os/exec: set PWD implicitly if Dir is non-empty and Env is nil

Fixes #50599.

Change-Id: I4e5dbb3972cdf21ede049567bfb98f2c992c5849
Reviewed-on: https://go-review.googlesource.com/c/go/+/401340
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

api/next/50599.txt

@@ -0,0 +1 @@
+pkg os/exec, method (*Cmd) Environ() []string #50599

doc/go1.19.html

@@ -132,6 +132,21 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- net -->
 
+<dl id="os/exec"><dt><a href="/pkg/os/exec/">os/exec</a></dt>
+  <dd><!-- https://go.dev/issue/50599 -->
+    <p>
+      An <code>exec.Cmd</code> with a non-empty <code>Dir</code> and a
+      nil <code>Env</code> now implicitly sets the <code>PWD</code> environment
+      variable for the subprocess to match <code>Dir</code>.
+    </p>
+    <p>
+      The new method <code>(*exec.Cmd).Environ</code> reports the
+      environment that would be used to run the command, including the
+      aforementioned <code>PWD</code> variable.
+    </p>
+  </dd>
+</dl> <!-- os/exec -->
+
 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p><!-- https://go.dev/issue/51461 -->

src/os/exec/example_test.go

@@ -144,6 +144,21 @@ func ExampleCmd_CombinedOutput() {
 	fmt.Printf("%s\n", stdoutStderr)
 }
 
+func ExampleCmd_Environ() {
+	cmd := exec.Command("pwd")
+
+	// Set Dir before calling cmd.Environ so that it will include an
+	// updated PWD variable (on platforms where that is used).
+	cmd.Dir = ".."
+	cmd.Env = append(cmd.Environ(), "POSIXLY_CORRECT=1")
+
+	out, err := cmd.Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s\n", out)
+}
+
 func ExampleCommandContext() {
 	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
 	defer cancel()

src/os/exec/exec.go

@@ -223,13 +223,6 @@ func interfaceEqual(a, b any) bool {
 	return a == b
 }
 
-func (c *Cmd) envv() ([]string, error) {
-	if c.Env != nil {
-		return c.Env, nil
-	}
-	return execenv.Default(c.SysProcAttr)
-}
-
 func (c *Cmd) argv() []string {
 	if len(c.Args) > 0 {
 		return c.Args
@@ -414,15 +407,15 @@ func (c *Cmd) Start() error {
 	}
 	c.childFiles = append(c.childFiles, c.ExtraFiles...)
 
-	envv, err := c.envv()
+	env, err := c.environ()
 	if err != nil {
 		return err
 	}
 
 	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
 		Dir:   c.Dir,
 		Files: c.childFiles,
-		Env:   addCriticalEnv(dedupEnv(envv)),
+		Env:   env,
 		Sys:   c.SysProcAttr,
 	})
 	if err != nil {
@@ -735,6 +728,54 @@ func minInt(a, b int) int {
 	return b
 }
 
+// environ returns a best-effort copy of the environment in which the command
+// would be run as it is currently configured. If an error occurs in computing
+// the environment, it is returned alongside the best-effort copy.
+func (c *Cmd) environ() ([]string, error) {
+	var err error
+
+	env := c.Env
+	if env == nil {
+		env, err = execenv.Default(c.SysProcAttr)
+		if err != nil {
+			env = os.Environ()
+			// Note that the non-nil err is preserved despite env being overridden.
+		}
+
+		if c.Dir != "" {
+			switch runtime.GOOS {
+			case "windows", "plan9":
+				// Windows and Plan 9 do not use the PWD variable, so we don't need to
+				// keep it accurate.
+			default:
+				// On POSIX platforms, PWD represents “an absolute pathname of the
+				// current working directory.” Since we are changing the working
+				// directory for the command, we should also update PWD to reflect that.
+				//
+				// Unfortunately, we didn't always do that, so (as proposed in
+				// https://go.dev/issue/50599) to avoid unintended collateral damage we
+				// only implicitly update PWD when Env is nil. That way, we're much
+				// less likely to override an intentional change to the variable.
+				if pwd, absErr := filepath.Abs(c.Dir); absErr == nil {
+					env = append(env, "PWD="+pwd)
+				} else if err == nil {
+					err = absErr
+				}
+			}
+		}
+	}
+
+	return addCriticalEnv(dedupEnv(env)), err
+}
+
+// Environ returns a copy of the environment in which the command would be run
+// as it is currently configured.
+func (c *Cmd) Environ() []string {
+	//  Intentionally ignore errors: environ returns a best-effort environment no matter what.
+	env, _ := c.environ()
+	return env
+}
+
 // dedupEnv returns a copy of env with any duplicates removed, in favor of
 // later values.
 // Items not of the normal environment "key=value" form are preserved unchanged.

