[filecache] Make it typed, add GetOrSet variants (#285)

## Summary

Makes filecache better by:

* Make it typed
* export `Cache` type
* Add `GetOrSet` variants.

## How was it tested?
builds

filecache/filecache.go

@@ -13,20 +13,20 @@ import (
 var NotFound = errors.New("not found")
 var Expired = errors.New("expired")
 
-type cache struct {
+type Cache[T any] struct {
 	domain   string
 	cacheDir string
 }
 
-type data struct {
-	Val []byte
+type data[T any] struct {
+	Val T
 	Exp time.Time
 }
 
-type Option func(*cache)
+type Option[T any] func(*Cache[T])
 
-func New(domain string, opts ...Option) *cache {
-	result := &cache{domain: domain}
+func New[T any](domain string, opts ...Option[T]) *Cache[T] {
+	result := &Cache[T]{domain: domain}
 
 	var err error
 	result.cacheDir, err = os.UserCacheDir()
@@ -41,56 +41,102 @@ func New(domain string, opts ...Option) *cache {
 	return result
 }
 
-func WithCacheDir(dir string) Option {
-	return func(c *cache) {
+func WithCacheDir[T any](dir string) Option[T] {
+	return func(c *Cache[T]) {
 		c.cacheDir = dir
 	}
 }
 
-func (c *cache) Set(key string, val []byte, dur time.Duration) error {
-	d, err := json.Marshal(data{Val: val, Exp: time.Now().Add(dur)})
+// Set stores a value in the cache with the given key and expiration duration.
+func (c *Cache[T]) Set(key string, val T, dur time.Duration) error {
+	d, err := json.Marshal(data[T]{Val: val, Exp: time.Now().Add(dur)})
 	if err != nil {
 		return errors.WithStack(err)
 	}
 
 	return errors.WithStack(os.WriteFile(c.filename(key), d, 0644))
 }
 
-func (c *cache) SetT(key string, val []byte, t time.Time) error {
-	d, err := json.Marshal(data{Val: val, Exp: t})
+// SetWithTime is like Set but it allows the caller to specify the expiration
+// time of the value.
+func (c *Cache[T]) SetWithTime(key string, val T, t time.Time) error {
+	d, err := json.Marshal(data[T]{Val: val, Exp: t})
 	if err != nil {
 		return errors.WithStack(err)
 	}
 
 	return errors.WithStack(os.WriteFile(c.filename(key), d, 0644))
 }
 
-func (c *cache) Get(key string) ([]byte, error) {
+// Get retrieves a value from the cache with the given key.
+func (c *Cache[T]) Get(key string) (T, error) {
 	path := c.filename(key)
+	resultData := data[T]{}
+
 	if _, err := os.Stat(path); errors.Is(err, os.ErrNotExist) {
-		return nil, NotFound
+		return resultData.Val, NotFound
 	}
 
 	content, err := os.ReadFile(path)
 	if err != nil {
-		return nil, errors.WithStack(err)
+		return resultData.Val, errors.WithStack(err)
 	}
-	d := data{}
-	if err := json.Unmarshal(content, &d); err != nil {
-		return nil, errors.WithStack(err)
+
+	if err := json.Unmarshal(content, &resultData); err != nil {
+		return resultData.Val, errors.WithStack(err)
 	}
-	if time.Now().After(d.Exp) {
-		return nil, Expired
+	if time.Now().After(resultData.Exp) {
+		return resultData.Val, Expired
 	}
-	return d.Val, nil
+	return resultData.Val, nil
 }
 
-func (c *cache) filename(key string) string {
-	dir := filepath.Join(c.cacheDir, c.domain)
-	_ = os.MkdirAll(dir, 0755)
-	return filepath.Join(dir, key)
+// GetOrSet is a convenience method that gets the value from the cache if it
+// exists, otherwise it calls the provided function to get the value and sets
+// it in the cache.
+// If the function returns an error, the error is returned and the value is not
+// cached.
+func (c *Cache[T]) GetOrSet(
+	key string,
+	f func() (T, time.Duration, error),
+) (T, error) {
+	if val, err := c.Get(key); err == nil || !IsCacheMiss(err) {
+		return val, err
+	}
+
+	val, dur, err := f()
+	if err != nil {
+		return val, err
+	}
+
+	return val, c.Set(key, val, dur)
+}
+
+// GetOrSetWithTime is like GetOrSet but it allows the caller to specify the
+// expiration time of the value.
+func (c *Cache[T]) GetOrSetWithTime(
+	key string,
+	f func() (T, time.Time, error),
+) (T, error) {
+	if val, err := c.Get(key); err == nil || !IsCacheMiss(err) {
+		return val, err
+	}
+
+	val, t, err := f()
+	if err != nil {
+		return val, err
+	}
+
+	return val, c.SetWithTime(key, val, t)
 }
 
+// IsCacheMiss returns true if the error is NotFound or Expired.
 func IsCacheMiss(err error) bool {
 	return errors.Is(err, NotFound) || errors.Is(err, Expired)
 }
+
+func (c *Cache[T]) filename(key string) string {
+	dir := filepath.Join(c.cacheDir, c.domain)
+	_ = os.MkdirAll(dir, 0755)
+	return filepath.Join(dir, key)
+}