From bc1e02e53e87ff7a7b199d40d05c0dd709328819 Mon Sep 17 00:00:00 2001
From: logandavies181 <logan.davies181@gmail.com>
Date: Tue, 29 Nov 2022 09:03:09 +0000
Subject: [PATCH] add ValidateHandlerFunc

---
 lambda/handler.go      | 25 ++++++++++++++++++
 lambda/handler_test.go | 59 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/lambda/handler.go b/lambda/handler.go
index 0fc82d6e..02f10cf3 100644
--- a/lambda/handler.go
+++ b/lambda/handler.go
@@ -99,6 +99,31 @@ func WithEnableSIGTERM(callbacks ...func()) Option {
 	})
 }
 
+// ValidateHandlerFunc validates the handler against the criteria for Start and returns an error
+// if the criteria are not met
+func ValidateHandlerFunc(handlerFunc interface{}) error {
+	if handlerFunc == nil {
+		return errors.New("handler is nil")
+	}
+
+	handlerType := reflect.TypeOf(handlerFunc)
+	if handlerType.Kind() != reflect.Func {
+		return fmt.Errorf("handler kind %s is not %s", handlerType.Kind(), reflect.Func)
+	}
+
+	_, err := validateArguments(handlerType)
+	if err != nil {
+		return err
+	}
+
+	err = validateReturns(handlerType)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func validateArguments(handler reflect.Type) (bool, error) {
 	handlerTakesContext := false
 	if handler.NumIn() > 2 {
diff --git a/lambda/handler_test.go b/lambda/handler_test.go
index 3c3c51d4..c8dc80ec 100644
--- a/lambda/handler_test.go
+++ b/lambda/handler_test.go
@@ -75,11 +75,68 @@ func TestInvalidHandlers(t *testing.T) {
 	}
 	for i, testCase := range testCases {
 		testCase := testCase
-		t.Run(fmt.Sprintf("testCase[%d] %s", i, testCase.name), func(t *testing.T) {
+		t.Run(fmt.Sprintf("testCase[%d] %s part 1", i, testCase.name), func(t *testing.T) {
 			lambdaHandler := NewHandler(testCase.handler)
 			_, err := lambdaHandler.Invoke(context.TODO(), make([]byte, 0))
 			assert.Equal(t, testCase.expected, err)
 		})
+
+		t.Run(fmt.Sprintf("testCase[%d] %s part 2", i, testCase.name), func(t *testing.T) {
+			err := ValidateHandlerFunc(testCase.handler)
+			assert.Equal(t, testCase.expected, err)
+		})
+	}
+}
+
+func TestValidateHandlerFuncValidHandlers(t *testing.T) {
+	testCases := []struct {
+		name    string
+		handler interface{}
+	}{
+		{
+			name:    "0 arg 0 return",
+			handler: func() {},
+		},
+		{
+			name:    "0 arg, 1 returns",
+			handler: func() error { return nil },
+		},
+		{
+			name:    "1 arg, 0 returns",
+			handler: func(any) {},
+		},
+		{
+			name:    "1 arg, 1 returns",
+			handler: func(any) error { return nil },
+		},
+		{
+			name:    "0 arg, 2 returns",
+			handler: func() (any, error) { return 1, nil },
+		},
+		{
+			name:    "1 arg, 2 returns",
+			handler: func(any) (any, error) { return 1, nil },
+		},
+		{
+			name:    "2 arg, 0 returns",
+			handler: func(context.Context, any) {},
+		},
+		{
+			name:    "2 arg, 1 returns",
+			handler: func(context.Context, any) error { return nil },
+		},
+		{
+			name:    "2 arg, 2 returns",
+			handler: func(context.Context, any) (any, error) { return 1, nil },
+		},
+	}
+
+	for i, testCase := range testCases {
+		testCase := testCase
+		t.Run(fmt.Sprintf("testCase[%d] %s", i, testCase.name), func(t *testing.T) {
+			err := ValidateHandlerFunc(testCase.handler)
+			assert.Nil(t, err)
+		})
 	}
 }