From e570f91611bf5991f7a2e5684ab39825a871dcd6 Mon Sep 17 00:00:00 2001
From: leej3 <johnleenimh@gmail.com>
Date: Sun, 11 Oct 2026 15:47:45 -0400
Subject: [PATCH] fix: make SIGINT attachment atomic and enable macOS setsid
 tests

Block SIGINT during PID attachment and pending-signal forwarding. Add a
regression test and install util-linux for macOS CI session coverage.

Co-Authored-By: Codex Desktop 26.1007.21159 (runtime codex-cli 0.162.0-alpha.17.2) / gpt-6-astra <codex@openai.com>
Codex-Reasoning-Effort: low
---
 .github/workflows/test.yaml    |  6 ++++++
 src/con_duct/_signals.py       | 12 +++++++++---
 test/duct_main/test_signals.py | 17 +++++++++++++++++
 3 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 3bd538d9..a00c31ee 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -72,6 +72,12 @@ jobs:
         name: Install libjpeg on Ubuntu
         run: sudo apt-get install -y libjpeg-dev
 
+      - name: Install setsid on macOS
+        if: runner.os == 'macOS'
+        run: |
+          brew install util-linux
+          echo "$(brew --prefix util-linux)/bin" >> "$GITHUB_PATH"
+
       - name: Install uv
         if: matrix.toxenv == 'oldestdeps'
         run: python -m pip install uv
diff --git a/src/con_duct/_signals.py b/src/con_duct/_signals.py
index fa198509..82cc907b 100644
--- a/src/con_duct/_signals.py
+++ b/src/con_duct/_signals.py
@@ -53,9 +53,15 @@ def attach(self, pid: int) -> None:
             The PID of the process monitored by duct. A SIGINT that arrived
             while it was being started is acted on now.
         """
-        self.pid = pid
-        if self.sigcount:
-            self._act()
+        # A SIGINT after publishing the PID must not be forwarded both by
+        # __call__ and by the pending-signal check below.
+        old_mask = signal.pthread_sigmask(signal.SIG_BLOCK, {signal.SIGINT})
+        try:
+            self.pid = pid
+            if self.sigcount:
+                self._act()
+        finally:
+            signal.pthread_sigmask(signal.SIG_SETMASK, old_mask)
 
     def __call__(self, _sig: int, _frame: Optional[FrameType]) -> None:
         self.sigcount += 1
diff --git a/test/duct_main/test_signals.py b/test/duct_main/test_signals.py
index 1fe67690..bdc84a11 100644
--- a/test/duct_main/test_signals.py
+++ b/test/duct_main/test_signals.py
@@ -82,6 +82,23 @@ def test_attach_acts_on_earlier_sigints(
     assert sent == [(1234, expected)]
 
 
+@pytest.mark.usefixtures("restore_sigint")
+def test_sigint_during_attach_is_forwarded_once(sent: list[tuple[int, int]]) -> None:
+    class InterruptOnAttach(SigIntHandler):
+        def __setattr__(self, name: str, value: Any) -> None:
+            super().__setattr__(name, value)
+            if name == "pid" and value is not None:
+                # Deliver a real SIGINT immediately after publishing the PID.
+                signal.raise_signal(signal.SIGINT)
+
+    handler = InterruptOnAttach()
+    signal.signal(signal.SIGINT, handler)
+    handler.attach(1234)
+
+    assert handler.sigcount == 1
+    assert sent == [(1234, signal.SIGINT)]
+
+
 @pytest.mark.usefixtures("stderr_locked")
 def test_forwards_even_if_logging_fails(sent: list[tuple[int, int]]) -> None:
     handler = SigIntHandler()