added multiple config loader

docs/setup.md

@@ -0,0 +1,16 @@
+---
+title: Setup
+---
+
+## Configuration
+
+By default, Sagittarius loads `config/sagittarius.yml`. Set
+`SAGITTARIUS_CONFIG_FILES` to a comma-separated list of configuration file
+paths to load multiple files:
+
+```shell
+SAGITTARIUS_CONFIG_FILES=config/sagittarius.yml,/etc/sagittarius/config.yml,/run/secrets/sagittarius.yml
+```
+
+Files are deep merged from left to right. Values in later files take
+precedence over earlier files and the built-in defaults.

lib/sagittarius/configuration.rb

@@ -4,15 +4,28 @@ module Sagittarius
   class Configuration
     extend Code0::ZeroTrack::Memoize
 
+    CONFIG_FILES_ENV = 'SAGITTARIUS_CONFIG_FILES'
+
     def self.config
       memoize(:config) do
-        file_config = YAML.safe_load_file(Rails.root.join('config/sagittarius.yml')).deep_symbolize_keys
-        defaults.deep_merge(file_config)
-      rescue Errno::ENOENT # config file does not exist
+        configured_files = ENV.fetch(CONFIG_FILES_ENV, nil)
+        config_files(configured_files).reduce(defaults) do |config, config_file|
+          file_config = YAML.safe_load_file(config_file).deep_symbolize_keys
+          config.deep_merge(file_config)
+        end
+      rescue Errno::ENOENT
+        raise if configured_files.present?
+
         defaults
       end
     end
 
+    def self.config_files(configured_files = ENV.fetch(CONFIG_FILES_ENV, nil))
+      return [Rails.root.join('config/sagittarius.yml')] if configured_files.blank?
+
+      configured_files.split(',').map(&:strip).reject(&:empty?)
+    end
+
     def self.application_setting_overrides
       config[:application_setting_overrides]
     end

spec/lib/sagittarius/configuration_spec.rb

@@ -4,32 +4,83 @@
 
 RSpec.describe Sagittarius::Configuration do
   describe '.config' do
-    let(:yaml_config) do
-      <<~CONFIG
-        rails:
-          web:
-            threads: 4
-          grpc:
-            threads: 8
-      CONFIG
-    end
+    let(:default_config_file) { Rails.root.join('config/sagittarius.yml') }
 
     before do
-      allow(File).to receive(:open).and_call_original
-      allow(File).to receive(:open)
-        .with(Rails.root.join('config/sagittarius.yml'), instance_of(String))
-        .and_yield(yaml_config)
+      allow(ENV).to receive(:fetch).and_call_original
+      allow(ENV).to receive(:fetch).with('SAGITTARIUS_CONFIG_FILES', nil).and_return(nil)
       described_class.clear_memoize(:config)
     end
 
-    it 'loads the config' do
+    it 'loads the default config file' do
+      allow(YAML).to receive(:safe_load_file).with(default_config_file).and_return(
+        'rails' => {
+          'web' => { 'threads' => 4 },
+          'grpc' => { 'threads' => 8 },
+        }
+      )
+
       expect(described_class.config).to include(
         rails: a_hash_including(
           web: a_hash_including(threads: 4),
           grpc: a_hash_including(threads: 8)
         )
       )
     end
+
+    it 'uses built-in defaults when the default config file does not exist' do
+      allow(YAML).to receive(:safe_load_file).with(default_config_file).and_raise(Errno::ENOENT)
+
+      expect(described_class.config).to eq(described_class.defaults)
+    end
+
+    context 'when SAGITTARIUS_CONFIG_FILES contains multiple paths' do
+      before do
+        configured_files = ' config/base.yml,config/environment.yml, config/runtime.yml '
+        allow(ENV).to receive(:fetch).with('SAGITTARIUS_CONFIG_FILES', nil)
+                                     .and_return(configured_files)
+        allow(YAML).to receive(:safe_load_file).with('config/base.yml').and_return(
+          'rails' => {
+            'web' => { 'threads' => 4, 'port' => 4000 },
+            'grpc' => { 'threads' => 4 },
+          }
+        )
+        allow(YAML).to receive(:safe_load_file).with('config/environment.yml').and_return(
+          'rails' => {
+            'web' => { 'threads' => 8 },
+            'grpc' => { 'host' => 'environment:50051' },
+          }
+        )
+        allow(YAML).to receive(:safe_load_file).with('config/runtime.yml').and_return(
+          'rails' => {
+            'web' => { 'threads' => 16 },
+          }
+        )
+      end
+
+      it 'deep merges files in order with the last file taking precedence' do
+        expect(described_class.config).to include(
+          rails: a_hash_including(
+            web: a_hash_including(threads: 16, port: 4000),
+            grpc: a_hash_including(threads: 4, host: 'environment:50051')
+          )
+        )
+      end
+
+      it 'loads each configured file in order' do
+        described_class.config
+
+        expect(YAML).to have_received(:safe_load_file).ordered.with('config/base.yml')
+        expect(YAML).to have_received(:safe_load_file).ordered.with('config/environment.yml')
+        expect(YAML).to have_received(:safe_load_file).ordered.with('config/runtime.yml')
+      end
+
+      it 'raises when a configured file does not exist' do
+        allow(YAML).to receive(:safe_load_file).with('config/environment.yml').and_raise(Errno::ENOENT)
+
+        expect { described_class.config }.to raise_error(Errno::ENOENT)
+      end
+    end
   end
 
   describe '.defaults' do