Avoid problems where PW prompt in enter_chroot.sh would block other terminals.

enter_chroot.sh

Index: enter_chroot.sh
diff --git a/enter_chroot.sh b/enter_chroot.sh
index 29ef561291cfc01f3ccc0d09c2c96a0486c8c60c..fd8b4f93e467811d34498c626ea0797453a97f06 100755
--- a/enter_chroot.sh
+++ b/enter_chroot.sh
@@ -107,6 +107,10 @@ sudo chmod 0777 "$FLAGS_chroot/var/lock"
 LOCKFILE="$FLAGS_chroot/var/lock/enter_chroot"
 
 function setup_env {
+  # Validate sudo timestamp before entering the critical section so that we
+  # don't stall for a password while we have the lockfile.
+  sudo -v
+
   (
     flock 200
     echo $$ >> "$LOCKFILE"
@@ -223,6 +227,10 @@ function setup_env {
 }
 
 function teardown_env {
+  # Validate sudo timestamp before entering the critical section so that we
+  # don't stall for a password while we have the lockfile.
+  sudo -v
+
   # Only teardown if we're the last enter_chroot to die
   (
     flock 200