Golang runtime (#125)

Minimal golang runtime.  

Functions are built as Go plugins, and the plugin is uploaded to fission.

See `environments/go/README.md` for instructions and `examples/go` for a usage example.

For now, we're re-using the 'code' field of the function to store the binary plugin. With v2 environments, this will be stored separately as a binary package.

environments/go/server.go

@@ -0,0 +1,83 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"plugin"
+
+	"github.com/fission/fission/environments/go/context"
+)
+
+const (
+	CODE_PATH = "/userfunc/user"
+)
+
+var userFunc http.HandlerFunc
+
+func loadPlugin() http.HandlerFunc {
+	p, err := plugin.Open(CODE_PATH)
+	if err != nil {
+		panic(err)
+	}
+	sym, err := p.Lookup("Handler")
+	if err != nil {
+		panic("Entry point not found")
+	}
+
+	switch h := sym.(type) {
+	case *http.Handler:
+		return (*h).ServeHTTP
+	case *http.HandlerFunc:
+		return *h
+	case func(http.ResponseWriter, *http.Request):
+		return h
+	case func(context.Context, http.ResponseWriter, *http.Request):
+		return func(w http.ResponseWriter, r *http.Request) {
+			c := context.New()
+			h(c, w, r)
+		}
+	default:
+		panic("Entry point not found: bad type")
+	}
+}
+
+func specializeHandler(w http.ResponseWriter, r *http.Request) {
+	if userFunc != nil {
+		w.WriteHeader(400)
+		w.Write([]byte("Not a generic container"))
+		return
+	}
+
+	_, err := os.Stat(CODE_PATH)
+	if err != nil {
+		if os.IsNotExist(err) {
+			w.WriteHeader(http.StatusNotFound)
+			w.Write([]byte(CODE_PATH + ": not found"))
+			return
+		} else {
+			panic(err)
+		}
+	}
+
+	fmt.Println("Specializing ...")
+	userFunc = loadPlugin()
+	fmt.Println("Done")
+}
+
+func main() {
+	http.HandleFunc("/specialize", specializeHandler)
+
+	// Generic route -- all http requests go to the user function.
+	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if userFunc == nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte("Generic container: no requests supported"))
+			return
+		}
+		userFunc(w, r)
+	})
+
+	fmt.Println("Listening on 8888 ...")
+	http.ListenAndServe(":8888", nil)
+}