cmd/dist, go/build: make cmd/dist generate cgoEnabled map for go/build

This reduces the amount of duplication. Now there is only one list
of platforms supporting cgo.

Update #12270.

Change-Id: I5dcd55cb6be7c5bb6ce560383c71d90ab1189dc9
Reviewed-on: https://go-review.googlesource.com/14278
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/build.go

@@ -463,6 +463,9 @@ var deptab = []struct {
 	{"runtime/internal/sys", []string{
 		"zversion.go",
 	}},
+	{"go/build", []string{
+		"zcgo.go",
+	}},
 }
 
 // depsuffix records the allowed suffixes for source files.
@@ -478,6 +481,7 @@ var gentab = []struct {
 }{
 	{"zdefaultcc.go", mkzdefaultcc},
 	{"zversion.go", mkzversion},
+	{"zcgo.go", mkzcgo},
 
 	// not generated anymore, but delete the file if we see it
 	{"enam.c", nil},
@@ -1061,9 +1065,9 @@ func cmdbootstrap() {
 	}
 }
 
-// Copied from go/build/build.go.
 // Cannot use go/build directly because cmd/dist for a new release
 // builds against an old release's go/build, which may be out of sync.
+// To reduce duplication, we generate the list for go/build from this.
 var cgoEnabled = map[string]bool{
 	"darwin/386":      true,
 	"darwin/amd64":    true,

src/cmd/dist/buildgo.go

@@ -4,7 +4,10 @@
 
 package main
 
-import "fmt"
+import (
+	"bytes"
+	"fmt"
+)
 
 /*
  * Helpers for building cmd/go and cmd/cgo.
@@ -37,3 +40,26 @@ func mkzdefaultcc(dir, file string) {
 	file = file[:i] + "c" + file[i:]
 	writefile(out, file, writeSkipSame)
 }
+
+// mkzcgo writes zcgo.go for go/build package:
+//
+//	package build
+//  var cgoEnabled = map[string]bool{}
+//
+// It is invoked to write go/build/zcgo.go.
+func mkzcgo(dir, file string) {
+	var buf bytes.Buffer
+
+	fmt.Fprintf(&buf,
+		"// auto generated by go tool dist\n"+
+			"\n"+
+			"package build\n"+
+			"\n"+
+			"var cgoEnabled = map[string]bool{\n")
+	for plat := range cgoEnabled {
+		fmt.Fprintf(&buf, "\t%q: true,\n", plat)
+	}
+	fmt.Fprintf(&buf, "}")
+
+	writefile(buf.String(), file, writeSkipSame)
+}

src/go/build/build.go

@@ -256,34 +256,6 @@ func (ctxt *Context) SrcDirs() []string {
 // if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
 var Default Context = defaultContext()
 
-// Also known to cmd/dist/build.go.
-var cgoEnabled = map[string]bool{
-	"darwin/386":      true,
-	"darwin/amd64":    true,
-	"darwin/arm":      true,
-	"darwin/arm64":    true,
-	"dragonfly/amd64": true,
-	"freebsd/386":     true,
-	"freebsd/amd64":   true,
-	"freebsd/arm":     true,
-	"linux/386":       true,
-	"linux/amd64":     true,
-	"linux/arm":       true,
-	"linux/arm64":     true,
-	"linux/ppc64le":   true,
-	"android/386":     true,
-	"android/amd64":   true,
-	"android/arm":     true,
-	"netbsd/386":      true,
-	"netbsd/amd64":    true,
-	"netbsd/arm":      true,
-	"openbsd/386":     true,
-	"openbsd/amd64":   true,
-	"solaris/amd64":   true,
-	"windows/386":     true,
-	"windows/amd64":   true,
-}
-
 func defaultContext() Context {
 	var c Context