cmd/go: add -json flag to go env

"go env" prints Go environment information as a shell script format by
default but it's difficult for some tools (e.g. editor packages) to
interpret it.

The -json flag prints the environment in JSON format which
can be easily interpreted by a lot of tools.

$ go env -json
{
        "CC": "gcc",
        "CGO_CFLAGS": "-g -O2",
        "CGO_CPPFLAGS": "",
        "CGO_CXXFLAGS": "-g -O2",
        "CGO_ENABLED": "1",
        "CGO_FFLAGS": "-g -O2",
        "CGO_LDFLAGS": "-g -O2",
        "CXX": "g++",
        "GCCGO": "gccgo",
        "GOARCH": "amd64",
        "GOBIN": "/home/haya14busa/go/bin",
        "GOEXE": "",
        "GOGCCFLAGS": "-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build498013955=/tmp/go-build -gno-record-gcc-switches",
        "GOHOSTARCH": "amd64",
        "GOHOSTOS": "linux",
        "GOOS": "linux",
        "GOPATH": "/home/haya14busa",
        "GORACE": "",
        "GOROOT": "/home/haya14busa/src/go.googlesource.com/go",
        "GOTOOLDIR": "/home/haya14busa/src/go.googlesource.com/go/pkg/tool/linux_amd64",
        "PKG_CONFIG": "pkg-config"
}

Also, it supports arguments with -json flag.

$ go env -json GOROOT GOPATH GOBIN
{
        "GOBIN": "/home/haya14busa/go/bin",
        "GOPATH": "/home/haya14busa",
        "GOROOT": "/home/haya14busa/src/go.googlesource.com/go"
}

Fixes #12567

Change-Id: I75db3780f14a8ab8c7fa58cc3c9cc488ef7b66a1
Reviewed-on: https://go-review.googlesource.com/38757Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/envcmd/env.go

@@ -6,6 +6,7 @@
 package envcmd
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"runtime"
@@ -18,8 +19,7 @@ import (
 )
 
 var CmdEnv = &base.Command{
-	Run:       runEnv,
-	UsageLine: "env [var ...]",
+	UsageLine: "env [-json] [var ...]",
 	Short:     "print Go environment information",
 	Long: `
 Env prints Go environment information.
@@ -28,9 +28,18 @@ By default env prints information as a shell script
 (on Windows, a batch file).  If one or more variable
 names is given as arguments,  env prints the value of
 each named variable on its own line.
+
+The -json flag prints the environment in JSON format
+instead of as a shell script.
 	`,
 }
 
+func init() {
+	CmdEnv.Run = runEnv // break init cycle
+}
+
+var envJson = CmdEnv.Flag.Bool("json", false, "")
+
 func MkEnv() []cfg.EnvVar {
 	var b work.Builder
 	b.Init()
@@ -107,12 +116,26 @@ func runEnv(cmd *base.Command, args []string) {
 	env := cfg.CmdEnv
 	env = append(env, ExtraEnvVars()...)
 	if len(args) > 0 {
-		for _, name := range args {
-			fmt.Printf("%s\n", findEnv(env, name))
+		if *envJson {
+			var es []cfg.EnvVar
+			for _, name := range args {
+				e := cfg.EnvVar{Name: name, Value: findEnv(env, name)}
+				es = append(es, e)
+			}
+			printEnvAsJSON(es)
+		} else {
+			for _, name := range args {
+				fmt.Printf("%s\n", findEnv(env, name))
+			}
 		}
 		return
 	}
 
+	if *envJson {
+		printEnvAsJSON(env)
+		return
+	}
+
 	for _, e := range env {
 		if e.Name != "TERM" {
 			switch runtime.GOOS {
@@ -138,3 +161,18 @@ func runEnv(cmd *base.Command, args []string) {
 		}
 	}
 }
+
+func printEnvAsJSON(env []cfg.EnvVar) {
+	m := make(map[string]string)
+	for _, e := range env {
+		if e.Name == "TERM" {
+			continue
+		}
+		m[e.Name] = e.Value
+	}
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "\t")
+	if err := enc.Encode(m); err != nil {
+		base.Fatalf("%s", err)
+	}
+}