fn: add tester image (#609)

* fn: add fn-test-utils image

New tester image that uses go-fdk for advanced test scenarios.
Right now, this is an enhanced 'hello/sleeper' rolled into one
that echos the received headers/env to allow writing test cases.

images/fn-test-utils/fn-test-utils.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log"
+	"time"
+
+	fdk "github.com/fnproject/fdk-go"
+	"net/http"
+)
+
+type AppRequest struct {
+	// if specified we 'sleep' the specified msecs
+	SleepTime int `json:"sleepTime,omitempty"`
+	// if specified, this is our response http status code
+	ResponseCode int `json:"responseCode,omitempty"`
+	// if specified, this is our response content-type
+	ResponseContentType string `json:"responseContentType,omitempty"`
+	// if specified, this is echoed back to client
+	EchoContent string `json:"echoContent,omitempty"`
+	// verbose mode
+	IsDebug bool `json:"isDebug,omitempty"`
+	// simulate crash
+	IsCrash bool `json:"isCrash,omitempty"`
+	// TODO: simulate slow read/slow write
+	// TODO: simulate partial write/read
+	// TODO: simulate mem leak
+	// TODO: simulate high cpu usage
+	// TODO: simulate high mem usage
+	// TODO: simulate large body upload/download
+}
+
+type AppResponse struct {
+	Request AppRequest        `json:"request"`
+	Headers http.Header       `json:"header"`
+	Config  map[string]string `json:"config"`
+}
+
+func AppHandler(ctx context.Context, in io.Reader, out io.Writer) {
+
+	fnctx := fdk.Context(ctx)
+
+	var request AppRequest
+	json.NewDecoder(in).Decode(&request)
+
+	if request.IsDebug {
+		log.Printf("Received request %v", request)
+		log.Printf("Received headers %v", fnctx.Header)
+		log.Printf("Received config %v", fnctx.Config)
+	}
+
+	// simulate load if requested
+	if request.SleepTime > 0 {
+		if request.IsDebug {
+			log.Printf("Sleeping %d", request.SleepTime)
+		}
+		time.Sleep(time.Duration(request.SleepTime) * time.Millisecond)
+	}
+
+	// simulate crash
+	if request.IsCrash {
+		panic("Crash requested")
+	}
+
+	// custom response code
+	if request.ResponseCode != 0 {
+		fdk.WriteStatus(out, request.ResponseCode)
+	} else {
+		fdk.WriteStatus(out, 200)
+	}
+
+	// custom content type
+	if request.ResponseContentType != "" {
+		fdk.SetHeader(out, "Content-Type", request.ResponseContentType)
+	} else {
+		fdk.SetHeader(out, "Content-Type", "application/json")
+	}
+
+	resp := AppResponse{
+		Request: request,
+		Headers: fnctx.Header,
+		Config:  fnctx.Config,
+	}
+
+	json.NewEncoder(out).Encode(&resp)
+}
+
+func main() {
+	fdk.Handle(fdk.HandlerFunc(AppHandler))
+}