Call SyscallConn() instead of Fd() to use file's fd

[os.File.SyscallConn] is the safer alternative to [os.File.Fd], without the pitfalls mentioned in Fd's doc comment.

The SyscallConn method is implemented on all platforms except Plan 9, where it returns [syscall.EPLAN9], which matches [errors.ErrUnsupported] and goes to the fallback path.

Author: database64128

reader.go

@@ -234,7 +234,7 @@ func Open(file string, options ...ReaderOption) (*Reader, error) {
 		return nil, errors.New("file too large")
 	}
 
-	data, err := mmap(int(mapFile.Fd()), size)
+	data, err := openMmap(mapFile, size)
 	if err != nil {
 		if errors.Is(err, errors.ErrUnsupported) {
 			data, err = openFallback(mapFile, size)
@@ -257,6 +257,21 @@ func Open(file string, options ...ReaderOption) (*Reader, error) {
 	return reader, nil
 }
 
+func openMmap(f *os.File, size int) (data []byte, err error) {
+	rawConn, err := f.SyscallConn()
+	if err != nil {
+		return nil, err
+	}
+
+	if cerr := rawConn.Control(func(fd uintptr) {
+		data, err = mmap(int(fd), size)
+	}); cerr != nil {
+		return nil, cerr
+	}
+
+	return data, err
+}
+
 func openFallback(f *os.File, size int) (data []byte, err error) {
 	data = make([]byte, size)
 	_, err = io.ReadFull(f, data)