深入Golang的接口interface

type eface struct {
    // _type 指向接口的动态类型元数据
    // 描述了实体类型、包括内存对齐方式、大小等
	_type *_type
    // data 指向接口的动态值
	data  unsafe.Pointer
}

type iface struct {
    // tab 接口表指针，指向一个itab实体，存储方法列表和接口动态类型信息
	tab  *itab
    // data 指向接口的动态值（一般是指向堆内存的）
	data unsafe.Pointer
}
// layout of Itab known to compilers
// allocated in non-garbage-collected memory
// Needs to be in sync with
// ../cmd/compile/internal/gc/reflect.go:/^func.dumptabs.
type itab struct {
    // inter 指向interface的类型元数据，描述了接口的类型
	inter *interfacetype
    // _type 描述了实体类型、包括内存对齐方式、大小等
	_type *_type
    // hash 从动态类型元数据中拷贝的hash值，用于快速判断类型是否相等
	hash  uint32 // copy of _type.hash. Used for type switches.
	_     [4]byte
    // fun 记录动态类型实现的那些接口方法的地址
    // 存储的是第一个方法的函数指针
    // 这些方法是按照函数名称的字典序进行排列的
	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
}

// interfacetype 接口的类型元数据
type interfacetype struct {
	typ     _type
    // 记录定义了接口的包名
	pkgpath name
    // mhdr 标识接口所定义的函数列表
	mhdr    []imethod
}

// Note: change the formula in the mallocgc call in itabAdd if you change these fields.
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
}
func itabHashFunc(inter *interfacetype, typ *_type) uintptr {
	// compiler has provided some good hash codes for us.
    // 用接口类型hash和动态类型hash进行异或
	return uintptr(inter.typ.hash ^ typ.hash)
}

type iface struct{
    itab, data uintptr
}
func main() {
	var a interface{} = nil
	var b interface{} = (*int)(nil)
	x := 5
	var c interface{} = (*int)(&x)
	ia := *(*iface)(unsafe.Pointer(&a))
	ib := *(*iface)(unsafe.Pointer(&b))
	ic := *(*iface)(unsafe.Pointer(&c))
	fmt.Println(ia, ib, ic)
	fmt.Println(*(*int)(unsafe.Pointer(ic.data)))
}
// 输出
// {0 0} {17426912 0} {17426912 842350714568}
// 5

// 检查 *myWriter 类型是否实现了 io.Writer 接口
var _ io.Writer = (*myWriter)(nil)
// 检查 myWriter 类型是否实现了 io.Writer 接口
var _ io.Writer = myWriter{}

// 接口间的赋值实际上是调用了runtime.convI2I
// 实际上是要找到新interface的tab和data
// convI2I returns the new itab to be used for the destination value
// when converting a value with itab src to the dst interface.
func convI2I(dst *interfacetype, src *itab) *itab {
	if src == nil {
		return nil
	}
	if src.inter == dst {
		return src
	}
	return getitab(dst, src._type, false)
}

// 关键函数，获取itab
// getitab根据interfacetype和_type去全局的itab哈希表中查找，如果找到了直接返回
// 否则根据inter和typ新生成一个itab，插入到全局itab哈希表中
// 
// 查找了两次，第二次上锁了，目的是可能会写入hash表，阻塞其余协程的第二次查找
func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
	// 函数列表为空
    if len(inter.mhdr) == 0 {
		throw("internal error - misuse of itab")
	}

	// easy case
	if typ.tflag&tflagUncommon == 0 {
		if canfail {
			return nil
		}
		name := inter.typ.nameOff(inter.mhdr[0].name)
		panic(&TypeAssertionError{nil, typ, &inter.typ, 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).
    // 使用原子性去保证我们能看见在该线程之前的任意写操作
    // 确保更新全局hash表的字段
	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
    // 遍历一次，找到了就返回
	if m = t.find(inter, typ); m != nil {
		goto finish
	}
    // 没找到就上锁，再试一次
	// Not found.  Grab the lock and try again.
	lock(&itabLock)
	if m = itabTable.find(inter, typ); m != nil {
		unlock(&itabLock)
		goto finish
	}
    // hash表中没找到itab就新生成一个itab
	// Entry doesn't exist yet. Make a new entry & add it.
	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-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
	m.init()
    // 加到全局的hash表中
	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.typ, missingMethod: m.init()})
}

