fix: make SIGINT attachment atomic and enable macOS setsid tests

.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

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

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()