Rewind IID on Ci::Pipelines

If no pipeline is created we currently have IID gap.
This is due to fact that we generate IID not on save,
but rather ahead of time. This results, us,
losing IIDs.

app/models/concerns/atomic_internal_id.rb

@@ -53,6 +53,20 @@ module AtomicInternalId
 
         value
       end
+
+      define_method("reset_#{scope}_#{column}") do
+        if value = read_attribute(column)
+          scope_value = association(scope).reader
+          scope_attrs = { scope_value.class.table_name.singularize.to_sym => scope_value }
+          usage = self.class.table_name.to_sym
+
+          if InternalId.reset(self, scope_attrs, usage, value)
+            write_attribute(column, nil)
+          end
+        end
+
+        read_attribute(column)
+      end
     end
   end
 end

app/models/internal_id.rb

@@ -55,7 +55,8 @@ class InternalId < ApplicationRecord
     def track_greatest(subject, scope, usage, new_value, init)
       return new_value unless available?
 
-      InternalIdGenerator.new(subject, scope, usage, init).track_greatest(new_value)
+      InternalIdGenerator.new(subject, scope, usage)
+        .track_greatest(init, new_value)
     end
 
     def generate_next(subject, scope, usage, init)
@@ -63,7 +64,15 @@ class InternalId < ApplicationRecord
       # This can be the case in other (unrelated) migration specs
       return (init.call(subject) || 0) + 1 unless available?
 
-      InternalIdGenerator.new(subject, scope, usage, init).generate
+      InternalIdGenerator.new(subject, scope, usage)
+        .generate(init)
+    end
+
+    def reset(subject, scope, usage, value)
+      return false unless available?
+
+      InternalIdGenerator.new(subject, scope, usage)
+        .reset(value)
     end
 
     # Flushing records is generally safe in a sense that those
@@ -103,14 +112,11 @@ class InternalId < ApplicationRecord
     # subject: The instance we're generating an internal id for. Gets passed to init if called.
     # scope: Attributes that define the scope for id generation.
     # usage: Symbol to define the usage of the internal id, see InternalId.usages
-    # init: Block that gets called to initialize InternalId record if not present
-    #       Make sure to not throw exceptions in the absence of records (if this is expected).
-    attr_reader :subject, :scope, :init, :scope_attrs, :usage
+    attr_reader :subject, :scope, :scope_attrs, :usage
 
-    def initialize(subject, scope, usage, init)
+    def initialize(subject, scope, usage)
       @subject = subject
       @scope = scope
-      @init = init
       @usage = usage
 
       raise ArgumentError, 'Scope is not well-defined, need at least one column for scope (given: 0)' if scope.empty?
@@ -121,23 +127,40 @@ class InternalId < ApplicationRecord
     end
 
     # Generates next internal id and returns it
-    def generate
+    # init: Block that gets called to initialize InternalId record if not present
+    #       Make sure to not throw exceptions in the absence of records (if this is expected).
+    def generate(init)
       subject.transaction do
         # Create a record in internal_ids if one does not yet exist
         # and increment its last value
         #
         # Note this will acquire a ROW SHARE lock on the InternalId record
-        (lookup || create_record).increment_and_save!
+        (lookup || create_record(init)).increment_and_save!
+      end
     end
+
+    # Reset tries to rewind to `value-1`. This will only succeed,
+    # if `value` stored in database is equal to `last_value`.
+    # value: The expected last_value to decrement
+    def reset(value)
+      return false unless value
+
+      updated =
+        InternalId
+          .where(**scope, usage: usage_value)
+          .where(last_value: value)
+          .update_all('last_value = last_value - 1')
+
+      updated > 0
     end
 
     # Create a record in internal_ids if one does not yet exist
     # and set its new_value if it is higher than the current last_value
     #
     # Note this will acquire a ROW SHARE lock on the InternalId record
-    def track_greatest(new_value)
+    def track_greatest(init, new_value)
       subject.transaction do
-        (lookup || create_record).track_greatest_and_save!(new_value)
+        (lookup || create_record(init)).track_greatest_and_save!(new_value)
       end
     end
 
@@ -158,7 +181,7 @@ class InternalId < ApplicationRecord
     # was faster in doing this, we'll realize once we hit the unique key constraint
     # violation. We can safely roll-back the nested transaction and perform
     # a lookup instead to retrieve the record.
-    def create_record
+    def create_record(init)
       subject.transaction(requires_new: true) do
         InternalId.create!(
           **scope,

app/services/ci/create_pipeline_service.rb

@@ -55,6 +55,10 @@ module Ci
         end
       end
 
+      # If pipeline is not persisted, try to recover IID
+      pipeline.reset_project_iid unless pipeline.persisted? ||
+          Feature.disabled?(:ci_pipeline_rewind_iid, project, default_enabled: true)
+
       pipeline
     end
 

changelogs/unreleased/rewind-iid-on-pipelines.yml

+---
+title: Rewind IID on Ci::Pipelines
+merge_request: 26490
+author:
+type: fixed

