Fetcher is a helper for function run containers

Fetcher is a tool to download functions from the controller.  It is
meant to be run in a container in the same pod as the function run
container, with a shared volume.

function-run/fetcher/Dockerfile

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+FROM alpine:3.4
+
+ADD fetcher /
+
+EXPOSE 8000

function-run/fetcher/build.sh

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+GOOS=linux GOARCH=386 go build -o fetcher .

function-run/fetcher/fetcher.go

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+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+)
+
+type FetchRequest struct {
+	Url      string `json:"url"`
+	Filename string `json:"filename"`
+}
+
+type Fetcher struct {
+	sharedVolumePath string
+}
+
+func MakeFetcher(sharedVolumePath string) *Fetcher {
+	return &Fetcher{sharedVolumePath: sharedVolumePath}
+}
+
+func (fetcher *Fetcher) handler(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		http.Error(w, "", 404)
+		return
+	}
+
+	// parse request
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+	req := FetchRequest{}
+	err = json.Unmarshal(body, &req)
+	if err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+
+	// fetch the file and save it to tmp path
+	resp, err := http.Get(req.Url)
+	if err != nil {
+		e := fmt.Sprintf("Failed to fetch from url: %v", err)
+		http.Error(w, e, 400)
+		return
+	}
+	defer resp.Body.Close()
+	body, err = ioutil.ReadAll(resp.Body)
+	if err != nil {
+		e := fmt.Sprintf("Failed to read from url: %v", err)
+		http.Error(w, e, 400)
+		return
+	}
+	tmpFile := req.Filename + ".tmp"
+	tmpPath := filepath.Join(fetcher.sharedVolumePath, tmpFile)
+	err = ioutil.WriteFile(tmpPath, body, 0600)
+	if err != nil {
+		e := fmt.Sprintf("Failed to write file: %v", err)
+		http.Error(w, e, 500)
+		return
+	}
+
+	// TODO: add signature verification
+
+	// move tmp file to requested filename
+	err = os.Rename(tmpPath, filepath.Join(fetcher.sharedVolumePath, req.Filename))
+	if err != nil {
+		e := fmt.Sprintf("Failed to move file: %v", err)
+		http.Error(w, e, 500)
+		return
+	}
+
+	// all done
+	w.WriteHeader(http.StatusOK)
+}
+
+// Usage: fetcher <shared volume path>
+func main() {
+	dir := os.Args[1]
+	if _, err := os.Stat(dir); err != nil {
+		if os.IsNotExist(err) {
+			err = os.MkdirAll(dir, os.ModeDir|0700)
+			if err != nil {
+				log.Fatalf("Error creating directory: %v", err)
+			}
+		}
+	}
+	fetcher := MakeFetcher(dir)
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", fetcher.handler)
+	http.ListenAndServe(":8000", mux)
+}