Return 'null' when reading from a closed pipe.

src/pipe.toit

@@ -98,7 +98,7 @@ class OpenPipe extends OpenPipe_:
 
 class OpenPipe_ implements Stream:
   resource_ := ?
-  state_ := ?
+  state_/monitor.ResourceState_? := ?
   pid := null
   child-process-name_ /string?
   input_ /int := UNKNOWN-DIRECTION_
@@ -152,7 +152,9 @@ class OpenPipe_ implements Stream:
 
   read_ -> ByteArray?:
     while true:
+      if not state_: return null
       state_.wait-for-state READ-EVENT_ | CLOSE-EVENT_
+      if not state_: return null
       result := read-from-pipe_ resource_
       if result != -1:
         if result == null:

tests/pipe3_exe.toit

@@ -0,0 +1,9 @@
+// Copyright (C) 2025 Toit contributors.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/TESTS_LICENSE file.
+
+import host.pipe
+
+main:
+  // Just wait for a message.
+  pipe.stdin.in.read

tests/pipe3_test.toit

@@ -0,0 +1,32 @@
+// Copyright (C) 2025 Toit contributors.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/TESTS_LICENSE file.
+
+import expect show *
+
+import host.pipe
+import system
+
+main args:
+  toit-exe := args[0]
+
+  my-path := system.program-path
+  end := my-path.index-of --last "test.toit"
+  if end == -1: throw "UNEXPECTED"
+  input := "$my-path[..end]input.toit"
+
+  process := pipe.fork
+      --create-stdout
+      --create-stdin
+      toit-exe
+      [toit-exe, input]
+
+  to-process := process.stdin
+  from-process := process.stdout
+
+  from-process.close
+
+  expect-null from-process.in.read
+  to-process.out.write "done"
+
+  process.wait