Make invocation URL dynamic

The API endpoint in Amazon's implementation of Lambda used the path
`/2015-03-31/functions/[func]/invocations` to invoke the function.
`[func]` the name of the function and is set as the value of the
`AWS_LAMBDA_FUNCTION_NAME` environment variable when the function
is executing on AWS.

The emulator uses a hard coded endpoint of
`/2015-03-31/functions/function/invocations`. This is even the case
when the `AWS_LAMBDA_FUNCTION_NAME` environment variable is set to
another value.

This patch changes the endpoint URL when the `AWS_LAMBDA_FUNCTION_NAME`
environment variable is set. In this case the invocation URL will
be `/2015-03-31/functions/${AWS_LAMBDA_FUNCTION_NAME}/invocations`.
When the environment variable isn't set, the current behaviour
persists and the function name is set to `function`.

The end result is the emulator behaviour is closer to the real
environment the functions execute in.

This PR fixes #43 I raised a few weeks ago.

cmd/aws-lambda-rie/http.go

@@ -11,15 +11,21 @@ import (
 	"go.amzn.com/lambda/rapidcore"
 )
 
-func startHTTPServer(ipport string, sandbox *rapidcore.SandboxBuilder, bs interop.Bootstrap) {
+func startHTTPServer(ipport string, sandbox *rapidcore.SandboxBuilder, bs interop.Bootstrap, funcName string) {
 	srv := &http.Server{
 		Addr: ipport,
 	}
 
-	// Pass a channel
-	http.HandleFunc("/2015-03-31/functions/function/invocations", func(w http.ResponseWriter, r *http.Request) {
-		InvokeHandler(w, r, sandbox.LambdaInvokeAPI(), bs)
-	})
+	var functions = []string{funcName}
+	if funcName != "function" {
+		functions = []string{"function", funcName}
+	}
+	for _, funcName := range functions {
+		// Pass a channel
+		http.HandleFunc("/2015-03-31/functions/"+funcName+"/invocations", func(w http.ResponseWriter, r *http.Request) {
+			InvokeHandler(w, r, sandbox.LambdaInvokeAPI(), bs)
+		})
+	}
 
 	// go routine (main thread waits)
 	if err := srv.ListenAndServe(); err != nil {

cmd/aws-lambda-rie/main.go

@@ -78,13 +78,15 @@ func main() {
 		sandbox.SetRuntimeAPIAddress(opts.RuntimeAPIAddress)
 	}
 
+	funcName := GetenvWithDefault("AWS_LAMBDA_FUNCTION_NAME", "function")
+
 	sandboxContext, internalStateFn := sandbox.Create()
 	// Since we have not specified a custom interop server for standalone, we can
 	// directly reference the default interop server, which is a concrete type
 	sandbox.DefaultInteropServer().SetSandboxContext(sandboxContext)
 	sandbox.DefaultInteropServer().SetInternalStateGetter(internalStateFn)
 
-	startHTTPServer(opts.RuntimeInterfaceEmulatorAddress, sandbox, bootstrap)
+	startHTTPServer(opts.RuntimeInterfaceEmulatorAddress, sandbox, bootstrap, funcName)
 }
 
 func getCLIArgs() (options, []string) {