cmd/api: use 'go list' to locate transitive dependencies of std

With standard-library modules and vendoring, the mapping from import
path to directory within the standard library is no longer entirely
trivial. Fortunately, 'go list' makes that mapping straightforward to
compute.

Updates #30241
Updates #30228

Change-Id: Iddd77c21a527b7acdb30c17bec8b4bbd43e23756
Reviewed-on: https://go-review.googlesource.com/c/go/+/165497
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/api/goapi.go

@@ -8,6 +8,7 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"go/ast"
@@ -153,6 +154,7 @@ func main() {
 	var featureCtx = make(map[string]map[string]bool) // feature -> context name -> true
 	for _, context := range contexts {
 		w := NewWalker(context, filepath.Join(build.Default.GOROOT, "src"))
+		w.loadImports(pkgNames, w.context)
 
 		for _, name := range pkgNames {
 			// Vendored packages do not contribute to our
@@ -349,12 +351,14 @@ func fileFeatures(filename string) []string {
 var fset = token.NewFileSet()
 
 type Walker struct {
-	context  *build.Context
-	root     string
-	scope    []string
-	current  *types.Package
-	features map[string]bool           // set
-	imported map[string]*types.Package // packages already imported
+	context   *build.Context
+	root      string
+	scope     []string
+	current   *types.Package
+	features  map[string]bool              // set
+	imported  map[string]*types.Package    // packages already imported
+	importMap map[string]map[string]string // importer dir -> import path -> canonical path
+	importDir map[string]string            // canonical import path -> dir
 }
 
 func NewWalker(context *build.Context, root string) *Walker {
@@ -434,11 +438,74 @@ func tagKey(dir string, context *build.Context, tags []string) string {
 	return key
 }
 
+func (w *Walker) loadImports(paths []string, context *build.Context) {
+	if context == nil {
+		context = &build.Default
+	}
+
+	var (
+		tags       = context.BuildTags
+		cgoEnabled = "0"
+	)
+	if context.CgoEnabled {
+		tags = append(tags[:len(tags):len(tags)], "cgo")
+		cgoEnabled = "1"
+	}
+
+	// TODO(golang.org/issue/29666): Request only the fields that we need.
+	cmd := exec.Command(goCmd(), "list", "-e", "-deps", "-json")
+	if len(tags) > 0 {
+		cmd.Args = append(cmd.Args, "-tags", strings.Join(tags, " "))
+	}
+	cmd.Args = append(cmd.Args, paths...)
+
+	cmd.Env = append(os.Environ(),
+		"GOOS="+context.GOOS,
+		"GOARCH="+context.GOARCH,
+		"CGO_ENABLED="+cgoEnabled,
+	)
+
+	stdout := new(bytes.Buffer)
+	cmd.Stdout = stdout
+	cmd.Stderr = new(strings.Builder)
+	err := cmd.Run()
+	if err != nil {
+		log.Fatalf("%s failed: %v\n%s", strings.Join(cmd.Args, " "), err, cmd.Stderr)
+	}
+
+	w.importDir = make(map[string]string)
+	w.importMap = make(map[string]map[string]string)
+	dec := json.NewDecoder(stdout)
+	for {
+		var pkg struct {
+			ImportPath, Dir string
+			ImportMap       map[string]string
+		}
+		if err := dec.Decode(&pkg); err == io.EOF {
+			break
+		} else if err != nil {
+			log.Fatalf("%s: invalid output: %v", strings.Join(cmd.Args, " "), err)
+		}
+
+		w.importDir[pkg.ImportPath] = pkg.Dir
+		w.importMap[pkg.Dir] = pkg.ImportMap
+	}
+}
+
 // Importing is a sentinel taking the place in Walker.imported
 // for a package that is in the process of being imported.
 var importing types.Package
 
 func (w *Walker) Import(name string) (*types.Package, error) {
+	return w.ImportFrom(name, "", 0)
+}
+
+func (w *Walker) ImportFrom(fromPath, fromDir string, mode types.ImportMode) (*types.Package, error) {
+	name := fromPath
+	if canonical, ok := w.importMap[fromDir][fromPath]; ok {
+		name = canonical
+	}
+
 	pkg := w.imported[name]
 	if pkg != nil {
 		if pkg == &importing {
@@ -449,7 +516,10 @@ func (w *Walker) Import(name string) (*types.Package, error) {
 	w.imported[name] = &importing
 
 	// Determine package files.
-	dir := filepath.Join(w.root, filepath.FromSlash(name))
+	dir := w.importDir[name]
+	if dir == "" {
+		dir = filepath.Join(w.root, filepath.FromSlash(name))
+	}
 	if fi, err := os.Stat(dir); err != nil || !fi.IsDir() {
 		log.Fatalf("no source in tree for import %q: %v", name, err)
 	}