Avoid double validation of payloads

The call to validate a schema is quite expensive:

```
[3] pry(#<GovukSchemas::RandomExample>)> puts Benchmark.measure { 100.times { JSON::Validator.fully_validate(@Schema, item, errors_as_objects: true) } }
  7.614167   1.532485   9.146652 ( 10.468765)
```

This updates the code to avoid making two calls to the validator when a
payload is generated with a block if the customised version is valid.

Before:

```
irb(main):010:0> puts Benchmark.measure { 100.times { GovukSchemas::RandomExample.for_schema(frontend_schema: "news_artic
le") { |schema| schema["title"] = "Custom"; schema } } }
 13.308641   3.087679  16.396320 ( 18.977005)
```

After:

```
irb(main):002:0> puts Benchmark.measure { 100.times { GovukSchemas::RandomExample.for_schema(frontend_schema: "news_artic
le") { |schema| schema["title"] = "Custom"; schema } } }
  7.654303   1.561620   9.215923 ( 10.604802)
```

The motivation for this was that the GOV.UK Chat test suite makes quite
heavy use of random schemas with customisations for checking supported
schemas. On a MBP with GOV.UK Docker this reduces the time of the
running the test suite from around ~55s to around ~35s.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Improve speed of random schema generation for customised content items
+
 # 5.0.0
 
 * BREAKING: Drop support for Ruby 3.0. The minimum required Ruby version is now 3.1.4.

lib/govuk_schemas/random_example.rb

@@ -79,24 +79,40 @@ def self.for_schema(schema_key_value, &block)
     #   the new payload. The new payload is then validated. (optional)
     # @return [Hash] A content item
     # @raise [GovukSchemas::InvalidContentGenerated]
-    def payload
+    def payload(&block)
       payload = @random_generator.payload
-      # ensure the base payload is valid
+
+      return customise_payload(payload, &block) if block
+
       errors = validation_errors_for(payload)
       raise InvalidContentGenerated, error_message(payload, errors) if errors.any?
 
-      if block_given?
-        payload = yield(payload)
-        # check the payload again after customisation
-        errors = validation_errors_for(payload)
-        raise InvalidContentGenerated, error_message(payload, errors, customised: true) if errors.any?
-      end
-
       payload
     end
 
   private
 
+    def customise_payload(payload)
+      original_payload = payload
+      customised_payload = yield(payload)
+      customised_errors = validation_errors_for(customised_payload)
+
+      if customised_errors.any?
+        # Check if the original payload had errors and report those over
+        # any from customisation. This is not done prior to generating the
+        # customised payload because validation is time expensive and we
+        # want to avoid it if possible.
+        original_errors = validation_errors_for(original_payload)
+        errors = original_errors.any? ? original_errors : customised_errors
+        payload = original_errors.any? ? original_payload : customised_payload
+        message = error_message(payload, errors, customised: original_errors.empty?)
+
+        raise InvalidContentGenerated, message
+      end
+
+      customised_payload
+    end
+
     def validation_errors_for(item)
       JSON::Validator.fully_validate(@schema, item, errors_as_objects: true)
     end

spec/lib/random_example_spec.rb

@@ -32,34 +32,44 @@
       expect(first_payload).to eql(second_payload)
     end
 
-    it "can customise the payload" do
-      schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
-
-      example = GovukSchemas::RandomExample.new(schema:).payload do |hash|
-        hash.merge("base_path" => "/some-base-path")
-      end
+    it "raises an error if the generated schema is invalid" do
+      generator = instance_double(GovukSchemas::RandomSchemaGenerator, payload: {})
+      allow(GovukSchemas::RandomSchemaGenerator).to receive(:new).and_return(generator)
 
-      expect(example["base_path"]).to eql("/some-base-path")
+      schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+      expect { GovukSchemas::RandomExample.new(schema:).payload }
+        .to raise_error(GovukSchemas::InvalidContentGenerated)
     end
 
-    it "failes when attempting to edit the hash in place" do
-      schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+    context "when the payload is customised" do
+      it "returns the customised payload" do
+        schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
 
-      expect {
-        GovukSchemas::RandomExample.new(schema:).payload do |hash|
-          hash["base_path"] = "/some-base-path"
+        example = GovukSchemas::RandomExample.new(schema:).payload do |hash|
+          hash.merge("base_path" => "/some-base-path")
         end
-      }.to raise_error(GovukSchemas::InvalidContentGenerated)
-    end
 
-    it "raises if the resulting content item won't be valid" do
-      schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+        expect(example["base_path"]).to eql("/some-base-path")
+      end
 
-      expect {
-        GovukSchemas::RandomExample.new(schema:).payload do |hash|
-          hash.merge("base_path" => nil)
-        end
-      }.to raise_error(GovukSchemas::InvalidContentGenerated)
+      it "raises if the resulting content item won't be valid" do
+        schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+
+        expect {
+          GovukSchemas::RandomExample.new(schema:).payload do |hash|
+            hash.merge("base_path" => nil)
+          end
+        }.to raise_error(GovukSchemas::InvalidContentGenerated, /The item was valid before being customised/)
+      end
+
+      it "raises if the non-customised content item was invalid" do
+        generator = instance_double(GovukSchemas::RandomSchemaGenerator, payload: {})
+        allow(GovukSchemas::RandomSchemaGenerator).to receive(:new).and_return(generator)
+
+        schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+        expect { GovukSchemas::RandomExample.new(schema:).payload { |h| h.merge("base_path" => "/test") } }
+          .to raise_error(GovukSchemas::InvalidContentGenerated, /An invalid content item was generated/)
+      end
     end
   end
 end