core: introduce new Type=exec service type

Users are often surprised that "systemd-run" command lines like
"systemd-run -p User=idontexist /bin/true" will return successfully,
even though the logs show that the process couldn't be invoked, as the
user "idontexist" doesn't exist. This is because Type=simple will only
wait until fork() succeeded before returning start-up success.

This patch adds a new service type Type=exec, which is very similar to
Type=simple, but waits until the child process completed the execve()
before returning success. It uses a pipe that has O_CLOEXEC set for this
logic, so that the kernel automatically sends POLLHUP on it when the
execve() succeeded but leaves the pipe open if not. This means PID 1
waits exactly until the execve() succeeded in the child, and not longer
and not shorter, which is the desired functionality.

Making use of this new functionality, the command line
"systemd-run -p User=idontexist -p Type=exec /bin/true" will now fail,
as expected.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/core/swap.c b/src/core/swap.c
index b78b1aa266..e01e61e56d 100644
--- a/src/core/swap.c
+++ b/src/core/swap.c
@@ -606,6 +606,7 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
                 .stdin_fd  = -1,
                 .stdout_fd = -1,
                 .stderr_fd = -1,
+                .exec_fd   = -1,
         };
         pid_t pid;
         int r;