feat(go-mmap): do some initial go mmap testing

src/mmap-go-prototype/mmap.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"os"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+type Mmap struct {
+	data []byte
+}
+
+func NewMmap(f string) *Mmap {
+	file, err := os.OpenFile(f, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		panic(err)
+	}
+	defer file.Close()
+
+	pageSize := syscall.Getpagesize()
+
+	// XXX: Passing `FALLOC_FL_ZERO_RANGE` as mode would zero the file, but
+	// I couldn't find this constant in `syscall`...
+	mode := uint32(0)
+
+	if err := syscall.Fallocate(int(file.Fd()), mode, 0, int64(pageSize)); err != nil {
+		panic(err)
+	}
+
+	data, err := syscall.Mmap(int(file.Fd()), 0, int(pageSize),
+		syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
+	if err != nil {
+		panic(err)
+	}
+	return &Mmap{data: data}
+}
+
+func (m *Mmap) Free() {
+	if err := syscall.Munmap(m.data); err != nil {
+		panic(err)
+	}
+}
+
+func main() {
+	f := "/tmp/test_mmap.txt"
+	m := NewMmap(f)
+	m.data[0] = 'A'
+	m.data[1] = 'B'
+	m.data[2] = 'C'
+	// `sync/atomic` doesn't have support for working against `byte[]`,
+	// hence the use of unsafe pointers.
+	uptr := unsafe.Pointer(&m.data[0])
+	atomic.AddUint32((*uint32)(uptr), 1)                            // A => B
+	atomic.AddUint32((*uint32)(unsafe.Pointer(uintptr(uptr)+2)), 2) // C => E
+
+	// XXX: couldn't get the following block of code to work yet:
+	bs := make([]byte, 4)
+	binary.BigEndian.PutUint32(bs[:4], math.MaxUint32-1)
+	fmt.Printf("math.MaxUint32-1: %d\n", math.MaxUint32-1)
+	copy(m.data[3:], bs)
+	atomic.AddUint32((*uint32)(unsafe.Pointer(uintptr(uptr)+3)), 1)
+	i := binary.BigEndian.Uint32(m.data[3:7])
+	fmt.Printf("i: %d\n", i) // prints 255 rather than `MaxUint32`
+
+	m.Free()
+	data, err := os.ReadFile(f)
+	if err != nil {
+		panic(err)
+	}
+	os.Stdout.Write(data) // Prints "BBE".
+}