Mock context.Context to test lambdacontext.FromContext - unit-testing

Question

I'm building an aws lambda using aws-sdk-go and aws-lambda-go and I'm stuck with a little problem.
I want to test my lambda handler. To do so, I need to mock a valid context.Context containing valid attributes for lamdacontext.LambdaContext and satisfy lambdacontext.FromContext.
I cannot seem to find a way to build such mock, since lambdacontext.FromContext always returns me _, false.
Here's my main, with a simple handler on a events.SNSEvent event:
package main
import (
"context"
"github.com/aws/aws-lambda-go/events"
"github.com/aws/aws-lambda-go/lambda"
"github.com/aws/aws-lambda-go/lambdacontext"
)
func main() {
lambda.Start(handleRequest)
}
func handleRequest(ctx context.Context, snsEvent events.SNSEvent) error {
lc, ok := lambdacontext.FromContext(ctx); if !ok {
// Always false
...
return someErr
}
. . .
return nil
}
And here's my test for handleRequest:
package main
import (
"context"
"github.com/aws/aws-lambda-go/events"
"github.com/aws/aws-lambda-go/lambdacontext"
"github.com/stretchr/testify/assert"
"gitlab.easy-network.it/meg/aml-rekognition/testdata"
"testing"
)
const imgMock = `
{
\"some_parameter\": \"some_value\"
}`
func TestHandleRequest(t *testing.T) {
c := context.Background()
ctxV := context.WithValue(c, "", map[string]interface{}{
"AwsRequestID" : "some_aws_id",
"InvokedFunctionArn" : "some_arn",
"Identity" : lambdacontext.CognitoIdentity{},
"ClientContext" : lambdacontext.ClientContext{},
})
snsEventMock := events.SNSEvent{
Records: []events.SNSEventRecord{
{
SNS: events.SNSEntity{
Message: imgMock,
},
},
},
}
err := handleRequest(ctxV, snsEventMock)
assert.NoError(t, err)
}
I also tried other mocks like passing it a struct with these parameters etc, but I always get false. For instance, I tried also:
type TestMock struct {
AwsRequestID string
InvokedFunctionArn string
Identity lambdacontext.CognitoIdentity
ClientContext lambdacontext.ClientContext
}
func TestHandleRequest(t *testing.T) {
c := context.Background()
testMock := TestMock{
AwsRequestID : "some_aws_id",
InvokedFunctionArn : "some_arn",
Identity : lambdacontext.CognitoIdentity{},
ClientContext : lambdacontext.ClientContext{},
}
ctxV := context.WithValue(c, "", testMock)
. . .
}
I checked out the source of FromContext and I've been scratching my head for a while.
// LambdaContext is the set of metadata that is passed for every Invoke.
type LambdaContext struct {
AwsRequestID string
InvokedFunctionArn string
Identity CognitoIdentity
ClientContext ClientContext
}
// An unexported type to be used as the key for types in this package.
// This prevents collisions with keys defined in other packages.
type key struct{}
// The key for a LambdaContext in Contexts.
// Users of this package must use lambdacontext.NewContext and
lambdacontext.FromContext
// instead of using this key directly.
var contextKey = &key{}
// FromContext returns the LambdaContext value stored in ctx, if any.
func FromContext(ctx context.Context) (*LambdaContext, bool) {
lc, ok := ctx.Value(contextKey).(*LambdaContext)
return lc, ok
}
Of course, it returns false even if I just pass a context.Background() to it.
Any idea on how should I build a valid context.Context to let lambdacontext.FromContext return true?

Answer 1

lambda.FromContext() checks if the passed context.Context contains a value with a "private" key hold inside the lambdacontext package:
// An unexported type to be used as the key for types in this package.
// This prevents collisions with keys defined in other packages.
type key struct{}
// The key for a LambdaContext in Contexts.
// Users of this package must use lambdacontext.NewContext and lambdacontext.FromContext
// instead of using this key directly.
var contextKey = &key{}
// FromContext returns the LambdaContext value stored in ctx, if any.
func FromContext(ctx context.Context) (*LambdaContext, bool) {
lc, ok := ctx.Value(contextKey).(*LambdaContext)
return lc, ok
}
You cannot access this key, and you can't "reproduce" it (you can't create a value that will be equal to this "private" key).
But there's an easy way, simply use the lambdacontext.NewContext() to derive a context which will have this key:
// NewContext returns a new Context that carries value lc.
func NewContext(parent context.Context, lc *LambdaContext) context.Context {
return context.WithValue(parent, contextKey, lc)
}
So the solution:
ctx := context.Background()
// Add keys to your liking, then:
lc := new(lambdacontext.LambdaContext)
ctx = lambdacontext.NewContext(ctx, lc)