cache: Always use cached file if it exists

A file is always cached whole. Thus, any out of bounds access will also
fail when directed at the backend. To handle case in which the cached
file is broken, then caller must call Cache.Forget(h) for the file in
question.

internal/cache/file.go

@@ -34,46 +34,48 @@ func (c *Cache) canBeCached(t backend.FileType) bool {
 
 // load returns a reader that yields the contents of the file with the
 // given handle. rd must be closed after use. If an error is returned, the
-// ReadCloser is nil.
-func (c *Cache) load(h backend.Handle, length int, offset int64) (io.ReadCloser, error) {
+// ReadCloser is nil. The bool return value indicates whether the requested
+// file exists in the cache. It can be true even when no reader is returned
+// because length or offset are out of bounds
+func (c *Cache) load(h backend.Handle, length int, offset int64) (io.ReadCloser, bool, error) {
 	debug.Log("Load(%v, %v, %v) from cache", h, length, offset)
 	if !c.canBeCached(h.Type) {
-		return nil, errors.New("cannot be cached")
+		return nil, false, errors.New("cannot be cached")
 	}
 
 	f, err := fs.Open(c.filename(h))
 	if err != nil {
-		return nil, errors.WithStack(err)
+		return nil, false, errors.WithStack(err)
 	}
 
 	fi, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
-		return nil, errors.WithStack(err)
+		return nil, true, errors.WithStack(err)
 	}
 
 	size := fi.Size()
 	if size <= int64(crypto.CiphertextLength(0)) {
 		_ = f.Close()
-		return nil, errors.Errorf("cached file %v is truncated", h)
+		return nil, true, errors.Errorf("cached file %v is truncated", h)
 	}
 
 	if size < offset+int64(length) {
 		_ = f.Close()
-		return nil, errors.Errorf("cached file %v is too short", h)
+		return nil, true, errors.Errorf("cached file %v is too short", h)
 	}
 
 	if offset > 0 {
 		if _, err = f.Seek(offset, io.SeekStart); err != nil {
 			_ = f.Close()
-			return nil, err
+			return nil, true, err
 		}
 	}
 
 	if length <= 0 {
-		return f, nil
+		return f, true, nil
 	}
-	return util.LimitReadCloser(f, int64(length)), nil
+	return util.LimitReadCloser(f, int64(length)), true, nil
 }
 
 // save saves a file in the cache.