refactor all file serve logic into internal/cache

internal/cache/helpers.go

@@ -0,0 +1,57 @@
+package cache
+
+import (
+	"io"
+	"log"
+	"net/http"
+	"os"
+)
+
+func (c *Cache) nextMirror() string {
+	idx := c.mirrorIdx.Add(1) - 1
+	return c.cfg.mirrorURLs[idx%uint32(len(c.cfg.mirrorURLs))]
+}
+
+func downloadToDisk(url, destPath string, c http.Client) error {
+	log.Printf("fetching %v", url)
+
+	// set the user agent
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		log.Printf("failed to create request: %v", err)
+		return &UpstreamError{StatusCode: http.StatusInternalServerError}
+	}
+	req.Header.Set("User-Agent", userAgent)
+
+	resp, err := c.Do(req)
+	if err != nil {
+		log.Printf("error fetching %s: %v", url, err)
+		return err
+	}
+	if resp.StatusCode != 200 {
+		log.Printf("GET %s returned %d", url, resp.StatusCode)
+		return &UpstreamError{StatusCode: resp.StatusCode}
+	}
+	defer resp.Body.Close()
+
+	// use a tmp file for the initial fetch in case it fails
+	tempPath := destPath + ".tmp"
+	tmpFile, err := os.Create(tempPath)
+	if err != nil {
+		return err
+	}
+	defer tmpFile.Close()
+
+	_, err = io.Copy(tmpFile, resp.Body)
+	if err != nil {
+		os.Remove(tempPath)
+		return err
+	}
+
+	// mv file to final location
+	if err := os.Rename(tempPath, destPath); err != nil {
+		os.Remove(tempPath)
+		return err
+	}
+	return nil
+}