eval: Introduce (*Port).Fork.

eval/compile.go

@@ -340,11 +340,7 @@ func (cp *compiler) redir(n *parse.Redir) Op {
 				ec.ports[dst] = &Port{}
 			} else {
 				fd := srcMust.zerothMustNonNegativeInt()
-				ec.ports[dst] = ec.ports[fd]
-				if ec.ports[dst] != nil {
-					ec.ports[dst].CloseFile = false
-					ec.ports[dst].CloseChan = false
-				}
+				ec.ports[dst] = ec.ports[fd].Fork()
 			}
 		} else {
 			f, err := os.OpenFile(src, flag, defaultFileRedirPerm)

eval/eval.go

@@ -111,13 +111,12 @@ func NewTopEvalCtx(ev *Evaler, name, text string, ports []*Port) *evalCtx {
 	}
 }
 
-// fork returns a modified copy of ec. The ports are copied deeply, with
-// shouldClose flags reset, and the context is changed to the given value.
-// Other fields are copied shallowly.
+// fork returns a modified copy of ec. The ports are forked, and the context is
+// changed to the given value. Other fields are copied shallowly.
 func (ec *evalCtx) fork(newContext string) *evalCtx {
 	newPorts := make([]*Port, len(ec.ports))
 	for i, p := range ec.ports {
-		newPorts[i] = &Port{p.File, p.Chan, false, false}
+		newPorts[i] = p.Fork()
 	}
 	return &evalCtx{
 		ec.Evaler,

eval/port.go

@@ -10,7 +10,12 @@ type Port struct {
 	CloseChan bool
 }
 
-// close closes a Port.
+// Fork returns a copy of a Port with the Close* flags unset.
+func (p *Port) Fork() *Port {
+	return &Port{p.File, p.Chan, false, false}
+}
+
+// Close closes a Port.
 func (p *Port) Close() {
 	if p == nil {
 		return