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3.6.4. go1.16 (released 2021/02/16)

Most of its changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility.

Ports

Darwin and iOS:

adds support of 64-bit ARM architecture on macOS (also known as Apple Silicon)
    with `GOOS=darwin, GOARCH=arm64`.
The iOS port, which was previously darwin/arm64, has been renamed to `ios/arm64. GOOS=ios`
adds an `ios/amd64` port, which targets the iOS simulator running on AMD64-based macOS.

Go 1.16 is the last release that will run on macOS 10.12 Sierra

其他:

NetBSD
OpenBSD
386RISC-V
等详见文档

Tools

Go command

Modules:

1. 默认GO111MODULE=on
Module-aware mode is enabled by default, regardless of whether a go.mod file
    is present in the current working directory or a parent directory.
    More precisely, the GO111MODULE environment variable now defaults to on.
    To switch to the previous behavior, set GO111MODULE to auto.

2. go build或go test默认不再直接修改go.mod或go.sum
Build commands like go build and go test no longer modify go.mod and go.sum by default.
    Instead, they report an error if a module requirement or checksum needs to be added or updated
    (as if the -mod=readonly flag were used).
    Module requirements and sums may be adjusted with `go mod tidy or go get`

3. go install支持安装指定版本,如: go install example.com/cmd@v1.0.0
go install now accepts arguments with version suffixes (example, go install example.com/cmd@v1.0.0).
    This causes go install to build and install packages in module-aware mode,
      ignoring the go.mod file in the current directory or any parent directory, if there is one.
    This is useful for installing executables without affecting the dependencies of the main module.

4. 推荐使用go install命令build和install packages(原来go get方式废弃)
go install, with or without a version suffix (as described above), is now the recommended way
    to build and install packages in module mode.
  go get should be used with the -d flag to adjust the current module's dependencies
    without building packages,
    and use of go get to build and install packages is deprecated.
  In a future release, the -d flag will always be enabled.

5. 在go.mod中增加retract标签
retract directives may now be used in a go.mod file to indicate that
    certain published versions of the module should not be used by other modules.
  A module author may retract a version after a severe problem is discovered
    or if the version was published unintentionally.

6. 禁止相对路径import
In module mode, the go command now disallows import paths that
    include non-ASCII characters or path elements with a leading dot character (.).
  Module paths with these characters were already disallowed (see Module paths and versions), so this change affects only paths within module subdirectories.

Embedding Files:

The go command now supports including static files and file trees as part of the final executable,
    using the new `//go:embed` directive.

go test:

原来os:Exit(0)是程序终止，现在变成测试失败
When using go test, a test that calls os.Exit(0) during execution of a test function
    will now be considered to fail.
This will help catch cases in which a test calls code that calls os.Exit(0)
    and thereby stops running all future tests.
If a TestMain function calls os.Exit(0) that is still considered to be a passing test.

go get:

The go get -insecure flag is deprecated and will be removed in a future version.
替换:
使用环境变量: GOINSECURE
To bypass module sum validation, use GOPRIVATE or GONOSUMDB

环境变量:

GOVCS:
  By default,
  1. git and hg may be used to download code from any repository.
  2. svn, bzr, and fossil may only be used to download code from repositories
      with module paths or package paths matching patterns in the GOPRIVATE environment variable.
  See `go help vcs<https://golang.org/cmd/go/#hdr-Controlling_version_control_with_GOVCS>`_ for details.

  其格式为:
  GOVCS=<module prefix>:<tool name>,[<module prefix>:<tool name>, ...]
  实例:
  GOVCS=github.com:git,evil.com:off,*:git|hg

Runtime

GODEBUG——inittrace=1:

为 GODEBUG 环境变量增加了 inittrace=1 选项
可以输出 init 函数的执行时间和内存消耗(不要在init()中处理耗时的事情)

例:
package main

func init() {
  println("hello")
}

func main() {
  println("bye")
}
运行:
$ GODEBUG=inittrace=1 go run main.go
init internal/bytealg @0.018 ms, 0 ms clock, 0 bytes, 0 allocs
init runtime @0.15 ms, 0.56 ms clock, 0 bytes, 0 allocs
init errors @1.7 ms, 0.010 ms clock, 0 bytes, 0 allocs
init sync @1.8 ms, 0.001 ms clock, 16 bytes, 1 allocs
init internal/oserror @1.8 ms, 0.001 ms clock, 80 bytes, 5 allocs
init syscall @1.9 ms, 0.010 ms clock, 944 bytes, 2 allocs
init time @1.9 ms, 0.014 ms clock, 400 bytes, 8 allocs
init context @2.0 ms, 0.002 ms clock, 128 bytes, 4 allocs
init math @2.0 ms, 0 ms clock, 0 bytes, 0 allocs
init strconv @2.1 ms, 0.006 ms clock, 32 bytes, 2 allocs
init unicode @2.1 ms, 0.086 ms clock, 23944 bytes, 27 allocs
...
init internal/bytealg @0 ms, 0 ms clock, 0 bytes, 0 allocs
init runtime @0.048 ms, 0.042 ms clock, 0 bytes, 0 allocs
hello
init main @0.21 ms, 0.004 ms clock, 0 bytes, 0 allocs
bye

