Revert "os/exec: return error when PATH lookup would use current directory"

This reverts CL 381374.

Reason for revert: broke tests for x/sys/execabs.

Updates #43724.
Updates #43947.

Change-Id: I9eb3adb5728dead66dbd20f6afe1e7a77e2a26f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/403058
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

1 files changed, 37 insertions, 3 deletions

diff --git a/src/internal/execabs/execabs.go b/src/internal/execabs/execabs.go
index 5f60fbb119..9a05d971da 100644
--- a/src/internal/execabs/execabs.go
+++ b/src/internal/execabs/execabs.go
@@ -12,7 +12,11 @@ package execabs
 
 import (
 	"context"
+	"fmt"
 	"os/exec"
+	"path/filepath"
+	"reflect"
+	"unsafe"
 )
 
 var ErrNotFound = exec.ErrNotFound
@@ -23,14 +27,44 @@ type (
 	ExitError = exec.ExitError
 )
 
+func relError(file, path string) error {
+	return fmt.Errorf("%s resolves to executable relative to current directory (.%c%s)", file, filepath.Separator, path)
+}
+
 func LookPath(file string) (string, error) {
-	return exec.LookPath(file)
+	path, err := exec.LookPath(file)
+	if err != nil {
+		return "", err
+	}
+	if filepath.Base(file) == file && !filepath.IsAbs(path) {
+		return "", relError(file, path)
+	}
+	return path, nil
+}
+
+func fixCmd(name string, cmd *exec.Cmd) {
+	if filepath.Base(name) == name && !filepath.IsAbs(cmd.Path) {
+		// exec.Command was called with a bare binary name and
+		// exec.LookPath returned a path which is not absolute.
+		// Set cmd.lookPathErr and clear cmd.Path so that it
+		// cannot be run.
+		lookPathErr := (*error)(unsafe.Pointer(reflect.ValueOf(cmd).Elem().FieldByName("lookPathErr").Addr().Pointer()))
+		if *lookPathErr == nil {
+			*lookPathErr = relError(name, cmd.Path)
+		}
+		cmd.Path = ""
+	}
 }
 
 func CommandContext(ctx context.Context, name string, arg ...string) *exec.Cmd {
-	return exec.CommandContext(ctx, name, arg...)
+	cmd := exec.CommandContext(ctx, name, arg...)
+	fixCmd(name, cmd)
+	return cmd
+
 }
 
 func Command(name string, arg ...string) *exec.Cmd {
-	return exec.Command(name, arg...)
+	cmd := exec.Command(name, arg...)
+	fixCmd(name, cmd)
+	return cmd
 }