Fix exception overwritten for parameters fetch method

When executing an `ActionController::Parameters#fetch` with a block
that raises a `KeyError` the raised `KeyError` will be rescued and
converted to an `ActionController::ParameterMissing` exception,
covering up the original exception.

[Jonas Schubert Erlandsson & Roque Pinel]

3 files changed, 25 insertions, 3 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 4b081591c0..6a68c057ac 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Fix `ActionController::Parameters#fetch` overwriting `KeyError` returned by
+    default block.
+
+    *Jonas Schuber Erlandsson*, *Roque Pinel*
+
 *   `ActionController::Parameters` no longer inherits from
     `HashWithIndifferentAccess`
 
diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index 06d625e4d5..cf6a64009f 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -380,11 +380,15 @@ module ActionController
     def fetch(key, *args, &block)
       convert_hashes_to_parameters(
         key,
-        @parameters.fetch(key, *args, &block),
+        @parameters.fetch(key) {
+          if block_given?
+            yield
+          else
+            args.fetch(0) { raise ActionController::ParameterMissing.new(key) }
+          end
+        },
         false
       )
-    rescue KeyError
-      raise ActionController::ParameterMissing.new(key)
     end
 
     # Returns a new <tt>ActionController::Parameters</tt> instance that
diff --git a/actionpack/test/controller/parameters/parameters_permit_test.rb b/actionpack/test/controller/parameters/parameters_permit_test.rb
index 05532ec21b..2dd2826196 100644
--- a/actionpack/test/controller/parameters/parameters_permit_test.rb
+++ b/actionpack/test/controller/parameters/parameters_permit_test.rb
@@ -194,6 +194,19 @@ class ParametersPermitTest < ActiveSupport::TestCase
     assert_equal "monkey", @params.fetch(:foo) { "monkey" }
   end
 
+  test "fetch doesnt raise ParameterMissing exception if there is a default that is nil" do
+    assert_equal nil, @params.fetch(:foo, nil)
+    assert_equal nil, @params.fetch(:foo) { nil }
+  end
+
+  test 'KeyError in fetch block should not be coverd up' do
+    params = ActionController::Parameters.new
+    e = assert_raises(KeyError) do
+      params.fetch(:missing_key) { {}.fetch(:also_missing) }
+    end
+    assert_match(/:also_missing$/, e.message)
+  end
+
   test "not permitted is sticky beyond merges" do
     assert !@params.merge(a: "b").permitted?
   end