cmd/go2go: add ability to specify build tags in go2go

You can specify build tags, so that imports properly work during translation,
by using the `-tags` option, e.g. `go tool go2go run -tags=appengine x.go2`.
The `-tags` option is available for all the `go2go` sub-commands.

Change-Id: Ib60e7542b10c6a561b61db23d35592b2bc7f63cd
GitHub-Last-Rev: 954cccfae819aaac1cb78b97905e2a97dfdcefa7
GitHub-Pull-Request: golang/go#45147
Reviewed-on: https://go-review.googlesource.com/c/go/+/303275
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

4 files changed, 277 insertions, 9 deletions

diff --git a/src/cmd/go2go/doc.go b/src/cmd/go2go/doc.go
index c78fd9d6a4..059b91b0bb 100644
--- a/src/cmd/go2go/doc.go
+++ b/src/cmd/go2go/doc.go
@@ -7,7 +7,7 @@
 //
 // Usage:
 //
-//	go2go [options] <command> [arguments]
+//	go2go [options] <command> [go2go flags] [arguments]
 //
 // Commands:
 //
@@ -25,6 +25,13 @@
 // imports will be looked up in the usual way. If an import includes
 // .go2 files, they will be translated into .go files.
 //
+// The go2go flags are shared by the build, run, test, and translate commands:
+//
+//	-tags tag,list
+//		a comma-separated list of build tags to consider satisfied during the
+//		build. For more information about build tags, see the description of
+//		build constraints in the documentation for the go/build package.
+//
 // There is a sample GO2PATH in cmd/go2go/testdata/go2path. It provides
 // several packages that serve as examples of using generics, and may
 // be useful in experimenting with your own generic code.
diff --git a/src/cmd/go2go/go2go_test.go b/src/cmd/go2go/go2go_test.go
index 296fa3edb7..4d78c271ba 100644
--- a/src/cmd/go2go/go2go_test.go
+++ b/src/cmd/go2go/go2go_test.go
@@ -321,3 +321,66 @@ func TestTransitiveGo1(t *testing.T) {
 		t.Fatalf(`error running "go2go build": %v`, err)
 	}
 }
+
+func TestBuildWithTags(t *testing.T) {
+	t.Parallel()
+	buildGo2go(t)
+
+	gopath := t.TempDir()
+	testFiles{
+		{
+			"a/a.go2",
+			`package a; func ident[T any](v T) T { return v }; func F1(s string) string { return ident(s) }`,
+		},
+		{
+			"b/b_appengine.go",
+			`// +build appengine
+
+package b; import "a"; func F2(s string) string { return a.F1(s) + " App Engine!" }`,
+		},
+		{
+			"b/b.go",
+			`// +build !appengine
+
+package b; import "a"; func F2(s string) string { return a.F1(s) + " World!" }`,
+		},
+		{
+			"c/c.go2",
+			`package main; import ("fmt"; "b"); func main() { fmt.Println(b.F2("Hello")) }`,
+		},
+	}.create(t, gopath)
+
+	t.Log("go2go build")
+	cmd := exec.Command(testGo2go, "build", "-tags=appengine", "c")
+	cmd.Dir = gopath
+	cmd.Env = append(os.Environ(),
+		"GO2PATH="+gopath,
+	)
+	out, err := cmd.CombinedOutput()
+	if len(out) > 0 {
+		t.Logf("%s", out)
+	}
+	if err != nil {
+		t.Fatalf(`error running "go2go build": %v`, err)
+	}
+
+	cmdName := "./c"
+	if runtime.GOOS == "windows" {
+		cmdName += ".exe"
+	}
+	cmd = exec.Command(cmdName)
+	cmd.Dir = gopath
+	out, err = cmd.CombinedOutput()
+	t.Log("./c")
+	if len(out) > 0 {
+		t.Logf("%s", out)
+	}
+	if err != nil {
+		t.Fatalf("error running c: %v", err)
+	}
+	got := strings.Split(strings.TrimSpace(string(out)), "\n")[0]
+	want := "Hello App Engine!"
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("c output %v, want %v", got, want)
+	}
+}
diff --git a/src/cmd/go2go/main.go b/src/cmd/go2go/main.go
index a3543613ab..d778b4502f 100644
--- a/src/cmd/go2go/main.go
+++ b/src/cmd/go2go/main.go
@@ -26,6 +26,26 @@ var cmds = map[string]bool{
 	"translate": true,
 }
 
