file: load should propage file not found error (#84)

So it could be handled by caller. For example here
VictoriaMetrics/VictoriaMetrics#8952

Author: makasim

file.go

@@ -141,7 +141,7 @@ func load(filePath string, maxBytes int) (*Cache, error) {
 	// Read bucket files from filePath dir.
 	d, err := os.Open(filePath)
 	if err != nil {
-		return nil, fmt.Errorf("cannot open %q: %s", filePath, err)
+		return nil, fmt.Errorf("cannot open %q: %w", filePath, err)
 	}
 	defer func() {
 		_ = d.Close()
@@ -206,7 +206,7 @@ func loadMetadata(dir string) (uint64, error) {
 	metadataPath := dir + "/metadata.bin"
 	metadataFile, err := os.Open(metadataPath)
 	if err != nil {
-		return 0, fmt.Errorf("cannot open %q: %s", metadataPath, err)
+		return 0, fmt.Errorf("cannot open %q: %w", metadataPath, err)
 	}
 	defer func() {
 		_ = metadataFile.Close()

file_test.go

@@ -1,6 +1,7 @@
 package fastcache
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -45,6 +46,19 @@ func TestSaveLoadSmall(t *testing.T) {
 	}
 }
 
+func TestLoadFileNotExist(t *testing.T) {
+	c, err := LoadFromFile(`non-existing-file`)
+	if err == nil {
+		t.Fatalf("LoadFromFile must return error; got nil")
+	}
+	if !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("LoadFromFile must return os.ErrNotExist; got: %s", err)
+	}
+	if c != nil {
+		t.Fatalf("LoadFromFile must return nil cache")
+	}
+}
+
 func TestSaveLoadFile(t *testing.T) {
 	for _, concurrency := range []int{0, 1, 2, 4, 10} {
 		t.Run(fmt.Sprintf("concurrency_%d", concurrency), func(t *testing.T) {