Add project level git depth setting

Introduce default_git_depth in project's CI/CD settings and set it to
50. Use it if there is no GIT_DEPTH variable specified. Apply this
default only to newly created projects and keep it nil for old ones
in order to not break pipelines that rely on non-shallow clones.

default_git_depth can be updated from CI/CD Settings in the UI, must be
either nil or integer between 0 and 1000 (incl).

Inherit default_git_depth from the origin project when forking projects.

MR pipelines are run on a MR ref (refs/merge-requests/:iid/merge) and it
contains unique commit (i.e. merge commit) which doesn't exist in the
other branch/tags refs. We need to add it cause otherwise it may break
pipelines for old projects that have already enabled Pipelines for merge
results and have git depth 0.

Document new default_git_depth project CI/CD setting

6 files changed, 55 insertions, 16 deletions

diff --git a/app/controllers/projects/settings/ci_cd_controller.rb b/app/controllers/projects/settings/ci_cd_controller.rb
index c4dff95a4b9..1b8d479209b 100644
--- a/app/controllers/projects/settings/ci_cd_controller.rb
+++ b/app/controllers/projects/settings/ci_cd_controller.rb
@@ -50,7 +50,8 @@ module Projects
           :runners_token, :builds_enabled, :build_allow_git_fetch,
           :build_timeout_human_readable, :build_coverage_regex, :public_builds,
           :auto_cancel_pending_pipelines, :ci_config_path,
-          auto_devops_attributes: [:id, :domain, :enabled, :deploy_strategy]
+          auto_devops_attributes: [:id, :domain, :enabled, :deploy_strategy],
+          ci_cd_settings_attributes: [:default_git_depth]
         )
       end
 
diff --git a/app/models/project.rb b/app/models/project.rb
index 78d54571d94..e64a4b313aa 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -292,6 +292,7 @@ class Project < ApplicationRecord
   accepts_nested_attributes_for :project_feature, update_only: true
   accepts_nested_attributes_for :import_data
   accepts_nested_attributes_for :auto_devops, update_only: true
+  accepts_nested_attributes_for :ci_cd_settings, update_only: true
 
   accepts_nested_attributes_for :remote_mirrors,
                                 allow_destroy: true,
@@ -310,6 +311,7 @@ class Project < ApplicationRecord
   delegate :root_ancestor, to: :namespace, allow_nil: true
   delegate :last_pipeline, to: :commit, allow_nil: true
   delegate :external_dashboard_url, to: :metrics_setting, allow_nil: true, prefix: true
+  delegate :default_git_depth, :default_git_depth=, to: :ci_cd_settings
 
   # Validations
   validates :creator, presence: true, on: :create
diff --git a/app/models/project_ci_cd_setting.rb b/app/models/project_ci_cd_setting.rb
index 1414164b703..cf1f80fd1ef 100644
--- a/app/models/project_ci_cd_setting.rb
+++ b/app/models/project_ci_cd_setting.rb
@@ -6,6 +6,18 @@ class ProjectCiCdSetting < ApplicationRecord
   # The version of the schema that first introduced this model/table.
   MINIMUM_SCHEMA_VERSION = 20180403035759
 
+  DEFAULT_GIT_DEPTH = 50
+
+  before_create :set_default_git_depth, unless: :default_git_depth?
+
+  validates :default_git_depth,
+    numericality: {
+      only_integer: true,
+      greater_than_or_equal_to: 0,
+      less_than_or_equal_to: 1000
+    },
+    allow_nil: true
+
   def self.available?
     @available ||=
       ActiveRecord::Migrator.current_version >= MINIMUM_SCHEMA_VERSION
@@ -15,4 +27,10 @@ class ProjectCiCdSetting < ApplicationRecord
     @available = nil
     super
   end
+
+  private
+
+  def set_default_git_depth
+    self.default_git_depth = DEFAULT_GIT_DEPTH
+  end
 end
diff --git a/app/presenters/ci/build_runner_presenter.rb b/app/presenters/ci/build_runner_presenter.rb
index 6d46e0bf18a..471b6d3b726 100644
--- a/app/presenters/ci/build_runner_presenter.rb
+++ b/app/presenters/ci/build_runner_presenter.rb
@@ -25,14 +25,16 @@ module Ci
     end
 
     def git_depth
-      strong_memoize(:git_depth) do
-        git_depth = variables&.find { |variable| variable[:key] == 'GIT_DEPTH' }&.dig(:value)
-        git_depth.to_i
-      end
+      if git_depth_variable
+        git_depth_variable[:value]
+      else
+        project.default_git_depth
+      end.to_i
     end
 
     def refspecs
       specs = []
+      specs << refspec_for_merge_request_ref if merge_request_ref?
 
