cmd/doc: use empty GOPATH when running the tests

Otherwise, a populated GOPATH might result in failures such as:

	$ go test
	[...] no buildable Go source files in [...]/gopherjs/compiler/natives/src/crypto/rand
	exit status 1

Move the initialization of the dirs walker out of the init func, so that
we can control its behavior in the tests.

Updates #24464.

Change-Id: I4b26a7d3d6809bdd8e9b6b0556d566e7855f80fe
Reviewed-on: https://go-review.googlesource.com/101836
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/doc/dirs.go

@@ -5,7 +5,6 @@
 package main
 
 import (
-	"go/build"
 	"log"
 	"os"
 	"path"
@@ -25,7 +24,7 @@ type Dirs struct {
 
 var dirs Dirs
 
-func init() {
+func dirsInit() {
 	dirs.paths = make([]string, 0, 1000)
 	dirs.scan = make(chan string)
 	go dirs.walk()
@@ -55,7 +54,7 @@ func (d *Dirs) Next() (string, bool) {
 
 // walk walks the trees in GOROOT and GOPATH.
 func (d *Dirs) walk() {
-	d.bfsWalkRoot(build.Default.GOROOT)
+	d.bfsWalkRoot(buildCtx.GOROOT)
 	for _, root := range splitGopath() {
 		d.bfsWalkRoot(root)
 	}

src/cmd/doc/doc_test.go

@@ -7,7 +7,6 @@ package main
 import (
 	"bytes"
 	"flag"
-	"go/build"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -16,6 +15,14 @@ import (
 	"testing"
 )
 
+func TestMain(m *testing.M) {
+	// otherwise the tests are brittle, as they may give unexpected
+	// output or errors when a suffix match with GOPATH takes place
+	buildCtx.GOPATH = ""
+	dirsInit()
+	os.Exit(m.Run())
+}
+
 func maybeSkip(t *testing.T) {
 	if strings.HasPrefix(runtime.GOOS, "nacl") {
 		t.Skip("nacl does not have a full file tree")
@@ -653,7 +660,7 @@ func TestDotSlashLookup(t *testing.T) {
 			t.Fatal(err)
 		}
 	}()
-	if err := os.Chdir(filepath.Join(build.Default.GOROOT, "src", "text")); err != nil {
+	if err := os.Chdir(filepath.Join(buildCtx.GOROOT, "src", "text")); err != nil {
 		t.Fatal(err)
 	}
 	var b bytes.Buffer

src/cmd/doc/main.go

@@ -69,6 +69,7 @@ func usage() {
 func main() {
 	log.SetFlags(0)
 	log.SetPrefix("doc: ")
+	dirsInit()
 	err := do(os.Stdout, flag.CommandLine, os.Args[1:])
 	if err != nil {
 		log.Fatal(err)
@@ -355,9 +356,11 @@ func findPackage(pkg string) (string, bool) {
 	}
 }
 
+var buildCtx = build.Default
+
 // splitGopath splits $GOPATH into a list of roots.
 func splitGopath() []string {
-	return filepath.SplitList(build.Default.GOPATH)
+	return filepath.SplitList(buildCtx.GOPATH)
 }
 
 // pwd returns the current directory.

src/cmd/doc/pkg.go

@@ -62,7 +62,7 @@ func (pkg *Package) prettyPath() string {
 	// Also convert everything to slash-separated paths for uniform handling.
 	path = filepath.Clean(filepath.ToSlash(pkg.build.Dir))
 	// Can we find a decent prefix?
-	goroot := filepath.Join(build.Default.GOROOT, "src")
+	goroot := filepath.Join(buildCtx.GOROOT, "src")
 	if p, ok := trim(path, filepath.ToSlash(goroot)); ok {
 		return p
 	}