document mysql_server_id fact and add spec tests

add spec tests, expand documentation to mention lo behaviour
also, rename fact, for, consistency.

README.md

@@ -768,6 +768,14 @@ The name of the MySQL plugin to manage.
 
 The library file name.
 
+###Facts
+
+####`mysql_server_id`
+
+Generates a unique id, based on the node's MAC address, which can be used as
+`server_id`. This fact will *always* return `0` on all nodes which only have
+loopback interfaces. Given those nodes' connnectivity that's probably okay.
+
 ##Limitations
 
 This module has been tested on:

spec/unit/facter/mysql_server_id_spec.rb

@@ -0,0 +1,27 @@
+require "spec_helper"
+
+describe Facter::Util::Fact do
+  before {
+    Facter.clear
+  }
+
+  describe "mysql_server_id" do
+    context "igalic's laptop" do
+      before :each do
+        Facter.fact(:macadddress).stubs(:value).returns('3c:97:0e:69:fb:e1')
+      end
+      it do
+        Facter.fact(:mysql_server_id).value.to_s.should == '72898961'
+      end
+    end
+
+    context "node with lo only" do
+      before :each do
+        Facter.fact(:macadddress).stubs(:value).returns('00:00:00:00:00:00')
+      end
+      it do
+        Facter.fact(:mysql_server_id).value.should == '0'
+      end
+    end
+  end
+end