Added Writer type.

writer.go

@@ -0,0 +1,102 @@
+package main
+
+import (
+	"container/list"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"sync"
+)
+
+// Status of download in progress.
+type Status int
+
+const (
+	StatusNone        = iota // Nothing has happened yet
+	StatusDownloading        // JSON and data files created
+	StatusError              // Error retrieving the data
+	StatusDone               // All data has been retrieved
+)
+
+// Writer connects to a remote URL via HTTP and retrieves the data, writing it
+// directly to disk and notifying subscribed readers of its status.
+type Writer struct {
+	mutex    sync.Mutex
+	channels *list.List
+	status   Status
+}
+
+func (w *Writer) sendStatus(statusChan chan Status, status Status) {
+	statusChan <- status
+}
+
+func (w *Writer) setStatus(status Status) {
+	w.mutex.Lock()
+	defer w.mutex.Unlock()
+	w.status = status
+	for e := w.channels.Front(); e != nil; e = e.Next() {
+		go w.sendStatus(e.Value.(chan Status), status)
+	}
+}
+
+// Create a new writer for the given URL. Status information is passed along to
+// subscribed channels. The done channel is used to notify the storage system
+// that the download is complete (either success or an error).
+func NewWriter(url, jsonFilename, dataFilename string, done chan<- *Writer) *Writer {
+	w := &Writer{
+		channels: make([]chan Status),
+		status:   StatusNone,
+	}
+	go func() {
+		r, err := http.Get(url)
+		if err != nil {
+			goto error
+		}
+		defer r.Body.Close()
+		e := &Entry{
+			URL:         url,
+			ContentType: r.Header.Get("Content-Type"),
+		}
+		if err = e.Save(jsonFilename); err != nil {
+			goto error
+		}
+		f, err := os.Create(dataFilename)
+		if err != nil {
+			goto error
+		}
+		defer f.Close()
+		w.setStatus(StatusDownloading)
+		_, err = io.Copy(f, r.Body)
+		if err != nil {
+			goto error
+		}
+		w.setStatus(StatusDone)
+		done <- w
+	error:
+		w.setStatus(StatusError)
+		done <- w
+	}()
+	return w
+}
+
+// Subscribe adds a channel to the list to be notified when the writer's status
+// changes. The channel will also immediately receive the current status.
+func (w *Writer) Subscribe(statusChan chan Status) {
+	w.mutex.Lock()
+	defer w.mutex.Unlock()
+	w.channels = append(w.channels, c)
+	w.sendStatus(c, w.status)
+}
+
+// Unsubscribe a channel from the list to be notified. This may occur when a
+// client cancels a request, for example.
+func (w *Writer) Unsubscribe(statusChan chan Status) {
+	w.mutex.Lock()
+	defer w.mutex.Unlock()
+	for i, c := range w.channels {
+		if c == statusChan {
+			w.channels = append(w.channels[:i], w.channels[i+1:]...)
+		}
+	}
+}