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serialize.go
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serialize.go
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package memdump
import (
"encoding/binary"
"fmt"
"io"
"reflect"
"sort"
"sync"
"unsafe"
)
// uintptrSize is the size in bytes of uintptr
const uintptrSize = unsafe.Sizeof(uintptr(0))
// byteType is the reflect.Type of byte
var (
byteType = reflect.TypeOf(byte(0))
typeCache = make(map[reflect.Type]*typeInfo)
typeCacheLock sync.Mutex
)
// block represents a value to be written to the stream
type block struct {
src reflect.Value
dest uintptr
}
// pointer represents the location of a pointer in a type
type pointer struct {
offset uintptr
typ reflect.Type
}
// typeInfo represents the location of the pointers in a type
type typeInfo struct {
pointers []pointer
}
// asBytes gets a byte slice with data pointer set to the address of the
// assigned value and length set to the sizeof the value.
func asBytes(v reflect.Value) []byte {
size := int(v.Type().Size())
hdr := reflect.SliceHeader{
Data: v.Addr().Pointer(),
Len: size,
Cap: size,
}
return *(*[]byte)(unsafe.Pointer(&hdr))
}
// isNil determines whether the pointer contained within v is nil.
// This is equivalent to checking x==nil, except for strings, where
// this method checks the data pointer inside the string header.
func isNil(v reflect.Value) bool {
if v.Kind() == reflect.String {
hdr := (*reflect.StringHeader)(unsafe.Pointer(v.Addr().Pointer()))
return hdr.Data == 0
}
return v.IsNil()
}
func readPointer(v reflect.Value) uintptr {
if v.Kind() == reflect.String {
hdr := (*reflect.StringHeader)(unsafe.Pointer(v.Addr().Pointer()))
return hdr.Data
}
return v.Pointer()
}
type byOffset []pointer
func (xs byOffset) Len() int { return len(xs) }
func (xs byOffset) Swap(i, j int) { xs[i], xs[j] = xs[j], xs[i] }
func (xs byOffset) Less(i, j int) bool { return xs[i].offset < xs[j].offset }
type countingWriter struct {
w io.Writer
offset int
}
func (w *countingWriter) Write(buf []byte) (int, error) {
n, err := w.w.Write(buf)
w.offset += n
return n, err
}
// memEncoder writes the in-memory representation of an object, together
// with all referenced objects.
type memEncoder struct {
w countingWriter
}
func newMemEncoder(w io.Writer) *memEncoder {
return &memEncoder{
w: countingWriter{w: w},
}
}
// memEncoderState contains the state that is local to a single Encode() call.
type memEncoderState struct {
ptrLocs []int64
next uintptr
}
// alloc makes room for N objects of the specified type, and returns the
// base offset for that object. It deals correctly with alignment.
func (e *memEncoderState) alloc(t reflect.Type, n int) uintptr {
align := uintptr(t.Align())
if e.next%align != 0 {
e.next += align - (e.next % align)
}
cur := e.next
e.next += t.Size() * uintptr(n)
return cur
}
// arrayFromString gets a fixed-size array representing the bytes pointed to
// by a string.
func arrayFromString(strval reflect.Value) reflect.Value {
if strval.Kind() != reflect.String {
panic(fmt.Sprintf("expected string type but got %s", strval.Type()))
}
hdr := (*reflect.StringHeader)(unsafe.Pointer(strval.Addr().Pointer()))
typ := reflect.ArrayOf(hdr.Len, byteType)
return reflect.NewAt(typ, unsafe.Pointer(hdr.Data)).Elem()
}
// arrayFromSlice gets a fixed-size array representing the data pointed to by
// a slice
func arrayFromSlice(sliceval reflect.Value) reflect.Value {
if sliceval.Kind() != reflect.Slice {
panic(fmt.Sprintf("expected string type but got %s", sliceval.Type()))
}
hdr := (*reflect.SliceHeader)(unsafe.Pointer(sliceval.Addr().Pointer()))
typ := reflect.ArrayOf(hdr.Len, sliceval.Type().Elem())
return reflect.NewAt(typ, unsafe.Pointer(hdr.Data)).Elem()
}
// Encode writes the in-memory representation of the object pointed to by ptr. It
// returns the offset of each pointer and an error.
