Go 底层原理丨interface

type iface struct {
	tab  *itab
	data unsafe.Pointer
}

type eface struct {
	_type *_type
	data  unsafe.Pointer
}

type iface struct {
    tab  *itab          // 接口表指针（8字节）
    data unsafe.Pointer // 实际数据指针（8字节）
}

type itab = abi.ITab

type ITab struct {
	Inter *InterfaceType
	Type  *Type
	Hash  uint32     // copy of Type.Hash. Used for type switches.
	Fun   [1]uintptr // variable sized. fun[0]==0 means Type does not implement Inter.
}

type Reader interface {
    Read(p []byte) (n int, err error)
}

var r1 Reader = &File{...}
var r2 Reader = &File{...}

type MyInt int

func (m MyInt) String() string {
    return strconv.Itoa(int(m))
}

var i fmt.Stringer = MyInt(42)

func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
	if len(inter.Methods) == 0 {
		throw("internal error - misuse of itab")
	}

	// easy case
	if typ.TFlag&abi.TFlagUncommon == 0 {
		if canfail {
			return nil
		}
		name := toRType(&inter.Type).nameOff(inter.Methods[0].Name)
		panic(&TypeAssertionError{nil, typ, &inter.Type, name.Name()})
	}

	var m *itab

	// First, look in the existing table to see if we can find the itab we need.
	// This is by far the most common case, so do it without locks.
	// Use atomic to ensure we see any previous writes done by the thread
	// that updates the itabTable field (with atomic.Storep in itabAdd).
	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
	if m = t.find(inter, typ); m != nil {
		goto finish
	}

	// Not found.  Grab the lock and try again.
	lock(&itabLock)
	if m = itabTable.find(inter, typ); m != nil {
		unlock(&itabLock)
		goto finish
	}

	// Entry doesn't exist yet. Make a new entry & add it.
	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.Methods)-1)*goarch.PtrSize, 0, &memstats.other_sys))
	m.Inter = inter
	m.Type = typ
	// The hash is used in type switches. However, compiler statically generates itab's
	// for all interface/type pairs used in switches (which are added to itabTable
	// in itabsinit). The dynamically-generated itab's never participate in type switches,
	// and thus the hash is irrelevant.
	// Note: m.Hash is _not_ the hash used for the runtime itabTable hash table.
	m.Hash = 0
	itabInit(m, true)
	itabAdd(m)
	unlock(&itabLock)
finish:
	if m.Fun[0] != 0 {
		return m
	}
	if canfail {
		return nil
	}
	// this can only happen if the conversion
	// was already done once using the , ok form
	// and we have a cached negative result.
	// The cached result doesn't record which
	// interface function was missing, so initialize
	// the itab again to get the missing function name.
	panic(&TypeAssertionError{concrete: typ, asserted: &inter.Type, missingMethod: itabInit(m, false)})
}

// itabInit fills in the m.Fun array with all the code pointers for
// the m.Inter/m.Type pair. If the type does not implement the interface,
// it sets m.Fun[0] to 0 and returns the name of an interface function that is missing.
// If !firstTime, itabInit will not write anything to m.Fun (see issue 65962).
// It is ok to call this multiple times on the same m, even concurrently
// (although it will only be called once with firstTime==true).
func itabInit(m *itab, firstTime bool) string {
	inter := m.Inter
	typ := m.Type
	x := typ.Uncommon()

