server: replace blob prefix separator from ':' to '-' (#3146)

This fixes issues with blob file names that contain ':' characters to be rejected by file systems that do not support them.

server/fixblobs.go

@@ -0,0 +1,26 @@
+package server
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// fixBlobs walks the provided dir and replaces (":") to ("-") in the file
+// prefix. (e.g. sha256:1234 -> sha256-1234)
+func fixBlobs(dir string) error {
+	return filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		baseName := filepath.Base(path)
+		typ, sha, ok := strings.Cut(baseName, ":")
+		if ok && typ == "sha256" {
+			newPath := filepath.Join(filepath.Dir(path), typ+"-"+sha)
+			if err := os.Rename(path, newPath); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}