Automatically kill processes in bootstrap inner mount

Minimal bug reproducer for this issue is:

$ mock --postinstall --isolation=simple https://github.com/rpm-software-management/mock-test-data/raw/main/daemontest-1-0.src.rpm
... successful build, mock exists ...
$ pgrep daemontest -a
41652 daemontest PID=41652

Fixes: #1165
Closes: #1166

mock/py/mock.py

@@ -800,7 +800,7 @@ def main():
                                  options="private"))
         buildroot.mounts.bootstrap_mounts.append(
             BindMountPoint(buildroot.make_chroot_path(), inner_mount,
-                           recursive=True, options="private"))
+                           recursive=True, options="private").treat_as_chroot())
 
     signal.signal(signal.SIGTERM, partial(handle_signals, buildroot))
     signal.signal(signal.SIGPIPE, partial(handle_signals, buildroot))

mock/py/mockbuild/mounts.py

@@ -19,6 +19,15 @@ def __init__(self, mountsource, mountpath):
         self.mountpath = mountpath
         self.mountsource = mountsource
         self.mounted = None
+        self._is_chroot = False
+
+    def treat_as_chroot(self):
+        """
+        If we use this directory as chroot, we might want to do special
+        actions while mounting and unmounting.
+        """
+        self._is_chroot = True
+        return self
 
     @traceLog()
     # pylint: disable=unused-argument
@@ -28,6 +37,12 @@ def umount(self, force=False, nowarn=False):
         """
         if not self.mounted:
             return None
+
+        if self._is_chroot:
+            # Don't keep background processes running with unmounted
+            # /proc/self/root directory.
+            util.orphansKill(self.mountpath)
+
         if self._do_umount():
             self.mounted = False
             return True