Let subprocess launched by run handle its own sigint (#246)

Otherwise, the subprocess is aborted by the parent process. This change
makes `spin run python -i ...` usable.

Tested on Windows, but on that platform no changes seem to be necessary.

Closes #235

spin/cmds/meson.py

@@ -4,6 +4,7 @@
 import os
 import re
 import shutil
+import signal
 import sys
 from enum import Enum
 from pathlib import Path
@@ -815,7 +816,27 @@ def run(ctx, *, args, build_dir=None):
             cmd_args = ["bash", "-c", cmd_args]
 
     _set_pythonpath(build_dir, quiet=True)
-    p = _run(cmd_args, echo=False, shell=shell, sys_exit=False)
+
+    is_windows = sys.platform == "win32"
+
+    if not is_windows:
+        # Let the subprocess handle its own signals
+        # Except on Windows, where it already seems to work as intended,
+        # and `preexec_fn` is not supported
+        signal.signal(signal.SIGINT, signal.SIG_IGN)
+
+    def attach_sigint():
+        # Reset SIGINT handler to default
+        signal.signal(signal.SIGINT, signal.SIG_DFL)
+
+    # --- launch subprocess ---
+    p = _run(
+        cmd_args,
+        echo=False,
+        shell=shell,
+        sys_exit=False,
+        preexec_fn=None if is_windows else attach_sigint,
+    )
 
     # Is the user trying to run a Python script, without calling the Python interpreter?
     executable = args[0]