GODEBUG——madvdontneed:

废弃: 原来依靠配置 GODEBUG=madvdontneed=1 来改善内存监控行为不再需要

本版本在 Linux 上，runtime 现在默认会迅速地（使用 MADV_DONTNEED）向操作系统释放内存，
    而不是在操作系统面临内存压力时（使用 MADV_FREE）惰性地释放内存。
On Linux, the runtime now defaults to
    releasing memory to the operating system promptly (using MADV_DONTNEED),
    rather than lazily when the operating system is under memory pressure (using MADV_FREE).

这意味着像 RSS 这样的进程级内存统计信息将更准确地反映 Go 进程所使用的物理内存数量
This means process-level memory statistics like RSS will more accurately reflect
    the amount of physical memory being used by Go processes.

因此以后不需要 GODEBUG=madvdontneed=1 配置了
Systems that are currently using GODEBUG=madvdontneed=1 to
    improve memory monitoring behavior no longer need to set this environment variable.

• 列举一些 Go1.16 中可能对大家有影响的变化: https://polarisxu.studygolang.com/posts/go/dynamic/go1.16-changes-2020/

• GODEBUG documentation: https://golang.org/pkg/runtime/#hdr-Environment_Variables

Linker

For a representative set of large Go programs, linking is 20-25% faster than 1.15 and requires 5-15% less memory on average for linux/amd64

Core library

Embedded Files:

详见下面的``内嵌资源文件``
对应`embed`库

File Systems:

新增`io/fs`库
模仿 Linux 的 vfs 做一套基于 fs 的 io 接口
目的有三个:
  1. os 包应该专注于和系统交互而不是包含一部分 io 接口
  2. io 包和 os 包分别包含了 io 接口的一部分，导致互相依赖职责不清晰
  3. 可以把有关联的一部分文件或者数据组成虚拟文件系统，供通用接口处理提升程序的可扩展性，比如 zip 打包的文件

fs 包中主要包含了下面几种数据类型（都是接口类型）:
名称              作用
FS              文件系统的抽象，有一个 Open 方法用来从 FS 打开获取文件数据
DirEntry        描述目录项目（包含目录自身）的数据结构
File            描述文件数据的结构，包含 Stat，Read，Close 方法
ReadDirFile     在 File 的基础上支持 ReadDir，可以代表目录自身
FileMode        描述文件类型，比如是通常文件还是套接字或者是管道
FileInfo        文件的元数据，例如创建时间等

Deprecation of io/ioutil:

废弃` io/ioutil`package，把相关功能迁移到`os和io`package

Discard => io.Discard
NopCloser => io.NopCloser
ReadAll => io.ReadAll
ReadDir => os.ReadDir
ReadFile => os.ReadFile
TempDir => os.MkdirTemp
TempFile => os.CreateTemp
WriteFile => os.WriteFile

警告

io/ioutil库废弃需要做迁移

template 允许跨行。例如:

{{"hello" |
   printf}}

简化的结构体字段标签写法:

原来:
type MyStruct struct {
  Field1 string `json:"field_1,omitempty" bson:"field_1,omitempty" xml:"field_1,omitempty"`
}
现在:
type MyStruct struct {
  Field1 string `json bson xml:"field_1,omitempty"`
}

切片扩容策略:

一个 append(x, values...) 调用需要扩容，则基础切片 x 的容量将决定结果切片的容量
（而不是原来的基础切片 x 的长度决定结果切片的容量

实例:
package main
func main() {
  const N = 1024
  var a [N]int
  x := append(a[:N-1:N], 0, 9)
  y := append(a[:N:N], 9)
  println(cap(x), cap(y))
}

结果:
Go 1.15 将输出 2048 1280，但是使用 Go 1.16 将输出 1280 1280。

• 相关代码改动

内嵌资源文件

原来使用``go-bindata``之类的工具，把内嵌资源文件(如html等)嵌入到``.go``文件中。这样做的缺点也很明显，生成的文件大，需要依赖第三方生成器。

实例:

package main

import (
    "fmt"
    _ "embed"
)

//go:embed test.tpl
var tpl []byte

func main() {
    fmt.Println(tpl)
}

参考

• Go 1.16 Release Notes: https://golang.org/doc/go1.16

• embed package documentation(开发): https://pkg.go.dev/embed

• embed package documentation: https://golang.org/pkg/embed/

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