filch: use F_NOCACHE on macOS

For #1320

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

logtail/filch/filch.go

@@ -131,11 +131,11 @@ func New(filePrefix string, opts Options) (f *Filch, err error) {
 	path1 := filePrefix + ".log1.txt"
 	path2 := filePrefix + ".log2.txt"
 
-	f1, err = os.OpenFile(path1, os.O_CREATE|os.O_RDWR, 0600)
+	f1, err = openFileSync(path1, os.O_CREATE|os.O_RDWR, 0600)
 	if err != nil {
 		return nil, err
 	}
-	f2, err = os.OpenFile(path2, os.O_CREATE|os.O_RDWR, 0600)
+	f2, err = openFileSync(path2, os.O_CREATE|os.O_RDWR, 0600)
 	if err != nil {
 		return nil, err
 	}

logtail/filch/openfilesync_darwin.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//+build darwin
+
+package filch
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func openFileSync(path string, flag int, perm os.FileMode) (*os.File, error) {
+	f, err := os.OpenFile(path, flag, perm)
+	if err != nil {
+		return nil, err
+	}
+	_, err = unix.FcntlInt(uintptr(f.Fd()), syscall.F_NOCACHE, 1)
+	if err != nil {
+		return nil, fmt.Errorf("openFileSync: %w", err)
+	}
+	return f, nil
+}

logtail/filch/openfilesync_notdarwin.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//+build !darwin
+
+package filch
+
+import (
+	"os"
+)
+
+func openFileSync(path string, flag int, perm os.FileMode) (*os.File, error) {
+	// TODO(crawshaw): on Linux and FreeBSD, use O_SYNC
+	return os.OpenFile(path, flag, perm)
+}