GAE Go "The request failed because the instance could not start successfully" - google-app-engine

Question

I've been developing with Google App Engine Go for awhile, but I haven't touched it in a year. I tried updating my application from "go1" (1.9?) to "go111", and I'm currently getting some weird errors without any explanation as to what's going on.
The errors include:
The request failed because the instance could not start successfully
Container called exit(1).
500 Internal server error
etc.
None of these point me to any specific line in my code where something would go wrong, nor explain anything more meaningful...
I'm guessing the error is stemming from me upgrading between the golang versions. I had to change the app package into main, add a main function to the application, update the appengine package to a newer version, update the gsuite app, add a cloud compilation widget thingy, change app.yaml script from go to auto, etc.
All in all, I'm lost. A Similar SE question yielded no good answers. Someone else suggested app.yaml might be at fault, so here is mine:
runtime: go111
handlers:
- url: /static
static_dir: static
- url: /res
static_dir: res
- url: /update
script: auto
secure: always
login: admin
- url: /.*
script: auto
secure: always
login: optional
Debug console log is very unhelpful:
And the main file looks essentially like:
package main
import (
"fmt"
"google.golang.org/appengine"
"html/template"
"log"
"net/http"
)
var MainTemplate *template.Template
func main() {
http.HandleFunc("/", hello)
var err error
MainTemplate, err = template.ParseFiles("html/main.html")
if err != nil {
log.Printf("ERROR! %v\n", err.Error())
panic(err)
}
log.Printf("Hello world!\n")
}
func hello(w http.ResponseWriter, r *http.Request) {
c := appengine.NewContext(r)
//................................//
MainTemplate.Execute(w, nil)
}
Anything else it could be?

Answer 1

Okay, after some help from a few comments, here are a few issues that I had to fix to get my code to work:
I converted my init() function into main() without adding a way to listen to requests. The code just ran through the main() function and exited without an error, hence the problems with debugging.
appengine.NewContext(r) is deprecated apparently, so I had to switch those statements to r.Context(). MEANWHILE, Appengine Datastore is still using golang.org/x/net/context and not just context, so if you want to use things like RunInTransaction(), DON'T update your imports, context casts into golang.org/x/net/context just fine
If you follow the official examples provided by Google, you will most likely run into errors like textPayload: "2019/10/20 22:32:46 http: panic serving 127.0.0.1:16051: not an App Engine context. Instead, your code needs to look like the below example.
Since the former app package is now the main package, make sure that any files that are referenced in app.yaml (favicon.ico for example) are in a proper position in relation to the main package (I had to move mine to a different folder to avoid errors popping up every request...).
package main
import (
"google.golang.org/appengine"
"html/template"
"net/http"
)
var MainTemplate *template.Template
func init() {
http.HandleFunc("/", hello)
MainTemplate, _= template.ParseFiles("html/main.html")
}
func main() {
appengine.Main()
}
func hello(w http.ResponseWriter, r *http.Request) {
c := r.Context()
//................................//
MainTemplate.Execute(w, nil)
}
This ought to work. appengine.Main() apparently connects you to all the appengine functionality needed to use the context for datastore / memcache / whatever operations.
Thank you to the commenters that helped me get over the first hump!

Answer 2

Your code is missing the part to start a listener. Add this to your code:
port := os.Getenv("PORT")
if port == "" {
port = "8080"
log.Printf("Defaulting to port %s", port)
}
log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))