Avoid coercing parameter with multiple types to an empty Array

If an optional parameter has multiple types (e.g. [File, String]) and a
nil parameter is passed, Grape would previously coerce the value to an
empty Array. This can break certain APIs that treat nil values
differently from an empty Array.

To fix this, we refactor the nil handling into a `coerce_nil` method and
remove the special case check for handling an Array of types.

Closes #2018

Author: stanhu

CHANGELOG.md

@@ -9,6 +9,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#2019](https://github.com/ruby-grape/grape/pull/2018): Avoid coercing parameter with multiple types to an empty Array - [@stanhu](https://github.com/stanhu).
 
 ### 1.3.1 (2020/03/11)
 

lib/grape/validations/validators/coerce.rb

@@ -67,11 +67,7 @@ def valid_type?(val)
       def coerce_value(val)
         # define default values for structures, the dry-types lib which is used
         # for coercion doesn't accept nil as a value, so it would fail
-        if val.nil?
-          return []      if type == Array || type.is_a?(Array)
-          return Set.new if type == Set
-          return {}      if type == Hash
-        end
+        return coerce_nil(val) if val.nil?
 
         converter.call(val)
 
@@ -80,6 +76,13 @@ def coerce_value(val)
         Types::InvalidValue.new
       end
 
+      def coerce_nil(val)
+        return []      if type == Array
+        return Set.new if type == Set
+        return {}      if type == Hash
+        return val
+      end
+
       # Type to which the parameter will be coerced.
       #
       # @return [Class]

spec/grape/validations/validators/coerce_spec.rb

@@ -180,6 +180,23 @@ def self.parsed?(value)
         expect(last_response.body).to eq(integer_class_name)
       end
 
+      it 'String' do
+        subject.params do
+          requires :string, coerce: String
+        end
+        subject.get '/string' do
+          params[:string].class
+        end
+
+        get '/string', string: 45
+        expect(last_response.status).to eq(200)
+        expect(last_response.body).to eq('String')
+
+        get '/string', string: nil
+        expect(last_response.status).to eq(200)
+        expect(last_response.body).to eq('NilClass')
+      end
+
       it 'is a custom type' do
         subject.params do
           requires :uri, coerce: SecureURIOnly
@@ -647,6 +664,19 @@ def self.parsed?(value)
         expect(last_response.body).to eq('String')
       end
 
+      it 'respects nil values' do
+        subject.params do
+          optional :a, types: [File, String]
+        end
+        subject.get '/' do
+          params[:a].class.to_s
+        end
+
+        get '/', a: nil
+        expect(last_response.status).to eq(200)
+        expect(last_response.body).to eq('NilClass')
+      end
+
       it 'fails when no coercion is possible' do
         subject.params do
           requires :a, types: [Boolean, Integer]