Fix crash on empty for expression ((;;)) (#123)

core/cmd_exec.py

@@ -997,17 +997,20 @@ def _Dispatch(self, node, fork_external):

    elif node.tag == command_e.ForExpr:
      status = 0
      self.arith_ev.Eval(node.init)
      init, cond, body, update = node.init, node.cond, node.body, node.update
      if init:
        self.arith_ev.Eval(init)

      self.loop_level += 1
      try:
        while True:
          b = self.arith_ev.Eval(node.cond)
          if not b:
            break
          if cond:
            b = self.arith_ev.Eval(cond)
            if not b:
              break

          try:
            status = self._Execute(node.body)
            status = self._Execute(body)
          except _ControlFlow as e:
            if e.IsBreak():
              status = 0
@@ -1017,7 +1020,8 @@ def _Dispatch(self, node, fork_external):
            else:  # return needs to pop up more
              raise

          self.arith_ev.Eval(node.update)
          if update:
            self.arith_ev.Eval(update)

      finally:
        self.loop_level -= 1

spec/for-expr.test.sh

@@ -42,3 +42,18 @@ done  # A construct borrowed from ksh93.
## N-I mksh status: 1
## N-I mksh stdout-json: ""

### For loop with empty head
a=1
for ((;;)); do
  if test $a = 4; then
    break
  fi
  echo $((a++))
done  # A construct borrowed from ksh93.
## status: 0
## STDOUT:
1
2
3
## N-I mksh status: 1
## N-I mksh stdout-json: ""