syscallcompat: move syscall wrapper to their own package

We will get more of them as OSX also lacks support for openat.

internal/fusefrontend/file.go

@@ -16,6 +16,7 @@ import (
 	"github.com/hanwen/go-fuse/fuse/nodefs"
 
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -100,7 +101,7 @@ func (f *file) createHeader() error {
 	buf := h.Pack()
 
 	// Prevent partially written (=corrupt) header by preallocating the space beforehand
-	err := prealloc(int(f.fd.Fd()), 0, contentenc.HEADER_LEN)
+	err := syscallcompat.Prealloc(int(f.fd.Fd()), 0, contentenc.HEADER_LEN)
 	if err != nil {
 		tlog.Warn.Printf("ino%d: createHeader: prealloc failed: %s\n", f.ino, err.Error())
 		return err
@@ -261,7 +262,7 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 			f.ino, uint64(len(blockData))-f.contentEnc.BlockOverhead(), b.BlockNo)
 
 		// Prevent partially written (=corrupt) blocks by preallocating the space beforehand
-		err := prealloc(int(f.fd.Fd()), int64(blockOffset), int64(len(blockData)))
+		err := syscallcompat.Prealloc(int(f.fd.Fd()), int64(blockOffset), int64(len(blockData)))
 		if err != nil {
 			tlog.Warn.Printf("ino%d fh%d: doWrite: prealloc failed: %s", f.ino, f.intFd(), err.Error())
 			status = fuse.ToStatus(err)

internal/syscallcompat/sys_darwin.go

@@ -1,8 +1,8 @@
-package fusefrontend
+package syscallcompat
 
 // prealloc - preallocate space without changing the file size. This prevents
 // us from running out of space in the middle of an operation.
-func prealloc(fd int, off int64, len int64) (err error) {
+func Prealloc(fd int, off int64, len int64) (err error) {
 	//
 	// Sorry, fallocate is not available on OSX at all and
 	// fcntl F_PREALLOCATE is not accessible from Go.

internal/syscallcompat/sys_linux.go

@@ -1,17 +1,19 @@
-package fusefrontend
+package syscallcompat
 
 import (
 	"sync"
 	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
-import "github.com/rfjakob/gocryptfs/internal/tlog"
+const FALLOC_FL_KEEP_SIZE = 0x01
 
 var preallocWarn sync.Once
 
 // prealloc - preallocate space without changing the file size. This prevents
 // us from running out of space in the middle of an operation.
-func prealloc(fd int, off int64, len int64) (err error) {
+func Prealloc(fd int, off int64, len int64) (err error) {
 	for {
 		err = syscall.Fallocate(fd, FALLOC_FL_KEEP_SIZE, off, len)
 		if err == syscall.EINTR {