+// tagsFlag is the implementation of the -tags flag.
+type tagsFlag []string
+
+var buildTags tagsFlag
+
+func (v *tagsFlag) Set(s string) error {
+	// Split on commas, ignore empty strings.
+	*v = []string{}
+	for _, s := range strings.Split(s, ",") {
+		if s != "" {
+			*v = append(*v, s)
+		}
+	}
+	return nil
+}
+
+func (v *tagsFlag) String() string {
+	return strings.Join(*v, ",")
+}
+
 func main() {
 	flag.Usage = usage
 	flag.Parse()
@@ -38,6 +58,13 @@ func main() {
 	if !cmds[args[0]] {
 		usage()
 	}
+	cmd := args[0]
+
+	fs := flag.NewFlagSet("", flag.ContinueOnError)
+	fs.Var((*tagsFlag)(&buildTags), "tags", "tag,list")
+	fs.Parse(args[1:])
+
+	args = fs.Args()
 
 	importerTmpdir, err := ioutil.TempDir("", "go2go")
 	if err != nil {
@@ -47,30 +74,42 @@ func main() {
 
 	importer := go2go.NewImporter(importerTmpdir)
 
+	if len(buildTags) > 0 {
+		importer.SetTags(buildTags)
+	}
+
 	var rundir string
-	if args[0] == "run" {
-		tmpdir := copyToTmpdir(args[1:])
+	if cmd == "run" {
+		tmpdir := copyToTmpdir(args)
 		defer os.RemoveAll(tmpdir)
 		translate(importer, tmpdir)
 		nargs := []string{"run"}
-		for _, arg := range args[1:] {
+		for _, arg := range args {
 			base := filepath.Base(arg)
 			f := strings.TrimSuffix(base, ".go2") + ".go"
 			nargs = append(nargs, f)
 		}
 		args = nargs
 		rundir = tmpdir
-	} else if args[0] == "translate" && isGo2Files(args[1:]...) {
-		for _, arg := range args[1:] {
+	} else if cmd == "translate" && isGo2Files(args...) {
+		for _, arg := range args {
 			translateFile(importer, arg)
 		}
 	} else {
-		for _, dir := range expandPackages(args[1:]) {
+		for _, dir := range expandPackages(args) {
 			translate(importer, dir)
 		}
 	}
 
-	if args[0] != "translate" {
+	if cmd != "translate" {
+		if len(buildTags) > 0 {
+			args = append([]string{
+				fmt.Sprintf("-tags=%s", strings.Join(buildTags, ",")),
+			}, args...)
+		}
+
+		args = append([]string{cmd}, args...)
+
 		cmd := exec.Command(gotool, args...)
 		cmd.Stdin = os.Stdin
 		cmd.Stdout = os.Stdout
diff --git a/src/go/go2go/importer.go b/src/go/go2go/importer.go
index 6bd2e8b602..c73dc7ba82 100644
--- a/src/go/go2go/importer.go
+++ b/src/go/go2go/importer.go
@@ -5,6 +5,7 @@
 package go2go
 
 import (
+	"bytes"
 	"fmt"
 	"go/ast"
 	"go/build"
@@ -17,10 +18,12 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"runtime"
 	"sort"
 	"strings"
+	"unicode"
 )
 
 // Importer implements the types.ImporterFrom interface.
@@ -58,6 +61,9 @@ type Importer struct {
 	// since it doesn't deal with import information,
 	// but Importer is a useful common location to store the data.
 	instantiations map[*types.Package]*instantiations
+
+	// build tags
+	tags map[string]bool
 }
 
 var _ types.ImporterFrom = &Importer{}
@@ -81,6 +87,13 @@ func NewImporter(tmpdir string) *Importer {
 		idToFunc:        make(map[types.Object]*ast.FuncDecl),
 		idToTypeSpec:    make(map[types.Object]*ast.TypeSpec),
 		instantiations:  make(map[*types.Package]*instantiations),
+		tags:            make(map[string]bool),
+	}
+}
+
+func (imp *Importer) SetTags(tags []string) {
+	for _, tag := range tags {
+		imp.tags[tag] = true
 	}
 }
 
