cmd/go: fix incorrect determining default value of CGO_ENABLED

The default value is the value obtained when
no environment variables are set and go env  -w is not used.

In the past,
we used the current value
(may be modified by an environment variable to a non-default value),
error was used as the default value.

For #69994

Change-Id: Iead3a6cacd04dc51a094ffb9f7bb7553320fcd78
Reviewed-on: https://go-review.googlesource.com/c/go/+/621995
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/dist/build.go

@@ -673,7 +673,6 @@ var gentab = []struct {
 	file string
 	gen  func(dir, file string)
 }{
-	{"go/build", "zcgo.go", mkzcgo},
 	{"cmd/go/internal/cfg", "zdefaultcc.go", mkzdefaultcc},
 	{"internal/runtime/sys", "zversion.go", mkzversion},
 	{"time/tzdata", "zzipdata.go", mktzdata},

src/cmd/dist/buildgo.go

@@ -7,7 +7,6 @@ package main
 import (
 	"fmt"
 	"io"
-	"os"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -108,22 +107,6 @@ func defaultCCFunc(name string, defaultcc map[string]string) string {
 	return buf.String()
 }
 
-// mkzcgo writes zcgo.go for the go/build package:
-//
-//	package build
-//	const defaultCGO_ENABLED = <CGO_ENABLED>
-//
-// It is invoked to write go/build/zcgo.go.
-func mkzcgo(dir, file string) {
-	var buf strings.Builder
-	writeHeader(&buf)
-	fmt.Fprintf(&buf, "package build\n")
-	fmt.Fprintln(&buf)
-	fmt.Fprintf(&buf, "const defaultCGO_ENABLED = %s\n", quote(os.Getenv("CGO_ENABLED")))
-
-	writefile(buf.String(), file, writeSkipSame)
-}
-
 // mktzdata src/time/tzdata/zzipdata.go:
 //
 //	package tzdata

src/cmd/dist/buildruntime.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"fmt"
+	"os"
 	"strings"
 )
 
@@ -66,6 +67,7 @@ func mkbuildcfg(file string) {
 	fmt.Fprintf(&buf, "const defaultGOOS = runtime.GOOS\n")
 	fmt.Fprintf(&buf, "const defaultGOARCH = runtime.GOARCH\n")
 	fmt.Fprintf(&buf, "const DefaultGOFIPS140 = `%s`\n", gofips140)
+	fmt.Fprintf(&buf, "const DefaultCGO_ENABLED = %s\n", quote(os.Getenv("CGO_ENABLED")))
 
 	writefile(buf.String(), file, writeSkipSame)
 }

src/cmd/go/internal/cfg/cfg.go

@@ -13,6 +13,7 @@ import (
 	"go/build"
 	"internal/buildcfg"
 	"internal/cfg"
+	"internal/platform"
 	"io"
 	"io/fs"
 	"os"
@@ -140,10 +141,12 @@ func defaultContext() build.Context {
 	// Recreate that logic here with the new GOOS/GOARCH setting.
 	// We need to run steps 2 and 3 to determine what the default value
 	// of CgoEnabled would be for computing CGOChanged.
-	defaultCgoEnabled := ctxt.CgoEnabled
-	if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
-		defaultCgoEnabled = false
-	} else {
+	defaultCgoEnabled := false
+	if buildcfg.DefaultCGO_ENABLED == "1" {
+		defaultCgoEnabled = true
+	} else if buildcfg.DefaultCGO_ENABLED == "0" {
+	} else if runtime.GOARCH == ctxt.GOARCH && runtime.GOOS == ctxt.GOOS {
+		defaultCgoEnabled = platform.CgoSupported(ctxt.GOOS, ctxt.GOARCH)
 		// Use built-in default cgo setting for GOOS/GOARCH.
 		// Note that ctxt.GOOS/GOARCH are derived from the preference list
 		// (1) environment, (2) go/env file, (3) runtime constants,

src/cmd/go/testdata/script/env_changed.txt

@@ -11,6 +11,7 @@ env GO111MODULE=auto
 env CGO_CFLAGS=nodefault
 env CGO_CPPFLAGS=nodefault
 env GOFIPS140=latest
+[cgo] env CGO_ENABLED=0
 
 go env -changed
 # linux output like GOTOOLCHAIN='local'
@@ -22,6 +23,7 @@ stdout 'GO111MODULE=''?auto''?'
 stdout 'CGO_CFLAGS=''?nodefault''?'
 stdout 'CGO_CPPFLAGS=''?nodefault''?'
 stdout 'GOFIPS140=''?latest''?'
+[cgo] stdout 'CGO_ENABLED=''?0''?'
 
 go env -changed -json
 stdout '"GOTOOLCHAIN": "local"'
@@ -31,6 +33,7 @@ stdout '"GO111MODULE": "auto"'
 stdout '"CGO_CFLAGS": "nodefault"'
 stdout '"CGO_CPPFLAGS": "nodefault"'
 stdout '"GOFIPS140": "latest"'
+[cgo] stdout '"CGO_ENABLED": "0"'
 
 [GOOS:windows] env GOOS=linux
 [!GOOS:windows] env GOOS=windows

src/go/build/build.go

@@ -359,7 +359,7 @@ func defaultContext() Context {
 
 	env := os.Getenv("CGO_ENABLED")
 	if env == "" {
-		env = defaultCGO_ENABLED
+		env = buildcfg.DefaultCGO_ENABLED
 	}
 	switch env {
 	case "1":