DEBUG-2334 duplicate mutable values when serializing for dynamic inst… (

#4009)

* DEBUG-2334 duplicate mutable values when serializing for dynamic instrumentation

When serializing parameter values at method entry, the resulting
snapshot should not change if the parameter is mutated during
method execution. This currently only applies to string values as
other values used as serializer output are not mutable.

This commit duplicates the string values which achieves immutability
with no additional API complexity but at the cost of duplicating
all strings, even if they are not entry parameters and thus can be
stored without duplication.

* only duplicate strings if they are not frozen and not truncated

* replace case with if

---------

Co-authored-by: Oleg Pudeyev <[email protected]>

lib/datadog/di/serializer.rb

@@ -26,6 +26,16 @@ module DI
     # All serialization methods take the names of the variables being
     # serialized in order to be able to redact values.
     #
+    # The result of serialization should not reference parameter values when
+    # the values are mutable (currently, this only applies to string values).
+    # Serializer will duplicate such mutable values, so that if method
+    # arguments are captured at entry and then modified during method execution,
+    # the serialized values from entry are correctly preserved.
+    # Alternatively, we could pass a parameter to the serialization methods
+    # which would control whether values are duplicated. This may be more
+    # efficient but there would be additional overhead from passing this
+    # parameter all the time and the API would get more complex.
+    #
     # @api private
     class Serializer
       def initialize(settings, redactor)
@@ -92,12 +102,24 @@ def serialize_value(value, name: nil, depth: settings.dynamic_instrumentation.ma
         when Integer, Float, TrueClass, FalseClass
           serialized.update(value: value.to_s)
         when String, Symbol
-          value = value.to_s
+          need_dup = false
+          value = if String === value
+            # This is the only place where we duplicate the value, currently.
+            # All other values are immutable primitives (e.g. numbers).
+            # However, do not duplicate if the string is frozen, or if
+            # it is later truncated.
+            need_dup = !value.frozen?
+            value
+          else
+            value.to_s
+          end
           max = settings.dynamic_instrumentation.max_capture_string_length
           if value.length > max
             serialized.update(truncated: true, size: value.length)
             value = value[0...max]
+            need_dup = false
           end
+          value = value.dup if need_dup
           serialized.update(value: value)
         when Array
           if depth < 0

spec/datadog/di/serializer_spec.rb

@@ -320,5 +320,79 @@ def self.define_cases(cases)
         end
       end
     end
+
+    context 'when positional arg is mutated' do
+      let(:args) do
+        ['hello', 'world']
+      end
+
+      let(:kwargs) { {} }
+
+      it 'preserves original value' do
+        serialized
+
+        args.first.gsub!('hello', 'bye')
+
+        expect(serialized).to eq(
+          arg1: {type: 'String', value: 'hello'},
+          arg2: {type: 'String', value: 'world'},
+        )
+      end
+    end
+
+    context 'when keyword arg is mutated' do
+      let(:args) do
+        []
+      end
+
+      let(:kwargs) do
+        {foo: 'bar'}
+      end
+
+      it 'preserves original value' do
+        serialized
+
+        kwargs[:foo].gsub!('bar', 'bye')
+
+        expect(serialized).to eq(
+          foo: {type: 'String', value: 'bar'},
+        )
+      end
+    end
+
+    context 'when positional arg is frozen' do
+      let(:frozen_string) { 'hello'.freeze }
+
+      let(:args) do
+        [frozen_string, 'world']
+      end
+
+      let(:kwargs) { {} }
+
+      it 'serializes without duplication' do
+        expect(serialized).to eq(
+          arg1: {type: 'String', value: 'hello'},
+          arg2: {type: 'String', value: 'world'},
+        )
+
+        expect(serialized[:arg1][:value]).to be frozen_string
+      end
+    end
+
+    context 'when keyword arg is frozen' do
+      let(:frozen_string) { 'hello'.freeze }
+
+      let(:args) { [] }
+
+      let(:kwargs) { {foo: frozen_string} }
+
+      it 'serializes without duplication' do
+        expect(serialized).to eq(
+          foo: {type: 'String', value: 'hello'},
+        )
+
+        expect(serialized[:foo][:value]).to be frozen_string
+      end
+    end
   end
 end