x11: use get_or_init instead of the set/get dance for the offset

This is how OnceLock is really meant to be used.. This will be helpful
for the custom init support, where various ways of starting the bridge
loop will be used.

Signed-off-by: Val Packett <[email protected]>

Author: valpackett

crates/muvm/src/guest/bridge/x11.rs

@@ -4,7 +4,6 @@ use std::ffi::{c_long, c_void, CString};
 use std::fs::{read_to_string, remove_file, File};
 use std::io::{IoSlice, Write};
 use std::os::fd::{AsFd, AsRawFd, BorrowedFd, OwnedFd};
-use std::process::exit;
 use std::ptr::NonNull;
 use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
 use std::sync::{Arc, OnceLock};
@@ -704,7 +703,7 @@ impl RemoteCaller {
 
         // Find the vDSO and the address of a syscall instruction within it
         let (vdso_start, _) = find_vdso(Some(pid))?;
-        let syscall_addr = vdso_start + SYSCALL_OFFSET.get().unwrap();
+        let syscall_addr = vdso_start + SYSCALL_OFFSET.get_or_init(find_syscall_offset);
 
         let mut regs = old_regs;
         arch::set_syscall_addr(&mut regs, syscall_addr);
@@ -847,9 +846,7 @@ fn find_vdso(pid: Option<Pid>) -> Result<(usize, usize), Errno> {
     Err(Errno::EINVAL)
 }
 
-pub fn start_x11bridge(display: u32) {
-    let sock_path = format!("/tmp/.X11-unix/X{display}");
-
+fn find_syscall_offset() -> usize {
     // Look for a syscall instruction in the vDSO. We assume all processes map
     // the same vDSO (which should be true if they are running under the same
     // kernel!)
@@ -858,14 +855,13 @@ pub fn start_x11bridge(display: u32) {
         let addr = vdso_start + off;
         let val = unsafe { std::ptr::read(addr as *const arch::SyscallInstr) };
         if val == arch::SYSCALL_INSTR {
-            SYSCALL_OFFSET.set(off).unwrap();
-            break;
+            return off;
         }
     }
-    if SYSCALL_OFFSET.get().is_none() {
-        eprintln!("Failed to find syscall instruction in vDSO");
-        exit(1);
-    }
+    panic!("Failed to find syscall instruction in vDSO");
+}
 
+pub fn start_x11bridge(display: u32) {
+    let sock_path = format!("/tmp/.X11-unix/X{display}");
     common::bridge_loop::<X11ProtocolHandler>(&sock_path)
 }