spec/models/internal_id_spec.rb

@@ -107,6 +107,57 @@ describe InternalId do
     end
   end
 
+  describe '.reset' do
+    subject { described_class.reset(issue, scope, usage, value) }
+
+    context 'in the absence of a record' do
+      let(:value) { 2 }
+
+      it 'does not revert back the value' do
+        expect { subject }.not_to change { described_class.count }
+        expect(subject).to be_falsey
+      end
+    end
+
+    context 'when valid iid is used to reset' do
+      let!(:value) { generate_next }
+
+      context 'and iid is a latest one' do
+        it 'does rewind and next generated value is the same' do
+          expect(subject).to be_truthy
+          expect(generate_next).to eq(value)
+        end
+      end
+
+      context 'and iid is not a latest one' do
+        it 'does not rewind' do
+          generate_next
+
+          expect(subject).to be_falsey
+          expect(generate_next).to be > value
+        end
+      end
+
+      def generate_next
+        described_class.generate_next(issue, scope, usage, init)
+      end
+    end
+
+    context 'with an insufficient schema version' do
+      let(:value) { 2 }
+
+      before do
+        described_class.reset_column_information
+        # Project factory will also call the current_version
+        expect(ActiveRecord::Migrator).to receive(:current_version).twice.and_return(InternalId::REQUIRED_SCHEMA_VERSION - 1)
+      end
+
+      it 'does not reset any of the iids' do
+        expect(subject).to be_falsey
+      end
+    end
+  end
+
   describe '.track_greatest' do
     let(:value) { 9001 }
     subject { described_class.track_greatest(issue, scope, usage, value, init) }

spec/services/ci/create_pipeline_service_spec.rb

@@ -25,7 +25,8 @@ describe Ci::CreatePipelineService do
       merge_request: nil,
       push_options: nil,
       source_sha: nil,
-      target_sha: nil)
+      target_sha: nil,
+      save_on_errors: true)
       params = { ref: ref,
                  before: '00000000',
                  after: after,
@@ -36,7 +37,7 @@ describe Ci::CreatePipelineService do
                  target_sha: target_sha }
 
       described_class.new(project, user, params).execute(
-        source, trigger_request: trigger_request, merge_request: merge_request)
+        source, save_on_errors: save_on_errors, trigger_request: trigger_request, merge_request: merge_request)
     end
     # rubocop:enable Metrics/ParameterLists
 
@@ -57,6 +58,7 @@ describe Ci::CreatePipelineService do
         expect(pipeline).to eq(project.ci_pipelines.last)
         expect(pipeline).to have_attributes(user: user)
         expect(pipeline).to have_attributes(status: 'pending')
+        expect(pipeline.iid).not_to be_nil
         expect(pipeline.repository_source?).to be true
         expect(pipeline.builds.first).to be_kind_of(Ci::Build)
       end
@@ -446,6 +448,43 @@ describe Ci::CreatePipelineService do
         expect(Ci::Build.all).to be_empty
         expect(Ci::Pipeline.count).to eq(0)
       end
+
+      describe '#iid' do
+        let(:internal_id) do
+          InternalId.find_by(project_id: project.id, usage: :ci_pipelines)
+        end
+
+        before do
+          expect_any_instance_of(Ci::Pipeline).to receive(:ensure_project_iid!)
+            .and_call_original
+        end
+
+        context 'when ci_pipeline_rewind_iid is enabled' do
+          before do
+            stub_feature_flags(ci_pipeline_rewind_iid: true)
+          end
+
+          it 'rewinds iid' do
+            result = execute_service
+
+            expect(result).not_to be_persisted
+            expect(internal_id.last_value).to eq(0)
+          end
+        end
+
+        context 'when ci_pipeline_rewind_iid is disabled' do
+          before do
+            stub_feature_flags(ci_pipeline_rewind_iid: false)
+          end
+
+          it 'does not rewind iid' do
+            result = execute_service
+
+            expect(result).not_to be_persisted
+            expect(internal_id.last_value).to eq(1)
+          end
+        end
+      end
     end
 
     context 'with manual actions' do

spec/support/shared_examples/models/atomic_internal_id_spec.rb

@@ -52,20 +52,20 @@ shared_examples_for 'AtomicInternalId' do |validate_presence: true|
 
         expect(InternalId).to receive(:generate_next).with(instance, scope_attrs, usage, any_args).and_return(iid)
         subject
-        expect(instance.public_send(internal_id_attribute)).to eq(iid)
+        expect(read_internal_id).to eq(iid)
       end
 
       it 'does not overwrite an existing internal id' do
-        instance.public_send("#{internal_id_attribute}=", 4711)
+        write_internal_id(4711)
 
-        expect { subject }.not_to change { instance.public_send(internal_id_attribute) }
+        expect { subject }.not_to change { read_internal_id }
       end
 
       context 'when the instance has an internal ID set' do
         let(:internal_id) { 9001 }
 
         it 'calls InternalId.update_last_value and sets the `last_value` to that of the instance' do
-          instance.send("#{internal_id_attribute}=", internal_id)
+          write_internal_id(internal_id)
 
           expect(InternalId)
             .to receive(:track_greatest)
@@ -75,5 +75,39 @@ shared_examples_for 'AtomicInternalId' do |validate_presence: true|
         end
       end
     end
+
+    describe "#reset_scope_internal_id_attribute" do
+      it 'rewinds the allocated IID' do
+        expect { ensure_scope_attribute! }.not_to raise_error
+        expect(read_internal_id).not_to be_nil
+
+        expect(reset_scope_attribute).to be_nil
+        expect(read_internal_id).to be_nil
+      end
+
+      it 'allocates the same IID' do
+        internal_id = ensure_scope_attribute!
+        reset_scope_attribute
+        expect(read_internal_id).to be_nil
+
+        expect(ensure_scope_attribute!).to eq(internal_id)
+      end
+    end
+
+    def ensure_scope_attribute!
+      instance.public_send(:"ensure_#{scope}_#{internal_id_attribute}!")
+    end
+
+    def reset_scope_attribute
+      instance.public_send(:"reset_#{scope}_#{internal_id_attribute}")
+    end
+
+    def read_internal_id
+      instance.public_send(internal_id_attribute)
+    end
+
+    def write_internal_id(value)
+      instance.public_send(:"#{internal_id_attribute}=", value)
+    end
   end
 end