fix(runtime): route late writes around closed capture buffers

src/nooa/runtime/stream_wrappers.py

@@ -44,25 +44,42 @@ def __init__(
         self.mode = getattr(original, "mode", "w")
 
     def write(self, data: str) -> int:
-        """Write to contextvar buffer if set, otherwise to original stream."""
+        """Write to the task buffer, falling back if a stale buffer was closed."""
         buffer = self._buffer_var.get()
         if buffer is not None:
-            return buffer.write(data)
+            try:
+                return buffer.write(data)
+            except ValueError:
+                # Background tasks inherit contextvars.  They can outlive the
+                # execution capture that installed this buffer, leaving a
+                # closed StringIO in their copied context.  Preserve logging
+                # and exception reporting by routing those late writes to the
+                # process stream instead.
+                if not getattr(buffer, "closed", False):
+                    raise
         return self._original.write(data)
 
     def writelines(self, lines: list[str]) -> None:
-        """Write multiple lines."""
+        """Write multiple lines, falling back if a stale buffer was closed."""
         buffer = self._buffer_var.get()
         if buffer is not None:
-            buffer.writelines(lines)
-        else:
-            self._original.writelines(lines)
+            try:
+                buffer.writelines(lines)
+                return
+            except ValueError:
+                if not getattr(buffer, "closed", False):
+                    raise
+        self._original.writelines(lines)
 
     def flush(self) -> None:
-        """Flush both buffer and original stream."""
+        """Flush the active buffer when usable, then the original stream."""
         buffer = self._buffer_var.get()
         if buffer is not None:
-            buffer.flush()
+            try:
+                buffer.flush()
+            except ValueError:
+                if not getattr(buffer, "closed", False):
+                    raise
         self._original.flush()
 
     def fileno(self) -> int:

tests/runtime/test_stream_wrappers.py

@@ -68,6 +68,17 @@ def test_write_returns_length_from_original(self):
         n = stream.write("xyz")
         assert n == 3
 
+    def test_write_falls_back_to_original_when_inherited_buffer_is_closed(self):
+        stream, buf_var, original = make_stream()
+        buf = io.StringIO()
+        token = buf_var.set(buf)
+        buf.close()
+        try:
+            assert stream.write("late log") == len("late log")
+            assert original.getvalue() == "late log"
+        finally:
+            buf_var.reset(token)
+
 
 class TestContextVarStreamWritelines:
     """Tests for ContextVarStream.writelines() routing."""
@@ -88,6 +99,17 @@ def test_writelines_to_original_when_no_buffer(self):
         stream.writelines(["x", "y", "z"])
         assert original.getvalue() == "xyz"
 
+    def test_writelines_falls_back_to_original_when_buffer_is_closed(self):
+        stream, buf_var, original = make_stream()
+        buf = io.StringIO()
+        token = buf_var.set(buf)
+        buf.close()
+        try:
+            stream.writelines(["late", " log"])
+            assert original.getvalue() == "late log"
+        finally:
+            buf_var.reset(token)
+
 
 class TestContextVarStreamFlush:
     """Tests for ContextVarStream.flush() behavior with and without a buffer."""
@@ -118,6 +140,18 @@ def test_flush_without_buffer_flushes_original(self):
         stream.flush()
         mock_original.flush.assert_called_once()
 
+    def test_flush_ignores_closed_inherited_buffer(self):
+        stream, buf_var, original = make_stream()
+        buf = io.StringIO()
+        token = buf_var.set(buf)
+        buf.close()
+        try:
+            stream.flush()  # must not mask late exception reporting
+            original.write("still usable")
+            assert original.getvalue() == "still usable"
+        finally:
+            buf_var.reset(token)
+
 
 class TestContextVarStreamFileno:
     """Tests for ContextVarStream.fileno() delegation to the original stream."""