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, 3 insertions, 0 deletions

diff --git a/src/core/execute.h b/src/core/execute.h
index aa6f1ccfda..cd5165c2d1 100644
--- a/src/core/execute.h
+++ b/src/core/execute.h
@@ -317,6 +317,9 @@ struct ExecParameters {
         int stdin_fd;
         int stdout_fd;
         int stderr_fd;
+
+        /* An fd that is closed by the execve(), and thus will result in EOF when the execve() is done */
+        int exec_fd;
 };
 
 #include "unit.h"