Could someone explain why the following gocode fails with goapp serve - google-app-engine

Question

package helloworld
import (
"fmt"
"net/http"
"appengine"
"appengine/user"
)
func init() {
fmt.Print("hello")
http.HandleFunc("/", handler)
}
func handler(w http.ResponseWriter, r *http.Request) {
c := appengine.NewContext(r)
u := user.Current(c)
if u == nil {
url, err := user.LoginURL(c, r.URL.String())
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
w.Header().Set("Location", url)
w.WriteHeader(http.StatusFound)
return
}
fmt.Fprintf(w, "Hello, %v!", u)
}
Throw the following error in goapp serve output
(saucy)adam#localhost:~/projects/ringed-land-605/default$ goapp serve -host 0.0.0.0 .
INFO 2014-06-08 23:57:47,862 devappserver2.py:716] Skipping SDK update check.
INFO 2014-06-08 23:57:47,877 api_server.py:171] Starting API server at: http://localhost:48026
INFO 2014-06-08 23:57:47,923 dispatcher.py:182] Starting module "default" running at: http://0.0.0.0:8080
INFO 2014-06-08 23:57:47,925 admin_server.py:117] Starting admin server at: http://localhost:8000
ERROR 2014-06-08 23:57:48,759 http_runtime.py:262] bad runtime process port ['hello46591\n']
Removing the fmt.Print() fixes the issue. My question is why does that happen?

Answer 1

When starting the runtime process, the Go Development Server (in the App Engine Go SDK) is reading the single line response found in your helloworld's init.
This modifies the _start_process_flavor flag in http_runtime.py; consequently, the HTTP runtime attempts to read the line for direction on which port to listen to.
Read the single line response expected in the start process file. [...] The START_PROCESS_FILE flavor uses a file for the runtime instance to report back the port it is listening on.
In this case, hello isn't a valid port to listen on.
Try using Go's log package instead:
log.Print("hello")