feat(mmap+go): Add Falloc for mac in the go mmap experiment

src/mmap-go-prototype/mac_falloc.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+// based on https://github.com/coilhq/tigerbeetle/blob/7e12fccc4859f035cd26af29b8e9f9749a0899a3/src/storage.zig#L430
+func MacFallocate(file *os.File, offset int64, length int64) error {
+	store := syscall.Fstore_t{
+		Flags:      syscall.F_ALLOCATECONTIG | syscall.F_ALLOCATEALL,
+		Posmode:    syscall.F_PEOFPOSMODE,
+		Offset:     0,
+		Length:     offset + length,
+		Bytesalloc: 0,
+	}
+	_, _, err := syscall.Syscall(syscall.SYS_FCNTL, file.Fd(), syscall.F_PREALLOCATE, uintptr(unsafe.Pointer(&store)))
+	if err != 0 {
+		store.Flags = syscall.F_ALLOCATEALL
+		_, _, err = syscall.Syscall(syscall.SYS_FCNTL, file.Fd(), syscall.F_PREALLOCATE, uintptr(unsafe.Pointer(&store)))
+		if err != 0 {
+			return err
+		}
+	}
+
+	return syscall.Ftruncate(int(file.Fd()), store.Length)
+}

src/mmap-go-prototype/mmap.go

@@ -25,9 +25,10 @@ func NewMmap(f string) *Mmap {
 
 	// XXX: Passing `FALLOC_FL_ZERO_RANGE` as mode would zero the file, but
 	// I couldn't find this constant in `syscall`...
-	mode := uint32(0)
+	// mode := uint32(0) // syscall.F_PREALLOCATE
 
-	if err := syscall.Fallocate(int(file.Fd()), mode, 0, int64(pageSize)); err != nil {
+	//if err := syscall.Fallocate(int(file.Fd()), mode, 0, int64(pageSize)); err != nil {
+	if err := MacFallocate(file, 0, int64(pageSize)); err != nil {
 		panic(err)
 	}