Try to dup non-frozen default params with each use (#1438)

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 #### Fixes
 
+* [#1438](/~https://github.com/ruby-grape/grape/pull/1439): Try to dup non-frozen default params with each use - [@jlfaber](/~https://github.com/jlfaber).
 * [#1430](/~https://github.com/ruby-grape/grape/pull/1430): Fix for `declared(params)` inside `route_param` - [@Arkanain](/~https://github.com/Arkanain).
 * [#1405](/~https://github.com/ruby-grape/grape/pull/1405): Fix priority of `rescue_from` clauses applying - [@hedgesky](/~https://github.com/hedgesky).
 * [#1365](/~https://github.com/ruby-grape/grape/pull/1365): Fix finding exception handler in error middleware - [@ktimothy](/~https://github.com/ktimothy).

lib/grape/validations/validators/default.rb

@@ -6,8 +6,30 @@ def initialize(attrs, options, required, scope)
         super
       end
 
+      # return true if we know we cannot dup this object
+      private def cannot_dup(obj)
+        obj.nil? ||
+        obj == true ||
+        obj == false ||
+        obj.is_a?(Symbol) ||
+        obj.is_a?(Numeric)
+      end
+
+      private def dup_if_possible(obj)
+        obj.dup
+      rescue TypeError
+        obj
+      end
+
       def validate_param!(attr_name, params)
-        params[attr_name] = @default.is_a?(Proc) ? @default.call : @default unless params.key?(attr_name)
+        return if params.key? attr_name
+        params[attr_name] = if @default.is_a? Proc
+                              @default.call
+                            elsif @default.frozen? || cannot_dup(@default)
+                              @default
+                            else
+                              dup_if_possible(@default)
+                            end
       end
 
       def validate!(params)

spec/grape/api/parameters_modification_spec.rb

@@ -0,0 +1,41 @@
+require 'spec_helper'
+
+describe Grape::Endpoint do
+  subject { Class.new(Grape::API) }
+
+  def app
+    subject
+  end
+
+  before do
+    subject.namespace :test do
+      params do
+        optional :foo, default: '-abcdef'
+      end
+      get do
+        params[:foo].slice!(0)
+        params[:foo]
+      end
+    end
+  end
+
+  context 'when route modifies param value' do
+    it 'param default should not change' do
+      get '/test'
+      expect(last_response.status).to eq 200
+      expect(last_response.body).to eq 'abcdef'
+
+      get '/test'
+      expect(last_response.status).to eq 200
+      expect(last_response.body).to eq 'abcdef'
+
+      get '/test?foo=-123456'
+      expect(last_response.status).to eq 200
+      expect(last_response.body).to eq '123456'
+
+      get '/test'
+      expect(last_response.status).to eq 200
+      expect(last_response.body).to eq 'abcdef'
+    end
+  end
+end