os/exec: always set SYSTEMROOT on Windows if not listed in Cmd.Env

Fixes #25210

Change-Id: If27b61776154dae9b9b67bf4e4f5faa785d98105
Reviewed-on: https://go-review.googlesource.com/c/go/+/174318Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec/exec.go

@@ -71,6 +71,8 @@ type Cmd struct {
 	// environment.
 	// If Env contains duplicate environment keys, only the last
 	// value in the slice for each duplicate key is used.
+	// As a special case on Windows, SYSTEMROOT is always added if
+	// missing and not explicitly set to the empty string.
 	Env []string
 
 	// Dir specifies the working directory of the command.
@@ -412,7 +414,7 @@ func (c *Cmd) Start() error {
 	c.Process, err = os.StartProcess(c.Path, c.argv(), &os.ProcAttr{
 		Dir:   c.Dir,
 		Files: c.childFiles,
-		Env:   dedupEnv(c.envv()),
+		Env:   addCriticalEnv(dedupEnv(c.envv())),
 		Sys:   c.SysProcAttr,
 	})
 	if err != nil {
@@ -756,3 +758,24 @@ func dedupEnvCase(caseInsensitive bool, env []string) []string {
 	}
 	return out
 }
+
+// addCriticalEnv adds any critical environment variables that are required
+// (or at least almost always required) on the operating system.
+// Currently this is only used for Windows.
+func addCriticalEnv(env []string) []string {
+	if runtime.GOOS != "windows" {
+		return env
+	}
+	for _, kv := range env {
+		eq := strings.Index(kv, "=")
+		if eq < 0 {
+			continue
+		}
+		k := kv[:eq]
+		if strings.EqualFold(k, "SYSTEMROOT") {
+			// We already have it.
+			return env
+		}
+	}
+	return append(env, "SYSTEMROOT="+os.Getenv("SYSTEMROOT"))
+}

src/os/exec/exec_test.go

@@ -1184,3 +1184,22 @@ func TestStringPathNotResolved(t *testing.T) {
 		t.Errorf("String(%q, %q) = %q, want %q", "makemeasandwich", "-lettuce", got, want)
 	}
 }
+
+// start a child process without the user code explicitly starting
+// with a copy of the parent's. (The Windows SYSTEMROOT issue: Issue
+// 25210)
+func TestChildCriticalEnv(t *testing.T) {
+	testenv.MustHaveExec(t)
+	if runtime.GOOS != "windows" {
+		t.Skip("only testing on Windows")
+	}
+	cmd := helperCommand(t, "echoenv", "SYSTEMROOT")
+	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.TrimSpace(string(out)) == "" {
+		t.Error("no SYSTEMROOT found")
+	}
+}