Evaluate dynamic fields in event when used with gsub

Closes #1529

lib/logstash/filters/mutate.rb

@@ -191,9 +191,10 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
         @logger.error("Invalid gsub configuration. gsub has to define 3 elements per config entry", :field => field, :needle => needle, :replacement => replacement)
         raise "Bad configuration, aborting."
       end
+
       @gsub_parsed << {
         :field        => field,
-        :needle       => Regexp.new(needle),
+        :needle       => (needle.index("%{").nil?? Regexp.new(needle): needle),
         :replacement  => replacement
       }
     end
@@ -303,7 +304,7 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
                           "skipping", :field => field, :value => v)
             v
           else
-            v.gsub(needle, replacement)
+            gsub_dynamic_fields(event, v, needle, replacement)
           end
         end
       else
@@ -312,11 +313,21 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
                         "skipping", :field => field, :value => event[field])
           next
         end
-        event[field] = event[field].gsub(needle, replacement)
+        event[field] = gsub_dynamic_fields(event, event[field], needle, replacement)
       end
     end # @gsub_parsed.each
   end # def gsub
 
+  private
+  def gsub_dynamic_fields(event, original, needle, replacement)
+    if needle.is_a? Regexp
+      original.gsub(needle, event.sprintf(replacement))
+    else
+      # we need to replace any dynamic fields
+      original.gsub(Regexp.new(event.sprintf(needle)), event.sprintf(replacement))
+    end
+  end
+
   private
   def uppercase(event)
     @uppercase.each do |field|

spec/filters/mutate.rb

@@ -178,5 +178,53 @@ describe LogStash::Filters::Mutate do
       insist { subject["[foo][bar]"] }.is_a?(Fixnum)
     end
   end
+
+  #LOGSTASH-1529
+  describe "gsub on a String with dynamic fields (%{}) in pattern" do
+    config '
+      filter {
+        mutate {
+          gsub => [ "unicorns", "of type %{unicorn_type}", "green" ]
+        }
+      }'
+
+    sample("unicorns" => "Unicorns of type blue are common", "unicorn_type" => "blue") do
+      insist { subject["unicorns"] } == "Unicorns green are common"
+    end
+  end
+
+  #LOGSTASH-1529
+  describe "gsub on a String with dynamic fields (%{}) in pattern and replace" do
+    config '
+      filter {
+        mutate {
+          gsub => [ "unicorns2", "of type %{unicorn_color}", "%{unicorn_color} and green" ]
+        }
+      }'
+
+    sample("unicorns2" => "Unicorns of type blue are common", "unicorn_color" => "blue") do
+      insist { subject["unicorns2"] } == "Unicorns blue and green are common"
+    end
+  end
+
+  #LOGSTASH-1529
+  describe "gsub on a String array with dynamic fields in pattern" do
+    config '
+      filter {
+        mutate {
+          gsub => [ "unicorns_array", "of type %{color}", "blue and green" ]
+        }
+      }'
+
+    sample("unicorns_array" => [
+        "Unicorns of type blue are found in Alaska", "Unicorns of type blue are extinct" ],
+           "color" => "blue"
+    ) do
+      insist { subject["unicorns_array"] } == [
+          "Unicorns blue and green are found in Alaska",
+          "Unicorns blue and green are extinct"
+      ]
+    end
+  end
 end