       if git_depth > 0
         specs << refspec_for_branch(ref) if branch? || legacy_detached_merge_request_pipeline?
@@ -42,8 +44,6 @@ module Ci
         specs << refspec_for_tag
       end
 
-      specs << refspec_for_merge_request_ref if merge_request_ref?
-
       specs
     end
 
@@ -89,5 +89,11 @@ module Ci
     def refspec_for_merge_request_ref
       "+#{ref}:#{ref}"
     end
+
+    def git_depth_variable
+      strong_memoize(:git_depth_variable) do
+        variables&.find { |variable| variable[:key] == 'GIT_DEPTH' }
+      end
+    end
   end
 end
diff --git a/app/services/projects/fork_service.rb b/app/services/projects/fork_service.rb
index fc234bafc57..14c1c98ea73 100644
--- a/app/services/projects/fork_service.rb
+++ b/app/services/projects/fork_service.rb
@@ -36,18 +36,19 @@ module Projects
 
     def fork_new_project
       new_params = {
-        visibility_level:       allowed_visibility_level,
-        description:            @project.description,
-        name:                   target_name,
-        path:                   target_path,
-        shared_runners_enabled: @project.shared_runners_enabled,
-        namespace_id:           target_namespace.id,
-        fork_network:           fork_network,
+        visibility_level:          allowed_visibility_level,
+        description:               @project.description,
+        name:                      target_name,
+        path:                      target_path,
+        shared_runners_enabled:    @project.shared_runners_enabled,
+        namespace_id:              target_namespace.id,
+        fork_network:              fork_network,
+        ci_cd_settings_attributes: { default_git_depth: @project.default_git_depth },
         # We need to assign the fork network membership after the project has
         # been instantiated to avoid ActiveRecord trying to create it when
         # initializing the project, as that would cause a foreign key constraint
         # exception.
-        relations_block:        -> (project) { build_fork_network_member(project) }
+        relations_block:           -> (project) { build_fork_network_member(project) }
       }
 
       if @project.avatar.present? && @project.avatar.image?
@@ -56,7 +57,10 @@ module Projects
 
       new_params.merge!(@project.object_pool_params)
 
-      new_project = CreateService.new(current_user, new_params).execute
+      new_project = CreateService.new(current_user, new_params).execute do |p|
+        p.build_ci_cd_settings(default_git_depth: @project.default_git_depth)
+      end
+
       return new_project unless new_project.persisted?
 
       # Set the forked_from_project relation after saving to avoid having to
diff --git a/app/views/projects/settings/ci_cd/_form.html.haml b/app/views/projects/settings/ci_cd/_form.html.haml
index b38b8e3f686..2d108a1cba5 100644
--- a/app/views/projects/settings/ci_cd/_form.html.haml
+++ b/app/views/projects/settings/ci_cd/_form.html.haml
@@ -26,6 +26,14 @@
 
         %hr
         .form-group
+          = f.fields_for :ci_cd_settings_attributes, @project.ci_cd_settings do |form|
+            = form.label :default_git_depth, _('Git shallow clone'), class: 'label-bold'
+            = form.number_field :default_git_depth, { class: 'form-control', min: 0, max: 1000 }
+            %p.form-text.text-muted
+              = _('The number of changes to be fetched from GitLab when cloning a repository. This can speed up Pipelines execution. Keep empty or set to 0 to disable shallow clone by default and make GitLab CI fetch all branches and tags each time.')
+
+        %hr
+        .form-group
           = f.label :build_timeout_human_readable, _('Timeout'), class: 'label-bold'
           = f.text_field :build_timeout_human_readable, class: 'form-control'
           %p.form-text.text-muted