cmd/gomobile: remove go/build usages from build.go

This CL is a pure refactoring. This removes a global variable ctx,
which is a build.Default.

Before this change, ctx was used to keep build tags and its state
affected go command executions. As the variable is mutable, the
code was not readable.

This changes introduces another global variable buildTags instead,
but this is more consistent with other build flags, and this is
immutable.

Updates golang/go#27234

Conflict: cmd/gomobile/build.go
Cherry picked from github.com/golang/mobile

cmd/fyne/internal/mobile/bind.go

@@ -15,7 +15,7 @@ import (
 	"golang.org/x/tools/go/packages"
 )
 
-// ctx, pkg, tmpdir in build.go
+// pkg, tmpdir in build.go
 
 func copyFile(dst, src string) error {
 	if buildX {
@@ -68,8 +68,12 @@ func writeFile(filename string, generate func(io.Writer) error) error {
 func packagesConfig(targetOS string) *packages.Config {
 	config := &packages.Config{}
 	config.Env = append(os.Environ(), "GOARCH=arm", "GOOS="+targetOS)
-	if len(ctx.BuildTags) > 0 {
-		config.BuildFlags = []string{"-tags=" + strings.Join(ctx.BuildTags, ",")}
+	tags := buildTags
+	if targetOS == "darwin" {
+		tags = append(tags, "ios")
+	}
+	if len(tags) > 0 {
+		config.BuildFlags = []string{"-tags=" + strings.Join(tags, ",")}
 	}
 	return config
 }

cmd/fyne/internal/mobile/build.go

@@ -21,7 +21,6 @@ import (
 	"golang.org/x/tools/go/packages"
 )
 
-var ctx = build.Default
 var tmpdir string
 
 var cmdBuild = &command{
@@ -93,21 +92,6 @@ func runBuildImpl(cmd *command) (*packages.Package, error) {	cleanup, err := bui
 		return nil, fmt.Errorf(`invalid -target=%q: %v`, buildTarget, err)
 	}
 
-	// TODO(hajimehoshi): ctx is now used only for recording build tags in build. Remove this.
-	oldCtx := ctx
-	defer func() {
-		ctx = oldCtx
-	}()
-
-	if targetOS == "darwin" {
-		ctx.BuildTags = append(ctx.BuildTags, "ios")
-
-		if buildRelease {
-			targetArchs = []string{"arm", "arm64"}
-			ctx.GOARCH = targetArchs[0]
-		}
-	}
-
 	var buildPath string
 	switch len(args) {
 	case 0:
@@ -157,6 +141,10 @@ func runBuildImpl(cmd *command) (*packages.Package, error) {	cleanup, err := bui
 		if !xcodeAvailable() {
 			return nil, fmt.Errorf("-target=ios requires XCode")
 		}
+		if buildRelease {
+			targetArchs = []string{"arm", "arm64"}
+		}
+
 		if pkg.Name != "main" {
 			for _, arch := range targetArchs {
 				env := darwinEnv[arch]
@@ -247,21 +235,22 @@ func printcmd(format string, args ...interface{}) {
 
 // "Build flags", used by multiple commands.
 var (
-	buildA          bool   // -a
-	buildI          bool   // -i
-	buildN          bool   // -n
-	buildV          bool   // -v
-	buildX          bool   // -x
-	buildO          string // -o
-	buildGcflags    string // -gcflags
-	buildLdflags    string // -ldflags
-	buildRelease    bool   // -release
-	buildTarget     string // -target
-	buildTrimpath   bool   // -trimpath
-	buildWork       bool   // -work
-	buildBundleID   string // -bundleid
-	buildIOSVersion string // -iosversion
-	buildAndroidAPI int    // -androidapi
+	buildA          bool        // -a
+	buildI          bool        // -i
+	buildN          bool        // -n
+	buildV          bool        // -v
+	buildX          bool        // -x
+	buildO          string      // -o
+	buildGcflags    string      // -gcflags
+	buildLdflags    string      // -ldflags
+	buildRelease    bool        // -release
+	buildTarget     string      // -target
+	buildTrimpath   bool        // -trimpath
+	buildWork       bool        // -work
+	buildBundleID   string      // -bundleid
+	buildIOSVersion string      // -iosversion
+	buildAndroidAPI int         // -androidapi
+	buildTags       stringsFlag // -tags
 )
 
 func RunNewBuild(target, appID, icon, name string, release bool) error {
@@ -288,7 +277,7 @@ func addBuildFlags(cmd *command) {
 	cmd.Flag.BoolVar(&buildA, "a", false, "")
 	cmd.Flag.BoolVar(&buildI, "i", false, "")
 	cmd.Flag.BoolVar(&buildTrimpath, "trimpath", false, "")
-	cmd.Flag.Var((*stringsFlag)(&ctx.BuildTags), "tags", "")
+	cmd.Flag.Var(&buildTags, "tags", "")
 }
 
 func addBuildFlagsNVXWork(cmd *command) {
@@ -323,8 +312,16 @@ func goCmd(subcmd string, srcs []string, env []string, args ...string) error {
 		goBin(),
 		subcmd,
 	)
-	if len(ctx.BuildTags) > 0 {
-		cmd.Args = append(cmd.Args, "-tags", strings.Join(ctx.BuildTags, " "))
+	tags := buildTags
+	targetOS, _, err := parseBuildTarget(buildTarget)
+	if err != nil {
+		return err
+	}
+	if targetOS == "darwin" {
+		tags = append(tags, "ios")
+	}
+	if len(tags) > 0 {
+		cmd.Args = append(cmd.Args, "-tags", strings.Join(tags, " "))
 	}
 	if buildV {
 		cmd.Args = append(cmd.Args, "-v")