internal/stdlib: find api directory more robustly

stdlib contains precomputed information about exported symbols in the
standard library. The old code looked in runtime.GOROOT()/api for
this information. This works on many systems, but not always.
The new code runs go env, and looks first in GOROOT/api, and failing
that, in GOPATH/api.

(The situation seems to depend on mysterious details of the history of
the user's Go environment.)

This CL is on the path to adding function snippets for the standard
library, for the new unimported completions code.

Change-Id: If2853a3f26edf9d0f1603563104592b95eb466a1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/664995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>

Author: pjweinbgo

internal/stdlib/generate.go

@@ -29,21 +29,22 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"slices"
 	"strings"
 
 	"golang.org/x/tools/go/packages"
 )
 
 func main() {
-	manifest()
+	log.SetFlags(log.Lshortfile) // to identify the source of the log messages
+	dir := apidir()
+	manifest(dir)
 	deps()
 }
 
 // -- generate std manifest --
 
-func manifest() {
+func manifest(apidir string) {
 	pkgs := make(map[string]map[string]symInfo) // package -> symbol -> info
 	symRE := regexp.MustCompile(`^pkg (\S+).*?, (var|func|type|const|method \([^)]*\)) ([\pL\p{Nd}_]+)(.*)`)
 
@@ -111,15 +112,15 @@ func manifest() {
 		if minor > 0 {
 			base = fmt.Sprintf("go1.%d.txt", minor)
 		}
-		filename := filepath.Join(runtime.GOROOT(), "api", base)
+		filename := filepath.Join(apidir, base)
 		data, err := os.ReadFile(filename)
 		if err != nil {
 			if errors.Is(err, fs.ErrNotExist) {
 				// All caught up.
 				// Synthesize one final file from any api/next/*.txt fragments.
 				// (They are consolidated into a go1.%d file some time between
 				// the freeze and the first release candidate.)
-				filenames, err := filepath.Glob(filepath.Join(runtime.GOROOT(), "api/next/*.txt"))
+				filenames, err := filepath.Glob(filepath.Join(apidir, "next", "*.txt"))
 				if err != nil {
 					log.Fatal(err)
 				}
@@ -177,6 +178,28 @@ var PackageSymbols = map[string][]Symbol{
 	}
 }
 
+// find the api directory, In most situations it is in GOROOT/api, but not always.
+// TODO(pjw): understand where it might be, and if there could be newer and older versions
+func apidir() string {
+	stdout := new(bytes.Buffer)
+	cmd := exec.Command("go", "env", "GOROOT", "GOPATH")
+	cmd.Stdout = stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		log.Fatal(err)
+	}
+	// Prefer GOROOT/api over GOPATH/api.
+	for line := range strings.SplitSeq(stdout.String(), "\n") {
+		apidir := filepath.Join(line, "api")
+		info, err := os.Stat(apidir)
+		if err == nil && info.IsDir() {
+			return apidir
+		}
+	}
+	log.Fatal("could not find api dir")
+	return ""
+}
+
 type symInfo struct {
 	kind  string // e.g. "func"
 	minor int    // go1.%d