This option is similar to --expose-pids, except that instead of making
the subsandbox use a nested pid namespace inside the parent's, it makes
the subsandbox share the parent's pid namespace as-is, so that process
IDs in the parent and the subsandbox are interchangeable. This will
be useful if the parent and the subsandbox communicate via protocols
that assume a global view of the process ID namespace, for example
passing process IDs across an AF_UNIX socket or in shared memory.
This option is similar to --expose-pids, except that instead of making the subsandbox use a nested pid namespace inside the parent's, it makes the subsandbox share the parent's pid namespace as-is, so that process IDs in the parent and the subsandbox are interchangeable. This will be useful if the parent and the subsandbox communicate via protocols that assume a global view of the process ID namespace, for example passing process IDs across an AF_UNIX socket or in shared memory.
See https://github.com/flatpak/flatpak/pull/4060 for the server side of this feature.