Kernel+LibCore: Make %sid path parsing not take ages

Before this patch, Core::SessionManagement::parse_path_with_sid() would
figure out the root session ID by sifting through /sys/kernel/processes.

That file can take quite a while to generate (sometimes up to 40ms on my
machine, which is a problem on its own!) and with no caching, many of
our programs were effectively doing this multiple times on startup when
unveiling something in /tmp/session/%sid/

While we should find ways to make generating /sys/kernel/processes fast
again, this patch addresses the specific problem by introducing a new
syscall: sys$get_root_session_id(). This extracts the root session ID
by looking directly at the process table and takes <1ms instead of 40ms.

This cuts WebContent process startup time by ~100ms on my machine. :^)

1 files changed, 21 insertions, 0 deletions

diff --git a/Kernel/Syscalls/setpgid.cpp b/Kernel/Syscalls/setpgid.cpp
index fffdecd749..fd7715c2b3 100644
--- a/Kernel/Syscalls/setpgid.cpp
+++ b/Kernel/Syscalls/setpgid.cpp
@@ -139,4 +139,25 @@ ErrorOr<FlatPtr> Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
     });
 }
 
+ErrorOr<FlatPtr> Process::sys$get_root_session_id(pid_t force_sid)
+{
+    pid_t sid = (force_sid == -1) ? this->sid().value() : force_sid;
+    if (sid == 0)
+        return 0;
+    while (true) {
+        auto sid_process = Process::from_pid_in_same_jail(sid);
+        if (!sid_process)
+            return ESRCH;
+        auto parent_pid = sid_process->ppid().value();
+        auto parent_process = Process::from_pid_in_same_jail(parent_pid);
+        if (!parent_process)
+            return ESRCH;
+        pid_t parent_sid = parent_process->sid().value();
+        if (parent_sid == 0)
+            break;
+        sid = parent_sid;
+    }
+    return sid;
+}
+
 }