implmented fetch on cache miss

internal/cache/cache.go

@@ -0,0 +1,76 @@
+package cache
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sync/singleflight"
+)
+
+type Cache struct {
+	repo      string
+	mirrorURL string
+	sf        singleflight.Group
+}
+
+func NewCache(repo string, mirrorURL string) *Cache {
+	return &Cache{
+		repo:      repo,
+		mirrorURL: mirrorURL,
+	}
+}
+
+func (c *Cache) Fetch(pkgPath string) error {
+	_, err, _ := c.sf.Do(pkgPath, func() (any, error) {
+		return nil, c.fetch(pkgPath)
+	})
+	return err
+}
+
+type UpstreamError struct {
+	StatusCode int
+}
+
+func (e *UpstreamError) Error() string {
+	return fmt.Sprintf("upstream returned %d", e.StatusCode)
+}
+
+func (c *Cache) fetch(pkgPath string) error {
+	// pkgPath is relative to the repo or mirror root
+	tempPkgPath := pkgPath + ".tmp"
+	tempPkgName := filepath.Join(c.repo, tempPkgPath) //full tmp write path
+	outPkg := filepath.Join(c.repo, pkgPath)
+	pkgURL := c.mirrorURL + pkgPath
+
+	resp, err := http.Get(pkgURL)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return &UpstreamError{StatusCode: resp.StatusCode}
+	}
+
+	outFile, err := os.Create(tempPkgName)
+	if err != nil {
+		return err
+	}
+	defer outFile.Close()
+
+	_, err = io.Copy(outFile, resp.Body)
+	if err != nil {
+		os.Remove(tempPkgName)
+		return err
+	}
+
+	if err := os.Rename(tempPkgName, outPkg); err != nil {
+		os.Remove(tempPkgName)
+		return err
+	}
+	return nil
+}