go/build: avoid setting Default.GOROOT when runtime.GOROOT() is empty

Previously, we called path.Clean on the value of runtime.GOROOT() even
if it was empty, which would set it explicitly to ".".
That would cause (*Context).importGo to assume that errors resolving
paths in GOROOT are fatal and return early:
https://cs.opensource.google/go/go/+/master:src/go/build/build.go;l=1121-1127;drc=38174b3a3514629b84dcd76878b2f536b189dd7b

If we instead leave it empty (and are in module mode), then importGo
will fall back to letting the 'go' command resolve the path, which may
succeed if the 'go' command can infer the correct GOROOT (from its own
stamped-in default GOROOT or executable path).

Fixes #51483

Change-Id: I44dce7cec6c3d1c86670e629ddfbca8be461130c
Reviewed-on: https://go-review.googlesource.com/c/go/+/391805
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/testdata/script/build_trimpath_goroot.txt

@@ -23,11 +23,11 @@ stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
 
 ! go run -trimpath .
 stdout '^GOROOT $'
-stderr '^package runtime is not in GOROOT \(src'${/}'runtime\)$'
+stderr 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)\nexit status 1\n\z'
 
 ! go test -trimpath -v .
 stdout '^GOROOT $'
-stdout '^package runtime is not in GOROOT \(src'${/}'runtime\)$'
+stdout 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)$'
 
 -- go.mod --
 module example

src/go/build/build.go

@@ -35,7 +35,7 @@ type Context struct {
 	GOARCH string // target architecture
 	GOOS   string // target operating system
 	GOROOT string // Go root
-	GOPATH string // Go path
+	GOPATH string // Go paths
 
 	// Dir is the caller's working directory, or the empty string to use
 	// the current directory of the running process. In module mode, this is used
@@ -302,7 +302,9 @@ func defaultContext() Context {
 
 	c.GOARCH = buildcfg.GOARCH
 	c.GOOS = buildcfg.GOOS
-	c.GOROOT = pathpkg.Clean(runtime.GOROOT())
+	if goroot := runtime.GOROOT(); goroot != "" {
+		c.GOROOT = filepath.Clean(goroot)
+	}
 	c.GOPATH = envOr("GOPATH", defaultGOPATH())
 	c.Compiler = runtime.Compiler
 
@@ -672,7 +674,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				}
 				return false
 			}
-			if ctxt.Compiler != "gccgo" && searchVendor(ctxt.GOROOT, true) {
+			if ctxt.Compiler != "gccgo" && ctxt.GOROOT != "" && searchVendor(ctxt.GOROOT, true) {
 				goto Found
 			}
 			for _, root := range gopath {
@@ -706,12 +708,12 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				}
 				tried.goroot = dir
 			}
-		}
-		if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
-			p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
-			p.Goroot = true
-			p.Root = ctxt.GOROOT
-			goto Found
+			if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
+				p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
+				p.Goroot = true
+				p.Root = ctxt.GOROOT
+				goto Found
+			}
 		}
 		for _, root := range gopath {
 			dir := ctxt.joinPath(root, "src", path)
@@ -1082,6 +1084,13 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode)
 		return errNoModules
 	}
 
+	// If ctxt.GOROOT is not set, we don't know which go command to invoke,
+	// and even if we did we might return packages in GOROOT that we wouldn't otherwise find
+	// (because we don't know to search in 'go env GOROOT' otherwise).
+	if ctxt.GOROOT == "" {
+		return errNoModules
+	}
+
 	// Predict whether module aware mode is enabled by checking the value of
 	// GO111MODULE and looking for a go.mod file in the source directory or
 	// one of its parents. Running 'go env GOMOD' in the source directory would
@@ -1119,11 +1128,8 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode)
 	}
 
 	// For efficiency, if path is a standard library package, let the usual lookup code handle it.
-	if ctxt.GOROOT != "" {
-		dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
-		if ctxt.isDir(dir) {
-			return errNoModules
-		}
+	if dir := ctxt.joinPath(ctxt.GOROOT, "src", path); ctxt.isDir(dir) {
+		return errNoModules
 	}
 
 	// If GO111MODULE=auto, look to see if there is a go.mod.
@@ -1165,7 +1171,8 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode)
 		}
 	}
 
-	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
+	goCmd := filepath.Join(ctxt.GOROOT, "bin", "go")
+	cmd := exec.Command(goCmd, "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
 
 	if ctxt.Dir != "" {
 		cmd.Dir = ctxt.Dir

src/go/build/build_test.go

@@ -5,7 +5,6 @@
 package build
 
 import (
-	"flag"
 	"internal/testenv"
 	"io"
 	"os"
@@ -17,10 +16,7 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	flag.Parse()
-	if goTool, err := testenv.GoTool(); err == nil {
-		os.Setenv("PATH", filepath.Dir(goTool)+string(os.PathListSeparator)+os.Getenv("PATH"))
-	}
+	Default.GOROOT = testenv.GOROOT(nil)
 	os.Exit(m.Run())
 }
 
@@ -84,7 +80,7 @@ func TestDotSlashImport(t *testing.T) {
 }
 
 func TestEmptyImport(t *testing.T) {
-	p, err := Import("", Default.GOROOT, FindOnly)
+	p, err := Import("", testenv.GOROOT(t), FindOnly)
 	if err == nil {
 		t.Fatal(`Import("") returned nil error.`)
 	}
@@ -658,7 +654,7 @@ func TestImportDirTarget(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
 	ctxt := Default
 	ctxt.GOPATH = ""
-	p, err := ctxt.ImportDir(filepath.Join(ctxt.GOROOT, "src/path"), 0)
+	p, err := ctxt.ImportDir(filepath.Join(testenv.GOROOT(t), "src/path"), 0)
 	if err != nil {
 		t.Fatal(err)
 	}