src/os/exec/exec_posix_test.go

@@ -7,9 +7,15 @@
 package exec_test
 
 import (
+	"internal/testenv"
+	"os"
+	"os/exec"
 	"os/user"
+	"path/filepath"
+	"reflect"
 	"runtime"
 	"strconv"
+	"strings"
 	"syscall"
 	"testing"
 	"time"
@@ -86,3 +92,146 @@ func TestWaitid(t *testing.T) {
 
 	<-ch
 }
+
+// https://go.dev/issue/50599: if Env is not set explicitly, setting Dir should
+// implicitly update PWD to the correct path, and Environ should list the
+// updated value.
+func TestImplicitPWD(t *testing.T) {
+	testenv.MustHaveExec(t)
+	_, pwdErr := exec.LookPath("pwd")
+
+	t.Parallel()
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := []struct {
+		name string
+		dir  string
+		want string
+	}{
+		{"empty", "", cwd},
+		{"dot", ".", cwd},
+		{"dotdot", "..", filepath.Dir(cwd)},
+		{"PWD", cwd, cwd},
+		{"PWDdotdot", cwd + string(filepath.Separator) + "..", filepath.Dir(cwd)},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Note: we're using the actual "pwd" command here (instead of helperCommand)
+			// because the implementation of helperCommand requires a non-empty Env.
+			// (We could perhaps refactor helperCommand to use a flag or switch on the
+			// value of argv[0] instead, but that doesn't seem worth the trouble at
+			// the moment.)
+			cmd := exec.Command("pwd")
+			cmd.Dir = tc.dir
+
+			var pwds []string
+			for _, kv := range cmd.Environ() {
+				if strings.HasPrefix(kv, "PWD=") {
+					pwds = append(pwds, strings.TrimPrefix(kv, "PWD="))
+				}
+			}
+
+			wantPWDs := []string{tc.want}
+			if tc.dir == "" {
+				if _, ok := os.LookupEnv("PWD"); !ok {
+					wantPWDs = nil
+				}
+			}
+			if !reflect.DeepEqual(pwds, wantPWDs) {
+				t.Errorf("PWD entries in cmd.Environ():\n\t%s\nwant:\n\t%s", strings.Join(pwds, "\n\t"), strings.Join(wantPWDs, "\n\t"))
+			}
+
+			if pwdErr != nil {
+				t.Skipf("not running `pwd` because it was not found: %v", pwdErr)
+			}
+			cmd.Stderr = new(strings.Builder)
+			out, err := cmd.Output()
+			if err != nil {
+				t.Fatalf("%v:\n%s", err, cmd.Stderr)
+			}
+			got := strings.Trim(string(out), "\r\n")
+			t.Logf("in\n\t%s\n`pwd` reported\n\t%s", tc.dir, got)
+			if got != tc.want {
+				t.Errorf("want\n\t%s", tc.want)
+			}
+		})
+	}
+}
+
+// However, if cmd.Env is set explicitly, setting Dir should not override it.
+// (This checks that the implementation for https://go.dev/issue/50599 doesn't
+// break existing users who may have explicitly mismatched the PWD variable.)
+func TestExplicitPWD(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	link := filepath.Join(t.TempDir(), "link")
+	if err := os.Symlink(cwd, link); err != nil {
+		t.Fatal(err)
+	}
+
+	// Now link is another equally-valid name for cwd. If we set Dir to one and
+	// PWD to the other, the subprocess should report the PWD version.
