Add panic handler tests, remove redundant "graphql:" err prefix

* Add panic handler tests
* Fix rebase bug
* Document new PanicHandler option in README

Author: JohnStarich

README.md

@@ -109,6 +109,7 @@ func (r *helloWorldResolver) Hello(ctx context.Context) (string, error) {
 - `Tracer(tracer trace.Tracer)` is used to trace queries and fields. It defaults to `trace.OpenTracingTracer`.
 - `ValidationTracer(tracer trace.ValidationTracer)` is used to trace validation errors. It defaults to `trace.NoopValidationTracer`.
 - `Logger(logger log.Logger)` is used to log panics during query execution. It defaults to `exec.DefaultLogger`.
+- `PanicHandler(panicHandler errors.PanicHandler)` is used to transform panics into errors during query execution. It defaults to `errors.DefaultPanicHandler`.
 - `DisableIntrospection()` disables introspection queries.
 
 ### Custom Errors

errors/panic_handler.go

@@ -14,5 +14,5 @@ type DefaultPanicHandler struct{}
 
 // MakePanicError creates a new QueryError from a panic that occurred during execution
 func (h *DefaultPanicHandler) MakePanicError(ctx context.Context, value interface{}) *QueryError {
-	return Errorf("graphql: panic occurred: %v", value)
+	return Errorf("panic occurred: %v", value)
 }

errors/panic_handler_test.go

@@ -0,0 +1,24 @@
+package errors
+
+import (
+	"context"
+	"testing"
+)
+
+func TestDefaultPanicHandler(t *testing.T) {
+	handler := &DefaultPanicHandler{}
+	qErr := handler.MakePanicError(context.Background(), "foo")
+	if qErr == nil {
+		t.Fatal("Panic error must not be nil")
+	}
+	const (
+		expectedMessage = "panic occurred: foo"
+		expectedError   = "graphql: " + expectedMessage
+	)
+	if qErr.Error() != expectedError {
+		t.Errorf("Unexpected panic error message: %q != %q", qErr.Error(), expectedError)
+	}
+	if qErr.Message != expectedMessage {
+		t.Errorf("Unexpected panic QueryError.Message: %q != %q", qErr.Message, expectedMessage)
+	}
+}

subscriptions.go

@@ -57,6 +57,7 @@ func (s *Schema) subscribe(ctx context.Context, queryString string, operationNam
 		Limiter:                  make(chan struct{}, s.maxParallelism),
 		Tracer:                   s.tracer,
 		Logger:                   s.logger,
+		PanicHandler:             s.panicHandler,
 		SubscribeResolverTimeout: s.subscribeResolverTimeout,
 	}
 	varTypes := make(map[string]*introspection.Type)