Extract auto retry logic out of build model

Adds a new Gitlab::Ci::Build::AutoRetry class with
the new retry logic. Moves the original build model
specs and makes them pass in the new location.

lib/gitlab/ci/build/auto_retry.rb

+class Gitlab::Ci::Build::AutoRetry
+  include Gitlab::Utils::StrongMemoize
+
+  DEFAULT_RETRIES = {
+    scheduler_failure: 2
+  }.freeze
+
+  def initialize(build)
+    @build = build
+  end
+
+  def allowed?
+    # return false unless build.retryable?
+
+    within_max_retry_limit?
+  end
+  
+  private
+
+  def within_max_retry_limit?
+    max_allowed_retries > 0 && max_allowed_retries > @build.retries_count
+  end
+
+  def max_allowed_retries
+    strong_memoize(:options_retry) do
+      options_retry_max || DEFAULT_RETRIES.fetch(@build.failure_reason.to_sym, 0)
+    end
+  end
+
+  def options_retry_max
+    options_retry[:max] if retry_on_reason_or_always?
+  end
+
+  def options_retry_when
+    options_retry.fetch(:when, ['always'])
+  end
+ 
+
+  def retry_on_reason_or_always?
+    options_retry_when.include?(@build.failure_reason.to_s) ||
+      options_retry_when.include?('always')
+  end
+
+  # The format of the retry option changed in GitLab 11.5: Before it was
+  # integer only, after it is a hash. New builds are created with the new
+  # format, but builds created before GitLab 11.5 and saved in database still
+  # have the old integer only format. This method returns the retry option
+  # normalized as a hash in 11.5+ format.
+  def options_retry
+    strong_memoize(:options_retry) do
+      value = @build.options&.dig(:retry)
+      value = value.is_a?(Integer) ? { max: value } : value.to_h
+      value.with_indifferent_access
+    end
+  end
+end

spec/lib/gitlab/ci/build/auto_retry_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Ci::Build::AutoRetry do
+  describe '#within_max_retry_limit?' do
+    using RSpec::Parameterized::TableSyntax
+
+    let(:build) { create(:ci_build) }
+
+    subject { Gitlab::Ci::Build::AutoRetry.new(build).allowed? }
+
+    where(:description, :retry_count, :options, :failure_reason, :result) do
+      "retries are disabled" | 0 | { max: 0 } | nil | false
+      "max equals count" | 2 | { max: 2 } | nil | false
+      "max is higher than count" | 1 | { max: 2 } | nil | true
+      "matching failure reason" | 0 | { when: %w[api_failure], max: 2 } | :api_failure | true
+      "not matching with always" | 0 | { when: %w[always], max: 2 } | :api_failure | true
+      "not matching reason" | 0 | { when: %w[script_error], max: 2 } | :api_failure | false
+      "scheduler failure override" | 1 | { when: %w[scheduler_failure], max: 1 } | :scheduler_failure | false
+      "default for scheduler failure" | 1 | {} | :scheduler_failure | true
+    end
+
+    with_them do
+      before do
+        allow(build).to receive(:retries_count) { retry_count }
+
+        build.options[:retry] = options
+        build.failure_reason = failure_reason
+      end
+
+      it { is_expected.to eq(result) }
+    end
+  end
+end
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