least 10 seconds until the
+    # lease is granted (other process finishes running). Returns an error
+    # ServiceResponse if the lease is not granted during this time.
+    #
     # Returns a ServiceResponse indicating merge_status is/became can_be_merged
     # and the merge-ref is synced. Success in case of being/becoming mergeable,
     # error otherwise.
-    def execute(recheck: false)
+    def execute(recheck: false, retry_lease: true)
       return ServiceResponse.error(message: 'Invalid argument') unless merge_request
       return ServiceResponse.error(message: 'Unsupported operation') if Gitlab::Database.read_only?
+      return check_mergeability(recheck) unless merge_ref_auto_sync_lock_enabled?
+
+      in_write_lock(retry_lease: retry_lease) do |retried|
+        # When multiple calls are waiting for the same lock (retry_lease),
+        # it's possible that when granted, the MR status was already updated for
+        # that object, therefore we reset if there was a lease retry.
+        merge_request.reset if retried
+
+        check_mergeability(recheck)
+      end
+    rescue FailedToObtainLockError => error
+      ServiceResponse.error(message: error.message)
+    end
+
+    private
+
+    attr_reader :merge_request
 
+    def check_mergeability(recheck)
       recheck! if recheck
       update_merge_status
 
@@ -46,9 +69,21 @@ module MergeRequests
       ServiceResponse.success(payload: payload)
     end
 
-    private
+    # It's possible for this service to send concurrent requests to Gitaly in order
+    # to "git update-ref" the same ref. Therefore we handle a light exclusive
+    # lease here.
+    #
+    def in_write_lock(retry_lease:, &block)
+      lease_key = "mergeability_check:#{merge_request.id}"
 
-    attr_reader :merge_request
+      lease_opts = {
+        ttl:       1.minute,
+        retries:   retry_lease ? 10 : 0,
+        sleep_sec: retry_lease ? 1.second : 0
+      }
+
+      in_lock(lease_key, lease_opts, &block)
+    end
 
     def payload
       strong_memoize(:payload) do
@@ -116,5 +151,9 @@ module MergeRequests
     def merge_ref_auto_sync_enabled?
       Feature.enabled?(:merge_ref_auto_sync, project, default_enabled: true)
     end
+
+    def merge_ref_auto_sync_lock_enabled?
+      Feature.enabled?(:merge_ref_auto_sync_lock, project, default_enabled: true)
+    end
   end
 end

changelogs/unreleased/osw-avoid-errors-due-to-concurrent-calls.yml

+---
+title: Add exclusive lease to mergeability check process
+merge_request: 31082
+author:
+type: fixed

lib/gitlab/exclusive_lease_helpers.rb

@@ -15,17 +15,18 @@ module Gitlab
       raise ArgumentError, 'Key needs to be specified' unless key
 
       lease = Gitlab::ExclusiveLease.new(key, timeout: ttl)
+      retried = false
 
       until uuid = lease.try_obtain
         # Keep trying until we obtain the lease. To prevent hammering Redis too
         # much we'll wait for a bit.
         sleep(sleep_sec)
-        break if (retries -= 1) < 0
+        (retries -= 1) < 0 ? break : retried ||= true
       end
 
       raise FailedToObtainLockError, 'Failed to obtain a lock' unless uuid
 
-      yield
+      yield(retried)
     ensure
       Gitlab::ExclusiveLease.cancel(key, uuid)
     end

spec/lib/gitlab/exclusive_lease_helpers_spec.rb

@@ -25,13 +25,13 @@ describe Gitlab::ExclusiveLeaseHelpers, :clean_gitlab_redis_shared_state do
       end
 
       it 'calls the given block' do
-        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_control.once
+        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_with_args(false)
       end
 
       it 'calls the given block continuously' do
-        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_control.once
-        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_control.once
-        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_control.once
+        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_with_args(false)
+        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_with_args(false)
+        expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_with_args(false)
       end
 
       it 'cancels the exclusive lease after the block' do
@@ -68,6 +68,15 @@ describe Gitlab::ExclusiveLeaseHelpers, :clean_gitlab_redis_shared_state do
 
           expect { subject }.to raise_error('Failed to obtain a lock')
         end
+
+        context 'when lease is granted after retry' do
+          it 'yields block with true' do
+            expect(lease).to receive(:try_obtain).exactly(3).times { nil }
+            expect(lease).to receive(:try_obtain).once { unique_key }
+
+            expect { |b| class_instance.in_lock(unique_key, &b) }.to yield_with_args(true)
+          end
+        end
       end
 
