Auto merge of #26792 - servo:static-mut, r=nox

Replace `static mut` with `const`, `static`+`AtomicPtr`, or `static`+`UnsafeCell`

Fixes https://github.com/servo/servo/issues/26550

components/background_hang_monitor/sampler_linux.rs

@@ -23,12 +23,19 @@ use unwind_sys::{
 #[link(name = "lzma")]
 extern "C" {}
 
-static mut SHARED_STATE: SharedState = SharedState {
-    msg2: None,
-    msg3: None,
-    msg4: None,
-    context: AtomicPtr::new(ptr::null_mut()),
-};
+struct UncheckedSyncUnsafeCell<T>(std::cell::UnsafeCell<T>);
+
+/// Safety: dereferencing the pointer from `UnsafeCell::get` must involve external synchronization
+unsafe impl<T> Sync for UncheckedSyncUnsafeCell<T> {}
+
+static SHARED_STATE: UncheckedSyncUnsafeCell<SharedState> =
+    UncheckedSyncUnsafeCell(std::cell::UnsafeCell::new(SharedState {
+        msg2: None,
+        msg3: None,
+        msg4: None,
+    }));
+
+static CONTEXT: AtomicPtr<libc::ucontext_t> = AtomicPtr::new(ptr::null_mut());
 
 type MonitoredThreadId = libc::pid_t;
 
@@ -37,25 +44,30 @@ struct SharedState {
     msg2: Option<PosixSemaphore>,
     msg3: Option<PosixSemaphore>,
     msg4: Option<PosixSemaphore>,
-    context: AtomicPtr<libc::ucontext_t>,
 }
 
 fn clear_shared_state() {
+    // Safety: this is only called from the sampling thread (there’s only one)
+    // Sampled threads only access SHARED_STATE in their signal handler.
+    // This signal and the semaphores in SHARED_STATE provide the necessary synchronization.
     unsafe {
-        SHARED_STATE.msg2 = None;
-        SHARED_STATE.msg3 = None;
-        SHARED_STATE.msg4 = None;
-        SHARED_STATE.context = AtomicPtr::new(ptr::null_mut());
+        let shared_state = &mut *SHARED_STATE.0.get();
+        shared_state.msg2 = None;
+        shared_state.msg3 = None;
+        shared_state.msg4 = None;
     }
+    CONTEXT.store(ptr::null_mut(), Ordering::SeqCst);
 }
 
 fn reset_shared_state() {
+    // Safety: same as clear_shared_state
     unsafe {
-        SHARED_STATE.msg2 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
-        SHARED_STATE.msg3 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
-        SHARED_STATE.msg4 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
-        SHARED_STATE.context = AtomicPtr::new(ptr::null_mut());
+        let shared_state = &mut *SHARED_STATE.0.get();
+        shared_state.msg2 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
+        shared_state.msg3 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
+        shared_state.msg4 = Some(PosixSemaphore::new(0).expect("valid semaphore"));
     }
+    CONTEXT.store(ptr::null_mut(), Ordering::SeqCst);
 }
 
 struct PosixSemaphore {
@@ -194,16 +206,18 @@ impl Sampler for LinuxSampler {
 
         // signal the thread, wait for it to tell us state was copied.
         send_sigprof(self.thread_id);
-        unsafe {
-            SHARED_STATE
-                .msg2
-                .as_ref()
-                .unwrap()
-                .wait_through_intr()
-                .expect("msg2 failed");
-        }
 
-        let context = unsafe { SHARED_STATE.context.load(Ordering::SeqCst) };
+        // Safety: non-exclusive reference only
+        // since sampled threads are accessing this concurrently
+        let shared_state = unsafe { &*SHARED_STATE.0.get() };
+        shared_state
+            .msg2
+            .as_ref()
+            .unwrap()
+            .wait_through_intr()
+            .expect("msg2 failed");
+
+        let context = CONTEXT.load(Ordering::SeqCst);
         let mut cursor = mem::MaybeUninit::uninit();
         let ret = unsafe { unw_init_local(cursor.as_mut_ptr(), context) };
         let result = if ret == UNW_ESUCCESS {
@@ -231,24 +245,23 @@ impl Sampler for LinuxSampler {
         };
 