@@ -217,7 +230,11 @@ func (imp *Importer) importGo1Package(importPath, dir string, mode types.ImportM
 
 	fset := token.NewFileSet()
 	filter := func(fi os.FileInfo) bool {
-		return !strings.HasSuffix(fi.Name(), "_test.go")
+		name := fi.Name()
+		if strings.HasSuffix(name, "_test.go") {
+			return false
+		}
+		return imp.shouldInclude(path.Join(pdir, name))
 	}
 	pkgs, err := parser.ParseDir(fset, pdir, filter, 0)
 	if err != nil {
@@ -412,3 +429,145 @@ func (imp *Importer) gatherTransitiveImports(path string, m map[string]bool) []s
 	sort.Strings(r)
 	return r
 }
+
+var slashslash = []byte("//")
+
+// shouldInclude reports whether it is okay to use this file,
+// The rule is that in the file's leading run of // comments
+// and blank lines, which must be followed by a blank line
+// (to avoid including a Go package clause doc comment),
+// lines beginning with '// +build' are taken as build directives.
+//
+// The file is accepted only if each such line lists something
+// matching the file. For example:
+//
+//	// +build windows linux
+//
+// marks the file as applicable only on Windows and Linux.
+//
+// If tags["*"] is true, then ShouldBuild will consider every
+// build tag except "ignore" to be both true and false for
+// the purpose of satisfying build tags, in order to estimate
+// (conservatively) whether a file could ever possibly be used
+// in any build.
+//
+// This code was copied from the go command internals.
+func (imp *Importer) shouldInclude(path string) bool {
+	content, err := os.ReadFile(path)
+	if err != nil {
+		return false
+	}
+
+	// Pass 1. Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	end := 0
+	p := content
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 { // Blank line
+			end = len(content) - len(p)
+			continue
+		}
+		if !bytes.HasPrefix(line, slashslash) { // Not comment line
+			break
+		}
+	}
+	content = content[:end]
+
+	// Pass 2.  Process each line in the run.
+	p = content
+	allok := true
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if !bytes.HasPrefix(line, slashslash) {
+			continue
+		}
+		line = bytes.TrimSpace(line[len(slashslash):])
+		if len(line) > 0 && line[0] == '+' {
+			// Looks like a comment +line.
+			f := strings.Fields(string(line))
+			if f[0] == "+build" {
+				ok := false
+				for _, tok := range f[1:] {
+					if matchTags(tok, imp.tags) {
+						ok = true
+					}
+				}
+				if !ok {
+					allok = false
+				}
+			}
+		}
+	}
+
+	return allok
+}
+
+// matchTags reports whether the name is one of:
+//
+//	tag (if tags[tag] is true)
+//	!tag (if tags[tag] is false)
+//	a comma-separated list of any of these
+//
+func matchTags(name string, tags map[string]bool) bool {
+	if name == "" {
+		return false
+	}
+	if i := strings.Index(name, ","); i >= 0 {
+		// comma-separated list
+		ok1 := matchTags(name[:i], tags)
+		ok2 := matchTags(name[i+1:], tags)
+		return ok1 && ok2
+	}
+	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
+		return false
+	}
+	if strings.HasPrefix(name, "!") { // negation
+		return len(name) > 1 && matchTag(name[1:], tags, false)
+	}
+	return matchTag(name, tags, true)
+}
+
+// matchTag reports whether the tag name is valid and satisfied by tags[name]==want.
+func matchTag(name string, tags map[string]bool, want bool) bool {
+	// Tags must be letters, digits, underscores or dots.
+	// Unlike in Go identifiers, all digits are fine (e.g., "386").
+	for _, c := range name {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+
+	if tags["*"] && name != "" && name != "ignore" {
+		// Special case for gathering all possible imports:
+		// if we put * in the tags map then all tags
+		// except "ignore" are considered both present and not
+		// (so we return true no matter how 'want' is set).
+		return true
+	}
+
+	have := tags[name]
+	if name == "linux" {
+		have = have || tags["android"]
+	}
+	if name == "solaris" {
+		have = have || tags["illumos"]
+	}
+	if name == "darwin" {
+		have = have || tags["ios"]
+	}
+	return have == want
+}