+	cases := []struct {
+		name string
+		dir  string
+		pwd  string
+	}{
+		{name: "original PWD", pwd: cwd},
+		{name: "link PWD", pwd: link},
+		{name: "in link with original PWD", dir: link, pwd: cwd},
+		{name: "in dir with link PWD", dir: cwd, pwd: link},
+		// Ideally we would also like to test what happens if we set PWD to
+		// something totally bogus (or the empty string), but then we would have no
+		// idea what output the subprocess should actually produce: cwd itself may
+		// contain symlinks preserved from the PWD value in the test's environment.
+	}
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			cmd := helperCommand(t, "pwd")
+			// This is intentionally opposite to the usual order of setting cmd.Dir
+			// and then calling cmd.Environ. Here, we *want* PWD not to match cmd.Dir,
+			// so we don't care whether cmd.Dir is reflected in cmd.Environ.
+			cmd.Env = append(cmd.Environ(), "PWD="+tc.pwd)
+			cmd.Dir = tc.dir
+
+			var pwds []string
+			for _, kv := range cmd.Environ() {
+				if strings.HasPrefix(kv, "PWD=") {
+					pwds = append(pwds, strings.TrimPrefix(kv, "PWD="))
+				}
+			}
+
+			wantPWDs := []string{tc.pwd}
+			if !reflect.DeepEqual(pwds, wantPWDs) {
+				t.Errorf("PWD entries in cmd.Environ():\n\t%s\nwant:\n\t%s", strings.Join(pwds, "\n\t"), strings.Join(wantPWDs, "\n\t"))
+			}
+
+			cmd.Stderr = new(strings.Builder)
+			out, err := cmd.Output()
+			if err != nil {
+				t.Fatalf("%v:\n%s", err, cmd.Stderr)
+			}
+			got := strings.Trim(string(out), "\r\n")
+			t.Logf("in\n\t%s\nwith PWD=%s\nsubprocess os.Getwd() reported\n\t%s", tc.dir, tc.pwd, got)
+			if got != tc.pwd {
+				t.Errorf("want\n\t%s", tc.pwd)
+			}
+		})
+	}
+}

src/os/exec/exec_test.go

@@ -57,12 +57,20 @@ func init() {
 func helperCommandContext(t *testing.T, ctx context.Context, s ...string) (cmd *exec.Cmd) {
 	testenv.MustHaveExec(t)
 
+	// Use os.Executable instead of os.Args[0] in case the caller modifies
+	// cmd.Dir: if the test binary is invoked like "./exec.test", it should
+	// not fail spuriously.
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	cs := []string{"-test.run=TestHelperProcess", "--"}
 	cs = append(cs, s...)
 	if ctx != nil {
-		cmd = exec.CommandContext(ctx, os.Args[0], cs...)
+		cmd = exec.CommandContext(ctx, exe, cs...)
 	} else {
-		cmd = exec.Command(os.Args[0], cs...)
+		cmd = exec.Command(exe, cs...)
 	}
 	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
 	return cmd
@@ -831,6 +839,14 @@ func TestHelperProcess(*testing.T) {
 		}
 		pipe.Close()
 		os.Exit(0)
+	case "pwd":
+		pwd, err := os.Getwd()
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		fmt.Println(pwd)
+		os.Exit(0)
 	default:
 		fmt.Fprintf(os.Stderr, "Unknown command %q\n", cmd)
 		os.Exit(2)