Add support for a system default credentials file.

The path is:
  - windows: %ProgramData%\Google\Auth\application_default_credentials.json
  - other: /etc/google/auth/application_default_credentials.json

spec/googleauth/get_application_default_spec.rb

@@ -32,6 +32,7 @@ $LOAD_PATH.unshift(spec_dir)
 $LOAD_PATH.uniq!
 
 require 'faraday'
+require 'fakefs/safe'
 require 'googleauth'
 require 'spec_helper'
 
@@ -140,6 +141,19 @@ describe '#get_application_default' do
       stubs.verify_stubbed_calls
     end
 
+    it 'succeeds with system default file' do
+      ENV.delete(@var_name) unless ENV[@var_name].nil?
+      FakeFS do
+        Dir.mktmpdir do |dir|
+          key_path = File.join('/etc/google/auth/', CREDENTIALS_FILE_NAME)
+          FileUtils.mkdir_p(File.dirname(key_path))
+          File.write(key_path, cred_json_text)
+          expect(Google::Auth.get_application_default(@scope)).to_not be_nil
+          File.delete(key_path)
+        end
+      end
+    end
+
     it 'succeeds if environment vars are valid' do
       ENV.delete(@var_name) unless ENV[@var_name].nil? # no env var
       ENV[PRIVATE_KEY_VAR] = cred_json[:private_key]

spec/googleauth/service_account_spec.rb

@@ -32,6 +32,7 @@ $LOAD_PATH.unshift(spec_dir)
 $LOAD_PATH.uniq!
 
 require 'apply_auth_examples'
+require 'fakefs/safe'
 require 'fileutils'
 require 'googleauth/service_account'
 require 'jwt'
@@ -231,6 +232,32 @@ describe Google::Auth::ServiceAccountCredentials do
       end
     end
   end
+
+  describe '#from_system_default_path' do
+    before(:example) do
+      @scope = 'https://www.googleapis.com/auth/userinfo.profile'
+      @path = File.join('/etc/google/auth/', CREDENTIALS_FILE_NAME)
+      @clz = ServiceAccountCredentials
+    end
+
+    it 'is nil if no file exists' do
+      FakeFS do
+        expect(ServiceAccountCredentials.from_system_default_path(@scope)).
+          to be_nil
+      end
+    end
+
+    it 'successfully loads the file when it is present' do
+      FakeFS do
+        Dir.mktmpdir do |dir|
+          FileUtils.mkdir_p(File.dirname(@path))
+          File.write(@path, cred_json_text)
+          expect(@clz.from_system_default_path(@scope)).to_not be_nil
+          File.delete(@path)
+        end
+      end
+    end
+  end
 end
 
 describe Google::Auth::ServiceAccountJwtHeaderCredentials do

spec/googleauth/user_refresh_spec.rb

@@ -32,6 +32,7 @@ $LOAD_PATH.unshift(spec_dir)
 $LOAD_PATH.uniq!
 
 require 'apply_auth_examples'
+require 'fakefs/safe'
 require 'fileutils'
 require 'googleauth/user_refresh'
 require 'jwt'
@@ -185,4 +186,44 @@ describe Google::Auth::UserRefreshCredentials do
       end
     end
   end
+
+  describe '#from_system_default_path' do
+    before(:example) do
+      @scope = 'https://www.googleapis.com/auth/userinfo.profile'
+      @path = File.join('/etc/google/auth/', CREDENTIALS_FILE_NAME)
+      @clz = UserRefreshCredentials
+    end
+
+    it 'is nil if no file exists' do
+      FakeFS do
+        expect(UserRefreshCredentials.from_system_default_path(@scope)).
+          to be_nil
+      end
+    end
+
+    it 'fails if the file is invalid' do
+      needed = %w(client_id client_secret refresh_token)
+      needed.each do |missing|
+        FakeFS do
+          Dir.mktmpdir do |dir|
+            FileUtils.mkdir_p(File.dirname(@path))
+            File.write(@path, cred_json_text(missing))
+            expect { @clz.from_system_default_path(@scope) }.to raise_error
+            File.delete(@path)
+          end
+        end
+      end
+    end
+
+    it 'successfully loads the file when it is present' do
+      FakeFS do
+        Dir.mktmpdir do |dir|
+          FileUtils.mkdir_p(File.dirname(@path))
+          File.write(@path, cred_json_text)
+          expect(@clz.from_system_default_path(@scope)).to_not be_nil
+          File.delete(@path)
+        end
+      end
+    end
+  end
 end