Delay KeyboardInterrupt() handling when running a subprocess

Because we weren't giving subprocess time to gracefully exit, we'd
often get "Device or resource is busy" errors when CTRL+C'ing mkosi
as we we'd try to unmount directories while a subprocess was still
running in it. By delaying handling of SIGINT while a subprocess is
running, the subprocess can exit gracefully which prevents EBUSY errors
from occurring when running cleanup code. To allow interrupting hanging
subprocesses, we only delay the first SIGINT we receive. If a user
presses CTRL+C again, we exit immediately as before.

diff --git a/mkosi b/mkosi
index 1d03f3437260e8f959ad099aabeaac9194ae77f0..9dc73ece00ce6ae92ca0163bd2a5ca1984635234 100755 (executable)
--- a/mkosi
+++ b/mkosi
@@ -21,6 +21,7 @@ import platform
 import re
 import shlex
 import shutil
+import signal
 import stat
 import string
 import subprocess
@@ -50,6 +51,7 @@ from typing import (
     cast,
     TYPE_CHECKING,
 )
+from types import FrameType
 
 __version__ = '5'
 
@@ -95,6 +97,31 @@ def print_error(text: str) -> None:
     sys.stderr.write(f"‣ \033[31;1m{text}\033[0m\n")
 
 
+@contextlib.contextmanager
+def delay_interrupt() -> Generator[None, None, None]:
+    # CTRL+C is sent to the entire process group. We delay its handling in mkosi itself so the subprocess can
+    # exit cleanly before doing mkosi's cleanup. If we don't do this, we get device or resource is busy
+    # errors when unmounting stuff later on during cleanup. We only delay a single CTRL+C interrupt so that a
+    # user can always exit mkosi even if a subprocess hangs by pressing CTRL+C twice.
+    interrupted = False
+    def handler(signal: int, frame: FrameType) -> None:
+        nonlocal interrupted
+        if interrupted:
+            raise KeyboardInterrupt()
+        else:
+            interrupted = True
+
+    s = signal.signal(signal.SIGINT, handler)
+
+    try:
+        yield
+    finally:
+        signal.signal(signal.SIGINT, s)
+
+        if interrupted:
+            die("Interrupted")
+
+
 def run(cmdline: List[str], execvp: bool = False, **kwargs: Any) -> CompletedProcess:
     if 'run' in arg_debug:
         sys.stderr.write('+ ' + ' '.join(shlex.quote(x) for x in cmdline) + '\n')
@@ -103,7 +130,8 @@ def run(cmdline: List[str], execvp: bool = False, **kwargs: Any) -> CompletedPro
             assert not kwargs
             os.execvp(cmdline[0], cmdline)
         else:
-            return subprocess.run(cmdline, **kwargs)
+            with delay_interrupt():
+                return subprocess.run(cmdline, **kwargs)
     except FileNotFoundError as e:
         die(f"{cmdline[0]} not found in PATH.")