Treat a process reaped mid-read as gone in the build-time watcher

python/lean_pool/build_time_limits.py

@@ -191,7 +191,8 @@ def _process_identity(pid: int) -> tuple[int, int, int] | None:
     """Read kernel start ticks, current parent and group for a candidate PID."""
     try:
         stat = Path(f"/proc/{pid}/stat").read_text()
-    except FileNotFoundError:
+    except (FileNotFoundError, ProcessLookupError):
+        # ProcessLookupError: the process was reaped between open and read.
         return None
     fields = stat.rsplit(")", 1)[1].split()
     return int(fields[19]), int(fields[1]), int(fields[2])
@@ -322,7 +323,7 @@ def _process_state(pid: int) -> str | None:
     """Read the leader state without reaping its PID or releasing its group ID."""
     try:
         stat = Path(f"/proc/{pid}/stat").read_text()
-    except FileNotFoundError:
+    except (FileNotFoundError, ProcessLookupError):
         return None
     return stat.rsplit(")", 1)[1].split()[0]
 

python/tests/test_build_time_limits.py

@@ -784,3 +784,14 @@ def test_workflow_rebuilds_changed_modules_after_all_compiled_restores() -> None
                 assert restore_index < index, (name, step.get("name"))
             if "-m lean_pool.queue_build" in run and "--receipts" in run:
                 assert restore_index > index
+
+
+@pytest.mark.parametrize("reader", [limits._process_identity, limits._process_state])
+def test_a_process_reaped_mid_read_counts_as_gone(monkeypatch, reader) -> None:
+    """Reading /proc/PID/stat of a just-reaped process raises ESRCH, not ENOENT."""
+
+    def reaped(self, *args, **kwargs):
+        raise ProcessLookupError(3, "No such process")
+
+    monkeypatch.setattr(limits.Path, "read_text", reaped)
+    assert reader(os.getpid()) is None