tests: Count bytes written in stdio_test.StdIOTest

This is mainly for peace of mind. With all this non-blocking IO investigation
I'm getting a bit paranoid wrt file objects.

refs #712

docs/changelog.rst

@@ -24,6 +24,7 @@ In progress (unreleased)
 * :gh:issue:`1306` CI: Report sudo version on Ansible targets
 * :gh:issue:`1306` CI: Move sudo test users defaults into ``/etc/sudoers.d``
 * :gh:issue:`1306` preamble_size: Fix variability of measured command size
+* :gh:issue:`1306` tests: Count bytes written in ``stdio_test.StdIOTest``
 
 
 v0.3.27 (2025-08-20)

tests/data/stdio_checks.py

@@ -3,9 +3,24 @@ import os
 import sys
 
 
+def _shout_stdout_py3(size):
+    nwritten = sys.stdout.write('A' * size)
+    return nwritten
+
+
+def _shout_stdout_py2(size):
+    shout = 'A' * size
+    nwritten = 0
+    while nwritten < size:
+        nwritten += os.write(sys.stdout.fileno(), shout[-nwritten:])
+    return nwritten
+
+
 def shout_stdout(size):
-    sys.stdout.write('A' * size)
-    return 'success'
+    if sys.version_info > (3, 0):
+        return _shout_stdout_py3(size)
+    else:
+        return _shout_stdout_py2(size)
 
 
 def file_is_blocking(fobj):

tests/stdio_test.py

@@ -15,8 +15,8 @@ class StdIOTest(testlib.RouterMixin, testlib.TestCase):
         """
         size = 1 * 2**20
         context = self.router.local()
-        result = context.call(stdio_checks.shout_stdout, size)
-        self.assertEqual('success', result)
+        nwritten = context.call(stdio_checks.shout_stdout, size)
+        self.assertEqual(nwritten, size)
 
     def test_stdio_is_blocking(self):
         context = self.router.local()