func (e *memEncoder) Encode(ptr interface{}) ([]int64, error) {
var state memEncoderState
ptrval := reflect.ValueOf(ptr)
objval := ptrval.Elem()
cache := make(map[uintptr]uintptr)
queue := []block{{
src: objval,
dest: state.alloc(objval.Type(), 1),
}}
for len(queue) > 0 {
cur := queue[0]
queue = queue[1:]
blockaddr := cur.src.Addr()
blockbytes := asBytes(cur.src)
// check the position of the writer
if cur.dest < uintptr(e.w.offset) {
panic(fmt.Sprintf("block.dest=%d but writer is at %d", cur.dest, e.w.offset))
}
// for byte-alignment purposes we may need to fill some bytes
if fill := cur.dest - uintptr(e.w.offset); fill > 0 {
_, err := e.w.Write(make([]byte, fill))
if err != nil {
return nil, err
}
}
// look up info about this type
info := lookupType(cur.src.Type())
// add each referenced object to the queue
var blockpos uintptr
for _, ptr := range info.pointers {
_, err := e.w.Write(blockbytes[blockpos:ptr.offset])
if err != nil {
return nil, err
}
ptrdata := unsafe.Pointer(blockaddr.Pointer() + ptr.offset)
ptrval := reflect.NewAt(ptr.typ, ptrdata).Elem()
var dest uintptr
if !isNil(ptrval) {
state.ptrLocs = append(state.ptrLocs, int64(cur.dest+ptr.offset))
var found bool
dest, found = cache[readPointer(ptrval)]
if !found {
switch ptr.typ.Kind() {
case reflect.Ptr:
dest = state.alloc(ptr.typ.Elem(), 1)
queue = append(queue, block{
src: ptrval.Elem(),
dest: dest,
})
case reflect.Slice:
dest = state.alloc(ptr.typ.Elem(), ptrval.Len())
arr := arrayFromSlice(ptrval)
queue = append(queue, block{
src: arr,
dest: dest,
})
case reflect.String:
dest = state.alloc(byteType, ptrval.Len())
arr := arrayFromString(ptrval)
queue = append(queue, block{
src: arr,
dest: dest,
})
}
cache[readPointer(ptrval)] = dest
}
}
err = binary.Write(&e.w, binary.LittleEndian, uint64(dest))
if err != nil {
return nil, err
}
blockpos = ptr.offset + uintptrSize
}
_, err := e.w.Write(blockbytes[blockpos:])
if err != nil {
return nil, err
}
}
return state.ptrLocs, nil
}
// pointerFinder gets the byte offset of each pointer in an object. It
// only considers the immediate value of an object (i.e. the bytes that
// would be copied in a simple assignment). It does not follow pointers
// to other objects.
type pointerFinder struct {
pointers []pointer
}
func (f *pointerFinder) visit(t reflect.Type, base uintptr) {
switch t.Kind() {
case reflect.Ptr, reflect.String, reflect.Slice:
// these four types all store one pointer at offset zero
f.pointers = append(f.pointers, pointer{
offset: base,
typ: t,
})
case reflect.Struct:
for i := 0; i < t.NumField(); i++ {
field := t.Field(i)
f.visit(field.Type, base+field.Offset)
}
case reflect.Array:
elemSize := t.Elem().Size()
elemPtrs := lookupType(t.Elem()).pointers
for _, elemPtr := range elemPtrs {
for i := 0; i < t.Len(); i++ {
f.pointers = append(f.pointers, pointer{
offset: base + uintptr(i)*elemSize + elemPtr.offset,
typ: elemPtr.typ,
})
}
}
case reflect.Map, reflect.Chan, reflect.Interface, reflect.UnsafePointer, reflect.Func:
panic(fmt.Sprintf("cannot serialize objects of %v kind (got %v)", t.Kind(), t))
}
}
// lookupType gets the type info for t.
func lookupType(t reflect.Type) *typeInfo {
typeCacheLock.Lock()
info, found := typeCache[t]
typeCacheLock.Unlock()
if !found {
var f pointerFinder
f.visit(t, 0)
info = &typeInfo{pointers: f.pointers}
sort.Sort(byOffset(info.pointers))
typeCacheLock.Lock()
typeCache[t] = info
typeCacheLock.Unlock()
}
return info
}