syscallcompat: OSX: add Fallocate and Openat wrappers

...and convert all calls to syscall.{Fallocate,Openat}
to syscallcompat .

Both syscalls are not available on OSX. We emulate Openat and just
return EOPNOTSUPP for Fallocate.

2 files changed, 47 insertions, 5 deletions

diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index d437170..81e1b15 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -1,5 +1,11 @@
 package syscallcompat
 
+import (
+	"os"
+	"sync"
+	"syscall"
+)
+
 // 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) {
@@ -10,3 +16,30 @@ func Prealloc(fd int, off int64, len int64) (err error) {
 	// See https://github.com/rfjakob/gocryptfs/issues/18 if you want to help.
 	return nil
 }
+
+var openatLock sync.Mutex
+
+// Poor man's Openat:
+// 1) fchdir to the dirfd
+// 2) open the file
+// 3) chdir back.
+func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error) {
+	openatLock.Lock()
+	defer openatLock.Unlock()
+
+	oldWd, err := os.Getwd()
+	if err != nil {
+		return -1, err
+	}
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return -1, err
+	}
+	defer os.Chdir(oldWd)
+
+	return syscall.Open(path, flags, mode)
+}
+
+func Fallocate(fd int, mode uint32, off int64, len int64) (err error) {
+	return syscall.EOPNOTSUPP
+}
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index 9fe9da5..ca6df95 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -11,9 +11,10 @@ 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) {
+// EnospcPrealloc preallocates ciphertext space without changing the file
+// size. This guarantees that we don't run out of space while writing a
+// ciphertext block (that would corrupt the block).
+func EnospcPrealloc(fd int, off int64, len int64) (err error) {
 	for {
 		err = syscall.Fallocate(fd, FALLOC_FL_KEEP_SIZE, off, len)
 		if err == syscall.EINTR {
@@ -23,8 +24,8 @@ func Prealloc(fd int, off int64, len int64) (err error) {
 			continue
 		}
 		if err == syscall.EOPNOTSUPP {
-			// ZFS does not support fallocate which caused gocryptfs to abort
-			// every write operation: https://github.com/rfjakob/gocryptfs/issues/22
+			// ZFS and ext3 do not support fallocate. Warn but continue anyway.
+			// https://github.com/rfjakob/gocryptfs/issues/22
 			preallocWarn.Do(func() {
 				tlog.Warn.Printf("Warning: The underlying filesystem " +
 					"does not support fallocate(2). gocryptfs will continue working " +
@@ -35,3 +36,11 @@ func Prealloc(fd int, off int64, len int64) (err error) {
 		return err
 	}
 }
+
+func Openat(dirfd int, path string, flags int, mode uint32) (fd int, err error) {
+	return syscall.Openat(dirfd, path, flags, mode)
+}
+
+func Fallocate(fd int, mode uint32, off int64, len int64) (err error) {
+	return syscall.Fallocate(fd, mode, off, len)
+}