uefi: less panics in system_table_boot/system_table_runtime

This gives callers a chance to react, whether boot services are available
or not.

Author: phip1611

uefi/src/table/mod.rs

@@ -37,42 +37,40 @@ pub unsafe fn set_system_table(ptr: *const uefi_raw::table::system::SystemTable)
 ///
 /// # Panics
 ///
-/// Panics if the system table has not been set with `set_system_table`, or if
-/// boot services are not available (e.g. if [`exit_boot_services`] has been
-/// called).
+/// Panics if the system table has not been set with [`set_system_table`].
 ///
 /// [`exit_boot_services`]: SystemTable::exit_boot_services
-pub fn system_table_boot() -> SystemTable<Boot> {
+pub fn system_table_boot() -> Option<SystemTable<Boot>> {
     let st = SYSTEM_TABLE.load(Ordering::Acquire);
     assert!(!st.is_null());
 
     // SAFETY: the system table is valid per the requirements of `set_system_table`.
     unsafe {
         if (*st).boot_services.is_null() {
-            panic!("boot services are not active");
+            None
+        } else {
+            Some(SystemTable::<Boot>::from_ptr(st.cast()).unwrap())
         }
-
-        SystemTable::<Boot>::from_ptr(st.cast()).unwrap()
     }
 }
 
-/// Get the system table while runtime services are active.
+/// Get the system table while runtime services are active. This only returns
+/// `Some` after boot services have been exited.
 ///
 /// # Panics
 ///
-/// Panics if the system table has not been set with `set_system_table`, or if
-/// runtime services are not available.
-pub fn system_table_runtime() -> SystemTable<Runtime> {
+/// Panics if the system table has not been set with [`set_system_table`].
+pub fn system_table_runtime() -> Option<SystemTable<Runtime>> {
     let st = SYSTEM_TABLE.load(Ordering::Acquire);
     assert!(!st.is_null());
 
     // SAFETY: the system table is valid per the requirements of `set_system_table`.
     unsafe {
         if (*st).runtime_services.is_null() {
-            panic!("runtime services are not active");
+            None
+        } else {
+            Some(SystemTable::<Runtime>::from_ptr(st.cast()).unwrap())
         }
-
-        SystemTable::<Runtime>::from_ptr(st.cast()).unwrap()
     }
 }