internal: add unified function to fallocate

Change-Id: I2b80c97a8b7bd4e93085ba6fe4a0bf2a852db78e

internal/fallocate/fallocate.go

+package fallocate
+
+// Fallocate is a wrapper around fallocate syscall.
+// On Linux, it is a wrapper around fallocate(2).
+// On Darwin, it is a wrapper around fnctl(2).
+// On FreeBSD, it is a wrapper around posix_fallocate(2).
+func Fallocate(fd int, mode uint32, off int64, len int64) (err error) {
+	return fallocate(fd, mode, off, len)
+}

internal/fallocate/fallocate_darwin.go

+package fallocate
+
+import (
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+func fallocate(fd int, mode uint32, off int64, len int64) error {
+	// TODO: Handle `mode` parameter.
+	_ = mode
+
+	// From `man fcntl` on OSX:
+	//     The F_PREALLOCATE command operates on the following structure:
+	//
+	//             typedef struct fstore {
+	//                 u_int32_t fst_flags;      /* IN: flags word */
+	//                 int       fst_posmode;    /* IN: indicates offset field */
+	//                 off_t     fst_offset;     /* IN: start of the region */
+	//                 off_t     fst_length;     /* IN: size of the region */
+	//                 off_t     fst_bytesalloc; /* OUT: number of bytes allocated */
+	//             } fstore_t;
+	//
+	//     The flags (fst_flags) for the F_PREALLOCATE command are as follows:
+	//
+	//           F_ALLOCATECONTIG   Allocate contiguous space.
+	//
+	//           F_ALLOCATEALL      Allocate all requested space or no space at all.
+	//
+	//     The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use the offset field.  The modes are as fol-
+	//     lows:
+	//
+	//           F_PEOFPOSMODE   Allocate from the physical end of file.
+	//
+	//           F_VOLPOSMODE    Allocate from the volume offset.
+
+	k := struct {
+		Flags      uint32 // u_int32_t
+		Posmode    int64  // int
+		Offset     int64  // off_t
+		Length     int64  // off_t
+		Bytesalloc int64  // off_t
+	}{
+		0,
+		0,
+		int64(off),
+		int64(len),
+		0,
+	}
+	_, _, errno := unix.Syscall(syscall.SYS_FCNTL, uintptr(fd), uintptr(unix.F_PREALLOCATE), uintptr(unsafe.Pointer(&k)))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

internal/fallocate/fallocate_freebsd.go

+package fallocate
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func fallocate(fd int, mode uint32, off int64, len int64) error {
+	// Ignore mode
+	_ = mode
+	_, _, errno := unix.Syscall(unix.SYS_POSIX_FALLOCATE, uintptr(fd), uintptr(off), uintptr(len))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

internal/fallocate/fallocate_linux.go

+package fallocate
+
+import "golang.org/x/sys/unix"
+
+func fallocate(fd int, mode uint32, off int64, len int64) error {
+	return unix.Fallocate(fd, mode, off, len)
+}