Prevent stdio deadlock in forked children (#79522)

* background threads writing to stdout/stderr can cause children to deadlock if a thread in the parent holds the internal lock on the BufferedWriter wrapper
* prevent writes to std handles during fork by monkeypatching stdout/stderr during display startup to require a mutex lock with fork(); this ensures no background threads can hold the lock during a fork operation
* add integration test that fails reliably on Linux without this fix

1 files changed, 13 insertions, 11 deletions

diff --git a/test/lib/ansible_test/_util/target/sanity/import/importer.py b/test/lib/ansible_test/_util/target/sanity/import/importer.py
index 3dcb8bf934..3180530c95 100644
--- a/test/lib/ansible_test/_util/target/sanity/import/importer.py
+++ b/test/lib/ansible_test/_util/target/sanity/import/importer.py
@@ -48,11 +48,7 @@ def main():
             __import__(name)
             return sys.modules[name]
 
-    try:
-        # noinspection PyCompatibility
-        from StringIO import StringIO
-    except ImportError:
-        from io import StringIO
+    from io import BytesIO, TextIOWrapper
 
     try:
         from importlib.util import spec_from_loader, module_from_spec
@@ -436,8 +432,9 @@ def main():
     class Capture:
         """Captured output and/or exception."""
         def __init__(self):
-            self.stdout = StringIO()
-            self.stderr = StringIO()
+            # use buffered IO to simulate StringIO; allows Ansible's stream patching to behave without warnings
+            self.stdout = TextIOWrapper(BytesIO())
+            self.stderr = TextIOWrapper(BytesIO())
 
     def capture_report(path, capture, messages):
         """Report on captured output.
@@ -445,12 +442,17 @@ def main():
         :type capture: Capture
         :type messages: set[str]
         """
-        if capture.stdout.getvalue():
-            first = capture.stdout.getvalue().strip().splitlines()[0].strip()
+        # since we're using buffered IO, flush before checking for data
+        capture.stdout.flush()
+        capture.stderr.flush()
+        stdout_value = capture.stdout.buffer.getvalue()
+        if stdout_value:
+            first = stdout_value.decode().strip().splitlines()[0].strip()
             report_message(path, 0, 0, 'stdout', first, messages)
 
-        if capture.stderr.getvalue():
-            first = capture.stderr.getvalue().strip().splitlines()[0].strip()
+        stderr_value = capture.stderr.buffer.getvalue()
+        if stderr_value:
+            first = stderr_value.decode().strip().splitlines()[0].strip()
             report_message(path, 0, 0, 'stderr', first, messages)
 
     def report_message(path, line, column, code, message, messages):