feat(dm): keep the display manager and its sessions out of the OOM killer's way

data/cutefish-dm.service

@@ -11,6 +11,8 @@ OnFailure=plymouth-quit.service
 ExecStart=/usr/bin/cutefish-dm
 # The daemon ends the login screen and the session itself.
 KillMode=mixed
+# Its end takes the login screen and every session with it.
+OOMScoreAdjust=-900
 Restart=always
 RestartSec=1s
 EnvironmentFile=-/etc/default/locale

helper/main.cpp

@@ -402,6 +402,18 @@ int main()
         arguments.push_back(const_cast<char *>(argument.c_str()));
     arguments.push_back(nullptr);
 
+    // Out of the OOM killer's way, inherited by the session and everything it
+    // starts; this process's end ends the session too. The session puts its
+    // applications back to 0. Not -1000: a runaway session process must remain
+    // the kernel's last resort. Written as root, it is also the floor no
+    // descendant can go below.
+    const int protection = open("/proc/self/oom_score_adj", O_WRONLY | O_CLOEXEC);
+    if (protection >= 0) {
+        if (write(protection, "-900", 4) != 4)
+            perror("oom_score_adj");
+        close(protection);
+    }
+
     // The session runs in a child; this process stays to close the PAM session.
     const pid_t child = fork();
     if (child == 0) {