os/exec: use environment variables for user token when present

Builds upon the changes from #32000 which supported sourcing environment
variables for a new process from the environment of a Windows user token
when supplied.

But due to the logic of os/exec, the Env field of a process was
always non-nil when it reached that change.

This change moves the logic up to os/exec, specifically when
os.ProcAttr is being built for the os.StartProcess call, this
ensures that if a user token has been supplied and no Env slice has
been provided on the command it will be sourced from the user's
environment.

If no token is provided, or the program is compiled for any other
platform than Windows, the default environment will be sourced from
syscall.Environ().

Fixes #35314

Change-Id: I4c1722e90b91945eb6980d5c5928183269b50487
GitHub-Last-Rev: 32216b7291418f9285147a93ed6d0ba028f94ef2
GitHub-Pull-Request: golang/go#37402
Reviewed-on: https://go-review.googlesource.com/c/go/+/220587
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 3474ae0ca4..0c49575511 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -24,6 +24,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"internal/syscall/execenv"
 	"io"
 	"os"
 	"path/filepath"
@@ -222,11 +223,11 @@ func interfaceEqual(a, b interface{}) bool {
 	return a == b
 }
 
-func (c *Cmd) envv() []string {
+func (c *Cmd) envv() ([]string, error) {
 	if c.Env != nil {
-		return c.Env
+		return c.Env, nil
 	}
-	return os.Environ()
+	return execenv.Default(c.SysProcAttr)
 }
 
 func (c *Cmd) argv() []string {
@@ -413,11 +414,15 @@ func (c *Cmd) Start() error {
 	}
 	c.childFiles = append(c.childFiles, c.ExtraFiles...)
 
-	var err error
+	envv, err := c.envv()
+	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(c.envv())),
+		Env:   addCriticalEnv(dedupEnv(envv)),
 		Sys:   c.SysProcAttr,
 	})
 	if err != nil {