	// both inter and typ have method sorted by name,
	// and interface names are unique,
	// so can iterate over both in lock step;
	// the loop is O(ni+nt) not O(ni*nt).
	ni := len(inter.Methods)
	nt := int(x.Mcount)
	xmhdr := (*[1 << 16]abi.Method)(add(unsafe.Pointer(x), uintptr(x.Moff)))[:nt:nt]
	j := 0
	methods := (*[1 << 16]unsafe.Pointer)(unsafe.Pointer(&m.Fun[0]))[:ni:ni]
	var fun0 unsafe.Pointer
imethods:
	for k := 0; k < ni; k++ {
		i := &inter.Methods[k]
		itype := toRType(&inter.Type).typeOff(i.Typ)
		name := toRType(&inter.Type).nameOff(i.Name)
		iname := name.Name()
		ipkg := pkgPath(name)
		if ipkg == "" {
			ipkg = inter.PkgPath.Name()
		}
		for ; j < nt; j++ {
			t := &xmhdr[j]
			rtyp := toRType(typ)
			tname := rtyp.nameOff(t.Name)
			if rtyp.typeOff(t.Mtyp) == itype && tname.Name() == iname {
				pkgPath := pkgPath(tname)
				if pkgPath == "" {
					pkgPath = rtyp.nameOff(x.PkgPath).Name()
				}
				if tname.IsExported() || pkgPath == ipkg {
					ifn := rtyp.textOff(t.Ifn)
					if k == 0 {
						fun0 = ifn // we'll set m.Fun[0] at the end
					} else if firstTime {
						methods[k] = ifn
					}
					continue imethods
				}
			}
		}
		// didn't find method
		// Leaves m.Fun[0] set to 0.
		return iname
	}
	if firstTime {
		m.Fun[0] = uintptr(fun0)
	}
	return ""
}

iface {
    tab:  指向 (fmt.Stringer, MyInt) 的 itab,
    data: 指向 MyInt(42) 的内存地址
}

i.String()  // 调用接口方法

// 1. 从 iface 中取出 tab
tab := i.tab

// 2. 从 tab.Fun 中取出第一个方法（String 是第0个方法）
fn := tab.Fun[0]

// 3. 调用该方法，传入 data 作为接收者
return fn(i.data)

type eface struct {
	_type *_type
	data  unsafe.Pointer
}

if v, ok := i.(MyInt); ok {
    // 使用 v
}

// 对于 iface
if i.tab.Type == TypeOf(MyInt) {
    return *(*MyInt)(i.data), true
}
return zero, false

// 对于 eface
if i._type == TypeOf(MyInt) {
    return *(*MyInt)(i.data), true
}
return zero, false

type Type struct {
	Size_       uintptr
	PtrBytes    uintptr // number of (prefix) bytes in the type that can contain pointers
	Hash        uint32  // hash of type; avoids computation in hash tables
	TFlag       TFlag   // extra type information flags
	Align_      uint8   // alignment of variable with this type
	FieldAlign_ uint8   // alignment of struct field with this type
	Kind_       Kind    // enumeration for C
	// function for comparing objects of this type
	// (ptr to object A, ptr to object B) -> ==?
	Equal func(unsafe.Pointer, unsafe.Pointer) bool
	// GCData stores the GC type data for the garbage collector.
	// Normally, GCData points to a bitmask that describes the
	// ptr/nonptr fields of the type. The bitmask will have at
	// least PtrBytes/ptrSize bits.
	// If the TFlagGCMaskOnDemand bit is set, GCData is instead a
	// **byte and the pointer to the bitmask is one dereference away.
	// The runtime will build the bitmask if needed.
	// (See runtime/type.go:getGCMask.)
	// Note: multiple types may have the same value of GCData,
	// including when TFlagGCMaskOnDemand is set. The types will, of course,
	// have the same pointer layout (but not necessarily the same size).
	GCData    *byte
	Str       NameOff // string form
	PtrToThis TypeOff // type for pointer to this type, may be zero
}

type InterfaceType struct {
	Type
	PkgPath Name      // import path
	Methods []Imethod // sorted by hash
}

const itabInitSize = 512

var (
	itabLock      mutex                               // lock for accessing itab table
	itabTable     = &itabTableInit                    // pointer to current table
	itabTableInit = itabTableType{size: itabInitSize} // starter table
)

type itabTableType struct {
	size    uintptr             // length of entries array. Always a power of 2.
	count   uintptr             // current number of filled entries.
	entries [itabInitSize]*itab // really [size] large
}

// 手动写
func (t Truck) Drive() {

}

// Go 底层会帮我们自动写这个
func (t *Truck) Drive() {

}

// 手动写
func (t *Truck) Drive() {

}

/*
   Go 底层不会帮我们写这个
func (t Truck) Drive() {

}
*/

// nil is a predeclared identifier representing the zero value for a
// pointer, channel, func, interface, map, or slice type.
var nil Type // Type must be a pointer, channel, func, interface, map, or slice type

// Type is here for the purposes of documentation only. It is a stand-in
// for any Go type, but represents the same type for any given function
// invocation.
type Type int