// 查找全局的hash表，有没有itab
// find finds the given interface/type pair in t.
// Returns nil if the given interface/type pair isn't present.
func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab {
	// Implemented using quadratic probing.
	// Probe sequence is h(i) = h0 + i*(i+1)/2 mod 2^k.
	// We're guaranteed to hit all table entries using this probe sequence.
	mask := t.size - 1
    // 根据inter，typ算出hash值
	h := itabHashFunc(inter, typ) & mask
	for i := uintptr(1); ; i++ {
		p := (**itab)(add(unsafe.Pointer(&t.entries), h*goarch.PtrSize))
		// Use atomic read here so if we see m != nil, we also see
		// the initializations of the fields of m.
		// m := *p
		m := (*itab)(atomic.Loadp(unsafe.Pointer(p)))
		if m == nil {
			return nil
		}
        // inter和typ指针都相同
		if m.inter == inter && m._type == typ {
			return m
		}
		h += i
		h &= mask
	}
}
// 核心函数。填充itab
// 检查_type是否符合interface_type并且创建对应的itab结构体将其放到hash表中
// init 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.
// It is ok to call this multiple times on the same m, even concurrently.
func (m *itab) init() 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).
    // inter和typ的方法都按方法名称进行排序
    // 并且方法名是唯一的，因此循环次数的固定的
    // 复杂度为O(ni+nt)，而不是O(ni*nt)
	ni := len(inter.mhdr)
	nt := int(x.mcount)
	xmhdr := (*[1 << 16]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.mhdr[k]
		itype := inter.typ.typeOff(i.ityp)
		name := inter.typ.nameOff(i.name)
		iname := name.name()
		ipkg := name.pkgPath()
		if ipkg == "" {
			ipkg = inter.pkgpath.name()
		}
        // 第二层循环是从上一次遍历到的位置开始的
		for ; j < nt; j++ {
			t := &xmhdr[j]
			tname := typ.nameOff(t.name)
            // 检查方法名字是否一致
			if typ.typeOff(t.mtyp) == itype && tname.name() == iname {
				pkgPath := tname.pkgPath()
				if pkgPath == "" {
					pkgPath = typ.nameOff(x.pkgpath).name()
				}
				if tname.isExported() || pkgPath == ipkg {
					if m != nil {
                        // 获取函数地址，放入itab.func数组中
						ifn := typ.textOff(t.ifn)
						if k == 0 {
							fun0 = ifn // we'll set m.fun[0] at the end
						} else {
							methods[k] = ifn
						}
					}
					continue imethods
				}
			}
		}
		// didn't find method
		m.fun[0] = 0
		return iname
	}
	m.fun[0] = uintptr(fun0)
	return ""
}

// 检查是否需要扩容，并调用方法将itab存入
// itabAdd adds the given itab to the itab hash table.
// itabLock must be held.
func itabAdd(m *itab) {
	// Bugs can lead to calling this while mallocing is set,
	// typically because this is called while panicing.
	// Crash reliably, rather than only when we need to grow
	// the hash table.
	if getg().m.mallocing != 0 {
		throw("malloc deadlock")
	}

	t := itabTable
    // 检查是否需要扩容
	if t.count >= 3*(t.size/4) { // 75% load factor
		// Grow hash table.
		// t2 = new(itabTableType) + some additional entries
		// We lie and tell malloc we want pointer-free memory because
		// all the pointed-to values are not in the heap.
		t2 := (*itabTableType)(mallocgc((2+2*t.size)*goarch.PtrSize, nil, true))
		t2.size = t.size * 2

		// Copy over entries.
		// Note: while copying, other threads may look for an itab and
		// fail to find it. That's ok, they will then try to get the itab lock
		// and as a consequence wait until this copying is complete.
		iterate_itabs(t2.add)
		if t2.count != t.count {
			throw("mismatched count during itab table copy")
		}
		// Publish new hash table. Use an atomic write: see comment in getitab.
		atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
		// Adopt the new table as our own.
		t = itabTable
		// Note: the old table can be GC'ed here.
	}
    // 核心函数
	t.add(m)
}

// 核心函数
// add adds the given itab to itab table t.
// itabLock must be held.
func (t *itabTableType) add(m *itab) {
	// See comment in find about the probe sequence.
	// Insert new itab in the first empty spot in the probe sequence.
    // 在探针序列第一个空白点插入itab
	mask := t.size - 1
    // 计算hash值
	h := itabHashFunc(m.inter, m._type) & mask
	for i := uintptr(1); ; i++ {
		p := (**itab)(add(unsafe.Pointer(&t.entries), h*goarch.PtrSize))
		m2 := *p
		if m2 == m {
            // itab已被插入
			// A given itab may be used in more than one module
			// and thanks to the way global symbol resolution works, the
			// pointed-to itab may already have been inserted into the
			// global 'hash'.
			return
		}
		if m2 == nil {
			// Use atomic write here so if a reader sees m, it also
			// sees the correctly initialized fields of m.
			// NoWB is ok because m is not in heap memory.
			// *p = m
            // 使用原子操作确保其余的goroutine下次查找的时候可以看到他
			atomic.StorepNoWB(unsafe.Pointer(p), unsafe.Pointer(m))
			t.count++
			return
		}
		h += i
		h &= mask
	}
}

// hash函数，编译期间提供较好的hash codes
func itabHashFunc(inter *interfacetype, typ *_type) uintptr {
	// compiler has provided some good hash codes for us.
	return uintptr(inter.typ.hash ^ typ.hash)
}