Move config load logic to its own method

Add a private method (that has to be publicly accessible within the gem
itself), to load the config. This avoids duplication in the diagnose CLI
when I update it to call this method as well.

Make it more safe by replicating the existing behavior: it will never
return a `nil` object for the Config once started. If it started with an
error or the config file is empty, initialize the config but set
`active` to `false`.

lib/appsignal.rb

@@ -92,7 +92,7 @@ def testing?
     #
     # @return [void]
     # @since 0.7.0
-    def start # rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity, Metrics/MethodLength, Metrics/CyclomaticComplexity
+    def start # rubocop:disable Metrics/AbcSize
       if ENV.fetch("_APPSIGNAL_DIAGNOSE", false)
         internal_logger.info("Skipping start in diagnose context")
         return
@@ -117,35 +117,10 @@ def start # rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity, Metric
 
       internal_logger.debug("Loading AppSignal gem")
 
-      context = Appsignal::Config::Context.new(
-        :env => Config.determine_env,
-        :root_path => Config.determine_root_path
-      )
-      # If there's a config/appsignal.rb file
-      if context.dsl_config_file?
-        if config
-          # Do not load it if a config is already present
-          internal_logger.warn(
-            "Ignoring `#{context.dsl_config_file}` file because " \
-              "`Appsignal.configure` has already been called."
-          )
-        else
-          # Load it when no config is present
-          load_dsl_config_file(context.dsl_config_file)
-          # Disable AppSignal on config file error
-          config[:active] = false if config && config_file_error?
-        end
-      else
-        # Load config if no config file was found and no config is present yet
-        # This will load the config/appsignal.yml file automatically
-        @config ||= Config.new(context.root_path, context.env)
-      end
-      # Validate the config, if present
-      config&.validate
-
+      _load_config!
       _start_logger
 
-      if config&.valid?
+      if config.valid?
         if config.active?
           @started = true
           internal_logger.info "Starting AppSignal #{Appsignal::VERSION} " \
@@ -172,6 +147,37 @@ def start # rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity, Metric
       end
     end
 
+    # PRIVATE METHOD. DO NOT USE.
+    #
+    # @param env_var [String, NilClass] Used by diagnose CLI to pass through
+    #   the environment CLI option value.
+    # @api private
+    def _load_config!(env_param = nil)
+      context = Appsignal::Config::Context.new(
+        :env => Config.determine_env(env_param),
+        :root_path => Config.determine_root_path
+      )
+      # If there's a config/appsignal.rb file
+      if context.dsl_config_file?
+        if config
+          # Do not load it if a config is already present
+          internal_logger.warn(
+            "Ignoring `#{context.dsl_config_file}` file because " \
+              "`Appsignal.configure` has already been called."
+          )
+        else
+          # Load it when no config is present
+          load_dsl_config_file(context.dsl_config_file)
+        end
+      else
+        # Load config if no config file was found and no config is present yet
+        # This will load the config/appsignal.yml file automatically
+        @config ||= Config.new(context.root_path, context.env)
+      end
+      # Validate the config, if present
+      config&.validate
+    end
+
     # Stop AppSignal's agent.
     #
     # Stops the AppSignal agent. Call this before the end of your program to
@@ -456,19 +462,34 @@ def params_match_loaded_config?(env_param, root_path_param)
     def load_dsl_config_file(path)
       return if defined?(@dsl_config_file_loaded)
 
-      ENV["_APPSIGNAL_CONFIG_FILE_CONTEXT"] = "true"
-      @dsl_config_file_loaded = true
-      require path
-    rescue => error
-      @config_file_error = error
-      message = "Not starting AppSignal because an error occurred while " \
-        "loading the AppSignal config file.\n" \
-        "File: #{path.inspect}\n" \
-        "#{error.class.name}: #{error}"
-      Kernel.warn "appsignal ERROR: #{message}"
-      internal_logger.error "#{message}\n#{error.backtrace.join("\n")}"
-    ensure
-      ENV.delete("_APPSIGNAL_CONFIG_FILE_CONTEXT")
+      begin
+        ENV["_APPSIGNAL_CONFIG_FILE_CONTEXT"] = "true"
+        @dsl_config_file_loaded = true
+        require path
+      rescue => error
+        @config_file_error = error
+        message = "Not starting AppSignal because an error occurred while " \
+          "loading the AppSignal config file.\n" \
+          "File: #{path.inspect}\n" \
+          "#{error.class.name}: #{error}"
+        Kernel.warn "appsignal ERROR: #{message}"
+        internal_logger.error "#{message}\n#{error.backtrace.join("\n")}"
+      ensure
+        unless Appsignal.config
+          # Ensure _a config object_ is present, even if something went wrong
+          # loading it or the file is empty. In this config file context, see
+          # the context env vars, it will intentionally not load the YAML file.
+          Appsignal.configure
+
+          # Disable if no config was loaded from the file but it is present
+          config[:active] = false
+        end
+
+        # Disable on config file error
+        config[:active] = false if defined?(@config_file_error)
+
+        ENV.delete("_APPSIGNAL_CONFIG_FILE_CONTEXT")
+      end
     end
 
     # Returns true if we're currently in the `config/appsignal.rb` file
@@ -477,12 +498,6 @@ def config_file_context?
       ENV.fetch("_APPSIGNAL_CONFIG_FILE_CONTEXT", nil) == "true"
     end
 
-    # Returns true if loading the `config/appsignal.rb` file resulted in an
-    # error.
-    def config_file_error?
-      defined?(@config_file_error) ? true : false
-    end
-
     def start_internal_stdout_logger
       @internal_logger = Appsignal::Utils::IntegrationLogger.new($stdout)
       internal_logger.formatter = log_formatter("appsignal")

spec/lib/appsignal_spec.rb

@@ -491,7 +491,9 @@ def on_load
         expect(Appsignal.dsl_config_file_loaded?).to be(true)
         # No Appsignal.configure was called, so it's misconfigured, but it
         # shouldn't fall back on the YAML file.
-        expect(Appsignal.config).to be_nil
+        expect(Appsignal.config[:active]).to be(false)
+        expect(Appsignal.config[:name]).to be_nil
+        expect(Appsignal.config[:ignore_errors]).to be_empty
       ensure
         FileUtils.rm_rf(test_path)
       end