       context 'when sleep second is specified' do

spec/requests/api/merge_requests_spec.rb

@@ -1571,7 +1571,7 @@ describe API::MergeRequests do
     end
   end
 
-  describe "GET /projects/:id/merge_requests/:merge_request_iid/merge_ref" do
+  describe "GET /projects/:id/merge_requests/:merge_request_iid/merge_ref", :clean_gitlab_redis_shared_state do
     before do
       merge_request.mark_as_unchecked!
     end

spec/services/merge_requests/mergeability_check_service_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe MergeRequests::MergeabilityCheckService do
+describe MergeRequests::MergeabilityCheckService, :clean_gitlab_redis_shared_state do
   shared_examples_for 'unmergeable merge request' do
     it 'updates or keeps merge status as cannot_be_merged' do
       subject
@@ -60,24 +60,69 @@ describe MergeRequests::MergeabilityCheckService do
 
     subject { described_class.new(merge_request).execute }
 
+    def execute_within_threads(amount:, retry_lease: true)
+      threads = []
+
+      amount.times do
+        # Let's use a different object for each thread to get closer
+        # to a real world scenario.
+        mr = MergeRequest.find(merge_request.id)
+
+        threads << Thread.new do
+          described_class.new(mr).execute(retry_lease: retry_lease)
+        end
+      end
+
+      threads.each(&:join)
+      threads
+    end
+
     before do
       project.add_developer(merge_request.author)
     end
 
     it_behaves_like 'mergeable merge request'
 
-    context 'when multiple calls to the service' do
-      it 'returns success' do
-        subject
-        result = subject
+    context 'when lock is disabled' do
+      before do
+        stub_feature_flags(merge_ref_auto_sync_lock: false)
+      end
 
-        expect(result).to be_a(ServiceResponse)
-        expect(result.success?).to be(true)
+      it_behaves_like 'mergeable merge request'
+    end
+
+    context 'when concurrent calls' do
+      it 'waits first lock and returns "cached" result in subsequent calls' do
+        threads = execute_within_threads(amount: 3)
+        results = threads.map { |t| t.value.status }
+
+        expect(results).to contain_exactly(:success, :success, :success)
+      end
+
+      it 'writes the merge-ref once' do
+        service = instance_double(MergeRequests::MergeToRefService)
+
+        expect(MergeRequests::MergeToRefService).to receive(:new).once { service }
+        expect(service).to receive(:execute).once.and_return(success: true)
+
+        execute_within_threads(amount: 3)
       end
 
-      it 'second call does not change the merge-ref' do
-        expect { subject }.to change(merge_request, :merge_ref_head).from(nil)
-        expect { subject }.not_to change(merge_request.merge_ref_head, :id)
+      it 'resets one merge request upon execution' do
+        expect_any_instance_of(MergeRequest).to receive(:reset).once
+
+        execute_within_threads(amount: 2)
+      end
+
+      context 'when retry_lease flag is false' do
+        it 'the first call succeeds, subsequent concurrent calls get a lock error response' do
+          threads = execute_within_threads(amount: 3, retry_lease: false)
+          results = threads.map { |t| [t.value.status, t.value.message] }
+
+          expect(results).to contain_exactly([:error, 'Failed to obtain a lock'],
+                                             [:error, 'Failed to obtain a lock'],
+                                             [:success, nil])
+        end
       end
     end
 
@@ -102,8 +147,7 @@ describe MergeRequests::MergeabilityCheckService do
 
     context 'when broken' do
       before do
-        allow(merge_request).to receive(:broken?) { true }
-        allow(project.repository).to receive(:can_be_merged?) { false }
+        expect(merge_request).to receive(:broken?) { true }
       end
 
       it_behaves_like 'unmergeable merge request'
@@ -117,10 +161,13 @@ describe MergeRequests::MergeabilityCheckService do
       end
     end
 
-    context 'when it has conflicts' do
-      before do
-        allow(merge_request).to receive(:broken?) { false }
-        allow(project.repository).to receive(:can_be_merged?) { false }
+    context 'when it cannot be merged on git' do
+      let(:merge_request) do
+        create(:merge_request,
+               merge_status: :unchecked,
+               source_branch: 'conflict-resolvable',
+               source_project: project,
+               target_branch: 'conflict-start')
       end
 
       it_behaves_like 'unmergeable merge request'