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/+/174318
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index 26be62dd92..a157810eed 100644
--- a/src/os/exec/exec_test.go
+++ b/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")
+	}
+}