refactor all file serve logic into internal/cache

internal/cache/fetch.go

@@ -0,0 +1,72 @@
+package cache
+
+import (
+	"log"
+	"os"
+	"path/filepath"
+)
+
+func (c *Cache) Fetch(relPath string) (*CacheFile, error) {
+	// return file directly if exists in cache
+	cf, err := getCachedFile(c.cfg.cacheRoot, relPath)
+	if err == nil {
+		return cf, nil
+	}
+
+	// fetch file from upstream
+	_, err, _ = c.sf.Do(relPath, func() (any, error) {
+		log.Print("calling fetch")
+		return nil, c.fetch(relPath)
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	cf, err = getCachedFile(c.cfg.cacheRoot, relPath)
+	if err != nil {
+		return nil, err
+	}
+	return cf, nil
+}
+
+func (c *Cache) fetch(relPath string) error {
+	// relPath is relative to the localRoot
+	// ie relPath includes /{repo}/os/{arch}/ and the actual name linux-x.x.x.pkg.tar.zst
+
+	// final file name and path
+	destPath := filepath.Join(c.cfg.cacheRoot, relPath)
+
+	// declare vars outside loop
+	var err error
+	// fetch pkgs from mirror with retry logic
+	for range len(c.cfg.mirrorURLs) {
+		url := c.nextMirror() + relPath
+		err = downloadToDisk(url, destPath, c.client)
+		if err == nil {
+			break
+		}
+	}
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func getCachedFile(cacheRoot, relPath string) (*CacheFile, error) {
+	filePath := filepath.Join(cacheRoot, relPath)
+	info, err := os.Stat(filePath)
+	if err != nil {
+		return nil, err
+	}
+
+	f, err := os.Open(filePath)
+	if err != nil {
+		return nil, err
+	}
+
+	return &CacheFile{
+		Reader:   f,
+		Size:     info.Size(),
+		Filename: filepath.Base(filePath),
+	}, nil
+}