- make 'update' set a field value if it exists but do nothing if not

- make 'replace' do what it should, which is replace a field (or create
  it if it doesn't exist).

lib/logstash/filters/mutate.rb

@@ -45,6 +45,18 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
   #     }
   config :replace, :validate => :hash
 
+  # Update an existing field with a new value. If the field does not exist,
+  # then no action will be taken.
+  #
+  # Example:
+  # 
+  #     filter {
+  #       mutate {
+  #         update => [ "sample", "My new message" ]
+  #       }
+  #     }
+  config :update, :validate => :hash
+
   # Convert a field's value to a different type, like turning a string to an
   # integer. If the field value is an array, all members will be converted.
   # If the field is a hash, no action will be taken.
@@ -220,12 +232,18 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
   end # def rename
 
   private
-  def replace(event)
-    # TODO(sissel): use event.sprintf on the field names?
-    @replace.each do |field, newvalue|
+  def update(event)
+    @update.each do |field, newvalue|
       next unless event.include?(field)
       event[field] = event.sprintf(newvalue)
     end
+  end # def update
+
+  private
+  def replace(event)
+    @replace.each do |field, newvalue|
+      event[field] = event.sprintf(newvalue)
+    end
   end # def replace
 
   def convert(event)

spec/filters/mutate.rb

@@ -13,6 +13,8 @@ describe LogStash::Filters::Mutate do
           convert => [ "intme", "integer", "floatme", "float" ]
           rename => [ "rename1", "rename2" ]
           replace => [ "replaceme", "hello world" ]
+          replace => [ "newfield", "newnew" ]
+          update => [ "nosuchfield", "weee" ]
           remove => [ "removeme" ]
         }
       }
@@ -37,6 +39,10 @@ describe LogStash::Filters::Mutate do
       reject { subject }.include?("rename1")
       insist { subject["rename2"] } == [ "hello world" ]
       reject { subject }.include?("removeme")
+
+      insist { subject }.include?("newfield")
+      insist { subject["newfield"] } == "newnew"
+      reject { subject }.include?("nosuchfield")
     end
   end