[Python] Avoid SIGTTOU when importing readline from background process groups

python/tvm/base.py

@@ -66,11 +66,33 @@
         _RUNTIME_ONLY = True
 
 
-try:
-    # The following import is needed for TVM to work with pdb
-    import readline  # pylint: disable=unused-import
-except ImportError:
-    pass
+def _is_background_process_group():
+    """Return whether this process group is background on its controlling TTY."""
+    if not hasattr(os, "tcgetpgrp"):
+        return False
+
+    try:
+        tty_fd = os.open("/dev/tty", os.O_RDONLY | os.O_NOCTTY)
+    except OSError:
+        return False
+
+    try:
+        return os.tcgetpgrp(tty_fd) != os.getpgrp()
+    except OSError:
+        return False
+    finally:
+        os.close(tty_fd)
+
+
+# Import readline for pdb compatibility. A libedit-backed readline touches the
+# terminal during import, so skip it while this process is in a background
+# process group.
+if not _is_background_process_group():
+    try:
+        import readline  # pylint: disable=unused-import
+    except ImportError:
+        pass
+
 
 # version number
 __version__ = libinfo.__version__

tests/python/all-platform-minimal-test/test_runtime_import.py

@@ -0,0 +1,95 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""TVM import behavior tests."""
+
+import errno
+import os
+import sys
+
+import pytest
+
+pytestmark = pytest.mark.skipif(os.name != "posix", reason="requires POSIX job control")
+
+
+def _run_in_pty(script):
+    import pty
+
+    pid, output_fd = pty.fork()
+    if pid == 0:
+        os.execl(sys.executable, sys.executable, "-c", script)
+
+    output = bytearray()
+    while True:
+        try:
+            chunk = os.read(output_fd, 4096)
+        except OSError as err:
+            if err.errno == errno.EIO:
+                break
+            raise
+        if not chunk:
+            break
+        output.extend(chunk)
+
+    _, status = os.waitpid(pid, 0)
+    os.close(output_fd)
+    assert os.waitstatus_to_exitcode(status) == 0, output.decode(errors="replace")
+
+
+def test_import_preloads_readline_for_foreground_tty():
+    _run_in_pty(
+        """
+import importlib.util
+import sys
+
+assert "readline" not in sys.modules
+readline_is_available = importlib.util.find_spec("readline") is not None
+import tvm
+assert ("readline" in sys.modules) == readline_is_available
+import pdb
+pdb.Pdb()
+"""
+    )
+
+
+def test_import_does_not_stop_background_process_group():
+    _run_in_pty(
+        """
+import os
+import signal
+import sys
+
+for redirect_stdin in (False, True):
+    pid = os.fork()
+    if pid == 0:
+        os.setpgid(0, 0)
+        if redirect_stdin:
+            stdin_fd = os.open(os.devnull, os.O_RDONLY)
+            os.dup2(stdin_fd, 0)
+            os.close(stdin_fd)
+        import tvm
+        assert "readline" not in sys.modules
+        os._exit(0)
+
+    _, status = os.waitpid(pid, os.WUNTRACED)
+    if os.WIFSTOPPED(status):
+        os.kill(pid, signal.SIGKILL)
+        os.waitpid(pid, 0)
+        raise AssertionError(f"TVM import stopped by signal {os.WSTOPSIG(status)}")
+    if os.waitstatus_to_exitcode(status) != 0:
+        raise AssertionError(f"TVM import exited with status {status}")
+"""
+    )