os/exec: add GODEBUG setting to opt out of ErrDot changes

The changes are likely to break users, and we need
to make it easy to unbreak without code changes.

For #43724.
Fixes #53962.

Change-Id: I105c5d6c801d354467e0cefd268189c18846858e
Reviewed-on: https://go-review.googlesource.com/c/go/+/419794
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/go/build/deps_test.go

@@ -177,7 +177,11 @@ var depsRules = `
 
 	os/signal, STR
 	< path/filepath
-	< io/ioutil, os/exec;
+	< io/ioutil;
+
+	os < internal/godebug;
+
+	path/filepath, internal/godebug < os/exec;
 
 	io/ioutil, os/exec, os/signal
 	< OS;
@@ -187,8 +191,6 @@ var depsRules = `
 	OS
 	< golang.org/x/sys/cpu;
 
-	os < internal/godebug;
-
 	# FMT is OS (which includes string routines) plus reflect and fmt.
 	# It does not include package log, which should be avoided in core packages.
 	strconv, unicode

src/os/exec/dot_test.go

@@ -56,6 +56,9 @@ func TestLookPath(t *testing.T) {
 
 	// Add "." to PATH so that exec.LookPath looks in the current directory on all systems.
 	// And try to trick it with "../testdir" too.
+	for _, errdot := range []string{"1", "0"} {
+		t.Run("GODEBUG=execerrdot="+errdot, func(t *testing.T) {
+			t.Setenv("GODEBUG", "execerrdot="+errdot)
 			for _, dir := range []string{".", "../testdir"} {
 				t.Run(pathVar+"="+dir, func(t *testing.T) {
 					t.Setenv(pathVar, dir+string(filepath.ListSeparator)+origPath)
@@ -70,19 +73,32 @@ func TestLookPath(t *testing.T) {
 						}
 					}
 
-			if _, err := LookPath("execabs-test"); err == nil {
+					_, err := LookPath("execabs-test")
+					if errdot == "1" {
+						if err == nil {
 							t.Fatalf("LookPath didn't fail when finding a non-relative path")
 						} else if !errors.Is(err, ErrDot) {
 							t.Fatalf("LookPath returned unexpected error: want Is ErrDot, got %q", err)
 						}
+					} else {
+						if err != nil {
+							t.Fatalf("LookPath failed unexpectedly: %v", err)
+						}
+					}
 
 					cmd := Command("execabs-test")
+					if errdot == "1" {
 						if cmd.Err == nil {
 							t.Fatalf("Command didn't fail when finding a non-relative path")
 						} else if !errors.Is(cmd.Err, ErrDot) {
 							t.Fatalf("Command returned unexpected error: want Is ErrDot, got %q", cmd.Err)
 						}
 						cmd.Err = nil
+					} else {
+						if cmd.Err != nil {
+							t.Fatalf("Command failed unexpectedly: %v", err)
+						}
+					}
 
 					// Clearing cmd.Err should let the execution proceed,
 					// and it should fail because it's not a valid binary.
@@ -93,6 +109,8 @@ func TestLookPath(t *testing.T) {
 					}
 				})
 			}
+		})
+	}
 
 	// Test the behavior when the first entry in PATH is an absolute name for the
 	// current directory.

src/os/exec/exec.go

@@ -80,6 +80,11 @@
 //		log.Fatal(err)
 //	}
 //
+// Setting the environment variable GODEBUG=execerrdot=0
+// disables generation of ErrDot entirely, temporarily restoring the pre-Go 1.19
+// behavior for programs that are unable to apply more targeted fixes.
+// A future version of Go may remove support for this variable.
+//
 // Before adding such overrides, make sure you understand the
 // security implications of doing so.
 // See https://go.dev/blog/path-security for more information.

src/os/exec/lp_plan9.go

@@ -6,6 +6,7 @@ package exec
 
 import (
 	"errors"
+	"internal/godebug"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -53,7 +54,7 @@ func LookPath(file string) (string, error) {
 	for _, dir := range filepath.SplitList(path) {
 		path := filepath.Join(dir, file)
 		if err := findExecutable(path); err == nil {
-			if !filepath.IsAbs(path) {
+			if !filepath.IsAbs(path) && godebug.Get("execerrdot") != "0" {
 				return path, &Error{file, ErrDot}
 			}
 			return path, nil

src/os/exec/lp_unix.go

@@ -8,6 +8,7 @@ package exec
 
 import (
 	"errors"
+	"internal/godebug"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -56,7 +57,7 @@ func LookPath(file string) (string, error) {
 		}
 		path := filepath.Join(dir, file)
 		if err := findExecutable(path); err == nil {
-			if !filepath.IsAbs(path) {
+			if !filepath.IsAbs(path) && godebug.Get("execerrdot") != "0" {
 				return path, &Error{file, ErrDot}
 			}
 			return path, nil

src/os/exec/lp_windows.go

@@ -6,6 +6,7 @@ package exec
 
 import (
 	"errors"
+	"internal/godebug"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -102,6 +103,9 @@ func LookPath(file string) (string, error) {
 	)
 	if _, found := syscall.Getenv("NoDefaultCurrentDirectoryInExePath"); !found {
 		if f, err := findExecutable(filepath.Join(".", file), exts); err == nil {
+			if godebug.Get("execerrdot") == "0" {
+				return f, nil
+			}
 			dotf, dotErr = f, &Error{file, ErrDot}
 		}
 	}
@@ -124,7 +128,7 @@ func LookPath(file string) (string, error) {
 				}
 			}
 
-			if !filepath.IsAbs(f) {
+			if !filepath.IsAbs(f) && godebug.Get("execerrdot") != "0" {
 				return f, &Error{file, ErrDot}
 			}
 			return f, nil