Simplify Ci::BuildTraceChunk.persistable_store

Before this change, Ci::BuildTraceChunk.persistable_store was
implemented by scanning an array with 3 values: [false, true,
BuildTraceChunks::Fog.new.available?]. What could be a single if-else
("if Fog is available, then X, else, Y") was presented as a loop.

This commit replaces the loop with a single if-else.

app/models/ci/build_trace_chunk.rb

@@ -25,8 +25,6 @@ module Ci
 
     FailedToPersistDataError = Class.new(StandardError)
 
-    # Note: The ordering of this hash is related to the precedence of persist store.
-    # The bottom item takes the highest precedence, and the top item takes the lowest precedence.
     DATA_STORES = {
       redis: 1,
       database: 2,
@@ -48,8 +46,7 @@ module Ci
       end
 
       def persistable_store
-        # get first available store from the back of the list
-        all_stores.reverse.find { |store| get_store_class(store).available? }
+        STORE_TYPES[:fog].available? ? :fog : :database
       end
 
       def get_store_class(store)

app/models/ci/build_trace_chunks/database.rb

@@ -3,10 +3,6 @@
 module Ci
   module BuildTraceChunks
     class Database
-      def available?
-        true
-      end
-
       def keys(relation)
         []
       end

app/models/ci/build_trace_chunks/fog.rb

@@ -3,10 +3,18 @@
 module Ci
   module BuildTraceChunks
     class Fog
-      def available?
+      def self.available?
         object_store.enabled
       end
 
+      def self.object_store
+        Gitlab.config.artifacts.object_store
+      end
+
+      def available?
+        self.class.available?
+      end
+
       def data(model)
         files.get(key(model))&.body
       rescue Excon::Error::NotFound
@@ -85,7 +93,7 @@ module Ci
       end
 
       def object_store
-        Gitlab.config.artifacts.object_store
+        self.class.object_store
       end
 
       def object_store_raw_config

app/models/ci/build_trace_chunks/redis.rb

@@ -31,10 +31,6 @@ module Ci
         end
       EOS
 
-      def available?
-        true
-      end
-
       def data(model)
         Gitlab::Redis::SharedState.with do |redis|
           redis.get(key(model))

spec/models/ci/build_trace_chunks/database_spec.rb

@@ -5,12 +5,6 @@ require 'spec_helper'
 RSpec.describe Ci::BuildTraceChunks::Database do
   let(:data_store) { described_class.new }
 
-  describe '#available?' do
-    subject { data_store.available? }
-
-    it { is_expected.to be_truthy }
-  end
-
   describe '#data' do
     subject { data_store.data(model) }
 

spec/models/ci/build_trace_chunks/redis_spec.rb

@@ -5,12 +5,6 @@ require 'spec_helper'
 RSpec.describe Ci::BuildTraceChunks::Redis, :clean_gitlab_redis_shared_state do
   let(:data_store) { described_class.new }
 
-  describe '#available?' do
-    subject { data_store.available? }
-
-    it { is_expected.to be_truthy }
-  end
-
   describe '#data' do
     subject { data_store.data(model) }