Custom context for function handlers result in panic

[d-tsuji]

Is your feature request related to a problem? Please describe.

In the AWS documentation, the handler for the function is shown to have the following signature

• func ()
• func () error
• func (TIn) error
• func () (TOut, error)
• func (TIn) (TOut, error)
• func (context.Context) error
• func (context.Context, TIn) error
• func (context.Context) (TOut, error)
• func (context.Context, TIn) (TOut, error)

https://docs.aws.amazon.com/lambda/latest/dg/golang-handler.html

But it also says the following.

The handler may take between 0 and 2 arguments. If there are two arguments, the first argument must implement context.Context.

---

The first argument in the signature of the function that takes two arguments is clearly context.Context, but the panic occurs when customContext is taken as an argument.

For example, deploy and execute the following code to AWS Lambda.

package main

import (
	"context"
	"log"

	"github.com/aws/aws-lambda-go/lambda"
)

func main() {
	lambda.Start(Handle)
}

func Handle(ctx customContext) {
	log.Println("hello")
	return
}

// customContext satisfies "context.Context"
type customContext struct {
	context.Context
}

When this Lambda is executed, a panic occurs as shown below.

reflect: Call using *context.valueCtx as type main.customContext: string
[
	{
		"path": "github.com/aws/aws-lambda-go@v1.24.0/lambda/errors.go",
		"line": 39,
		"label": "lambdaPanicResponse"
	},
	{
		"path": "github.com/aws/aws-lambda-go@v1.24.0/lambda/function.go",
		"line": 36,
		"label": "(*Function).Invoke.func1"
	},
	{
		"path": "runtime/panic.go",
		"line": 965,
		"label": "gopanic"
	},
	{
		"path": "reflect/value.go",
		"line": 406,
		"label": "Value.call"
	},
	{
		"path": "reflect/value.go",
		"line": 337,
		"label": "Value.Call"
	},
	{
		"path": "github.com/aws/aws-lambda-go@v1.24.0/lambda/handler.go",
		"line": 124,
		"label": "NewHandler.func1"
	},
	{
		"path": "github.com/aws/aws-lambda-go@v1.24.0/lambda/handler.go",
		"line": 24,
		"label": "lambdaHandler.Invoke"
	},
	{
		"path": "github.com/aws/aws-lambda-go@v1.24.0/lambda/function.go",
		"line": 64,
		"label": "(*Function).Invoke"
	},
	{
		"path": "reflect/value.go",
		"line": 476,
		"label": "Value.call"
	},
	{
		"path": "reflect/value.go",
		"line": 337,
		"label": "Value.Call"
	},
	{
		"path": "net/rpc/server.go",
		"line": 377,
		"label": "(*service).call"
	},
	{
		"path": "runtime/asm_amd64.s",
		"line": 1371,
		"label": "goexit"
	}
]

Describe the solution you'd like

If we take context.Context as an argument, we need to make sure that the first argument of the function signature is context.Context explicitly.

aws-lambda-go/lambda/handler.go

Lines 48 to 50 in 5d64132

	contextType := reflect.TypeOf((*context.Context)(nil)).Elem()
	argumentType := handler.In(0)
	handlerTakesContext = argumentType.Implements(contextType)

The above implementation only checks if the argument satisfies context.Context, so if a custom context that satisfies context.Context is the first argument, it cannot be checked by the above validation.

Describe alternatives you've considered

I wasn't sure if it was correct to explicitly accept only context.Context as a signature, or if we will accept a custom context satisfying context.Context as a function signature.

Additional context

Nothing.

[shogo82148]

The document and the implementation of the validator are wrong.

The document says:

https://docs.aws.amazon.com/lambda/latest/dg/golang-handler.html#golang-handler-structs
The handler may take between 0 and 2 arguments. If there are two arguments, the first argument must implement context.Context.

it should be:

The handler may take between 0 and 2 arguments. If there are two arguments, the first argument must be an interface and a subset of context.Context.

#365 will fix this.

[d-tsuji]

@shogo82148

Thank you. I got the latest module as follows, and I can confirm that the panic doesn't happen in the custom context.

• go.mod

module sample

go 1.16

require github.com/aws/aws-lambda-go v1.25.0

replace github.com/aws/aws-lambda-go => github.com/shogo82148/aws-lambda-go v1.19.2-0.20210522080913-8845e52c0275