Read the whole bwrap info object before parsing the child pid

bwrap writes --info-fd JSON in several writes. A single os.read could
return only '{"child-pid": N' and fail with a JSON error, observed
reproducibly on railiance01 for the rebuilt owner runtime. Read until a
complete object or EOF within the existing 10 s bound; a truncated object
at EOF is still refused. 207 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 272244@bnt-lap001
Assistant-Session: c8962fa7-b290-47df-865f-403ddb6c77e9

src/sandboxer/extensions/bwrap.py

@@ -113,14 +113,31 @@ class BwrapExtension(SandboxExtension):
 
     @staticmethod
     def _read_child_pid(proc: subprocess.Popen, info_fd: int) -> int:
-        ready, _, _ = select.select([info_fd], [], [], 10)
-        if not ready:
-            proc.kill()
-            raise RuntimeError("timed out waiting for bwrap namespace child pid")
-        raw = os.read(info_fd, 16_384)
+        # bwrap writes the info object in several writes; one read can return
+        # only its first line. Read until EOF or a complete object, bounded.
+        deadline = time.monotonic() + 10
+        raw = b""
+        info = None
+        while info is None:
+            remaining = deadline - time.monotonic()
+            ready, _, _ = select.select([info_fd], [], [], max(remaining, 0))
+            if not ready:
+                proc.kill()
+                raise RuntimeError("timed out waiting for bwrap namespace child pid")
+            chunk = os.read(info_fd, 16_384)
+            raw += chunk
+            if len(raw) > 16_384:
+                proc.kill()
+                raise RuntimeError("bwrap did not report a valid namespace child pid")
+            try:
+                info = json.loads(raw)
+            except json.JSONDecodeError as exc:
+                if not chunk:
+                    proc.kill()
+                    raise RuntimeError("bwrap did not report a valid namespace child pid") from exc
         try:
-            child_pid = int(json.loads(raw)["child-pid"])
-        except (json.JSONDecodeError, KeyError, TypeError, ValueError) as exc:
+            child_pid = int(info["child-pid"])
+        except (KeyError, TypeError, ValueError) as exc:
             proc.kill()
             raise RuntimeError("bwrap did not report a valid namespace child pid") from exc
         if child_pid <= 0:

tests/test_bwrap.py

@@ -362,3 +362,52 @@ def test_runner_bounds_output_and_normalizes_timeout(tmp_path) -> None:
     assert result["stdout"] == "123"
     assert result["stderr"] == "abc"
     assert result["output_truncated"] is True
+
+
+class _FakeProc:
+    def __init__(self) -> None:
+        self.killed = False
+
+    def kill(self) -> None:
+        self.killed = True
+
+
+def test_read_child_pid_accepts_info_split_across_writes() -> None:
+    import os
+    import threading
+    import time
+
+    read_fd, write_fd = os.pipe()
+
+    def writer() -> None:
+        os.write(write_fd, b'{\n    "child-pid": 1234567')
+        time.sleep(0.05)
+        os.write(write_fd, b',\n    "cgroup-namespace": 4026531835\n}\n')
+        os.close(write_fd)
+
+    thread = threading.Thread(target=writer)
+    thread.start()
+    try:
+        proc = _FakeProc()
+        assert BwrapExtension._read_child_pid(proc, read_fd) == 1234567
+        assert not proc.killed
+    finally:
+        thread.join()
+        os.close(read_fd)
+
+
+def test_read_child_pid_refuses_truncated_info_at_eof() -> None:
+    import os
+
+    import pytest
+
+    read_fd, write_fd = os.pipe()
+    os.write(write_fd, b'{\n    "child-pid": 1234567')
+    os.close(write_fd)
+    proc = _FakeProc()
+    try:
+        with pytest.raises(RuntimeError, match="valid namespace child pid"):
+            BwrapExtension._read_child_pid(proc, read_fd)
+        assert proc.killed
+    finally:
+        os.close(read_fd)