Add support for setting a logger

Adds support for setting `Stripe.logger` to a logger that's compatible
with `Logger` from Ruby's standard library. In set, the library will no
longer log to stdout, and instead emit straight to the logger and defer
decision on what log level to print to it.

Addresses a request in #566.

lib/stripe.rb

@@ -84,6 +84,7 @@ module Stripe
   @uploads_base = 'https://uploads.stripe.com'
 
   @log_level = nil
+  @logger = nil
 
   @max_network_retries = 0
   @max_network_retry_delay = 2
@@ -151,6 +152,9 @@ module Stripe
   # what it's doing. For example, it'll produce information about requests,
   # responses, and errors that are received. Valid log levels are `debug` and
   # `info`, with `debug` being a little more verbose in places.
+  #
+  # Use of this configuration is only useful when `.logger` is _not_ set. When
+  # it is, the decision what levels to print is entirely deferred to the logger.
   def self.log_level
     @log_level
   end
@@ -162,6 +166,21 @@ module Stripe
     @log_level = val
   end
 
+  # Sets a logger to which logging output will be sent. The logger should
+  # support the same interface as the `Logger` class that's part of Ruby's
+  # standard library (hint, anything in `Rails.logger` will likely be
+  # suitable).
+  #
+  # If `.logger` is set, the value of `.log_level` is ignored. The decision on
+  # what levels to print is entirely deferred to the logger.
+  def self.logger
+    @logger
+  end
+
+  def self.logger=(val)
+    @logger = val
+  end
+
   def self.max_network_retries
     @max_network_retries
   end

lib/stripe/util.rb

@@ -90,16 +90,19 @@ module Stripe
     end
 
     def self.log_info(message, data = {})
-      if Stripe.log_level == Stripe::LEVEL_DEBUG ||Stripe.log_level == Stripe::LEVEL_INFO
+      if !Stripe.logger.nil? ||
+          Stripe.log_level == Stripe::LEVEL_DEBUG ||
+          Stripe.log_level == Stripe::LEVEL_INFO
         log_internal(message, data, color: :cyan,
-          level: Stripe::LEVEL_INFO, out: $stdout)
+          level: Stripe::LEVEL_INFO, logger: Stripe.logger, out: $stdout)
       end
     end
 
     def self.log_debug(message, data = {})
-      if Stripe.log_level == Stripe::LEVEL_DEBUG
+      if !Stripe.logger.nil? ||
+          Stripe.log_level == Stripe::LEVEL_DEBUG
         log_internal(message, data, color: :blue,
-          level: Stripe::LEVEL_DEBUG, out: $stdout)
+          level: Stripe::LEVEL_DEBUG, logger: Stripe.logger, out: $stdout)
       end
     end
 
@@ -340,16 +343,24 @@ module Stripe
 
     # TODO: Make these named required arguments when we drop support for Ruby
     # 2.0.
-    def self.log_internal(message, data = {}, color: nil, level: nil, out: nil)
+    def self.log_internal(message, data = {}, color: nil, level: nil, logger: nil, out: nil)
       data_str = data.select { |k,v| !v.nil? }.
         map { |(k,v)|
           "%s=%s" % [
-            colorize(k, color, out.isatty),
+            colorize(k, color, !out.nil? && out.isatty),
             wrap_logfmt_value(v)
           ]
         }.join(" ")
 
-      if out.isatty
+      if !logger.nil?
+        str = "message=%s %s" % [wrap_logfmt_value(message), data_str]
+        case level
+        when Stripe::LEVEL_DEBUG
+          logger.debug(str)
+        else # Stripe::LEVEL_INFO (there should be no other values)
+          logger.info(str)
+        end
+      elsif out.isatty
         out.puts "%s %s %s" %
           [colorize(level[0, 4].upcase, color, out.isatty), message, data_str]
       else

test/stripe/util_test.rb

@@ -1,3 +1,4 @@
+require "logger"
 require File.expand_path('../../test_helper', __FILE__)
 
 module Stripe
@@ -216,6 +217,35 @@ module Stripe
       end
     end
 
+    context ".log_* with a logger" do
+      setup do
+        @out = StringIO.new
+        logger = ::Logger.new(@out)
+
+        # Set a really simple formatter to make matching output as easy as
+        # possible.
+        logger.formatter = proc { |_severity, _datetime, _progname, message|
+          message
+        }
+
+        Stripe.logger = logger
+      end
+
+      context ".log_debug" do
+        should "log to the logger" do
+          Util.log_debug("foo")
+          assert_equal "message=foo ", @out.string
+        end
+      end
+
+      context ".log_info" do
+        should "log to the logger" do
+          Util.log_info("foo")
+          assert_equal "message=foo ", @out.string
+        end
+      end
+    end
+
     context ".normalize_headers" do
       should "normalize the format of a header key" do
         assert_equal({ "Request-Id" => nil },
@@ -267,7 +297,7 @@ module Stripe
         end
 
         Util.send(:log_internal, "message", { foo: "bar" },
-          color: :green, level: Stripe::LEVEL_DEBUG, out: out)
+          color: :green, level: Stripe::LEVEL_DEBUG, logger: nil, out: out)
         assert_equal "\e[0;32;49mDEBU\e[0m message \e[0;32;49mfoo\e[0m=bar\n",
           out.string
       end
@@ -275,10 +305,26 @@ module Stripe
       should "log in a data friendly way" do
         out = StringIO.new
         Util.send(:log_internal, "message", { foo: "bar" },
-          color: :green, level: Stripe::LEVEL_DEBUG, out: out)
+          color: :green, level: Stripe::LEVEL_DEBUG, logger: nil, out: out)
         assert_equal "message=message level=debug foo=bar\n",
           out.string
       end
+
+      should "log to a logger if set" do
+        out = StringIO.new
+        logger = ::Logger.new(out)
+
+        # Set a really simple formatter to make matching output as easy as
+        # possible.
+        logger.formatter = proc { |_severity, _datetime, _progname, message|
+          message
+        }
+
+        Util.send(:log_internal, "message", { foo: "bar" },
+          color: :green, level: Stripe::LEVEL_DEBUG, logger: logger, out: nil)
+        assert_equal "message=message foo=bar",
+          out.string
+      end
     end
 
     context ".wrap_logfmt_value" do