         // signal the thread to continue.
-        unsafe {
-            SHARED_STATE
-                .msg3
-                .as_ref()
-                .unwrap()
-                .post()
-                .expect("msg3 failed");
-        }
+        shared_state
+            .msg3
+            .as_ref()
+            .unwrap()
+            .post()
+            .expect("msg3 failed");
 
         // wait for thread to continue.
-        unsafe {
-            SHARED_STATE
-                .msg4
-                .as_ref()
-                .unwrap()
-                .wait_through_intr()
-                .expect("msg4 failed");
-        }
+        shared_state
+            .msg4
+            .as_ref()
+            .unwrap()
+            .wait_through_intr()
+            .expect("msg4 failed");
+
+        // No-op, but marks the end of the shared borrow
+        drop(shared_state);
 
         clear_shared_state();
 
@@ -271,26 +284,27 @@ extern "C" fn sigprof_handler(
     ctx: *mut libc::c_void,
 ) {
     assert_eq!(sig, libc::SIGPROF);
-    unsafe {
-        // copy the context.
-        SHARED_STATE
-            .context
-            .store(ctx as *mut libc::ucontext_t, Ordering::SeqCst);
-        // Tell the sampler we copied the context.
-        SHARED_STATE.msg2.as_ref().unwrap().post().expect("posted");
+    // copy the context.
+    CONTEXT.store(ctx as *mut libc::ucontext_t, Ordering::SeqCst);
 
-        // Wait for sampling to finish.
-        SHARED_STATE
-            .msg3
-            .as_ref()
-            .unwrap()
-            .wait_through_intr()
-            .expect("msg3 wait succeeded");
+    // Safety: non-exclusive reference only
+    // since the sampling thread is accessing this concurrently
+    let shared_state = unsafe { &*SHARED_STATE.0.get() };
 
-        // OK we are done!
-        SHARED_STATE.msg4.as_ref().unwrap().post().expect("posted");
-        // DO NOT TOUCH shared state here onwards.
-    }
+    // Tell the sampler we copied the context.
+    shared_state.msg2.as_ref().unwrap().post().expect("posted");
+
+    // Wait for sampling to finish.
+    shared_state
+        .msg3
+        .as_ref()
+        .unwrap()
+        .wait_through_intr()
+        .expect("msg3 wait succeeded");
+
+    // OK we are done!
+    shared_state.msg4.as_ref().unwrap().post().expect("posted");
+    // DO NOT TOUCH shared state here onwards.
 }
 
 fn send_sigprof(to: libc::pid_t) {

components/script/dom/bindings/codegen/CodegenRust.py

@@ -2750,7 +2750,9 @@ class CGWrapMethod(CGAbstractMethod):
         unforgeable = CopyUnforgeablePropertiesToInstance(self.descriptor)
         if self.descriptor.proxy:
             create = """
-let handler = RegisterBindings::PROXY_HANDLERS[PrototypeList::Proxies::%(concreteType)s as usize];
+let handler: *const libc::c_void =
+    RegisterBindings::proxy_handlers::%(concreteType)s
+    .load(std::sync::atomic::Ordering::Acquire);
 rooted!(in(*cx) let obj = NewProxyObject(
     *cx,
     handler,
@@ -6744,7 +6746,10 @@ class CGRegisterProxyHandlersMethod(CGAbstractMethod):
 
     def definition_body(self):
         return CGList([
-            CGGeneric("PROXY_HANDLERS[Proxies::%s as usize] = Bindings::%s::DefineProxyHandler();"
+            CGGeneric("proxy_handlers::%s.store(\n"
+                      "    Bindings::%s::DefineProxyHandler() as *mut _,\n"
+                      "    std::sync::atomic::Ordering::Release,\n"
+                      ");"
                       % (desc.name, '::'.join([desc.name + 'Binding'] * 2)))
             for desc in self.descriptors
         ], "\n")
@@ -6753,10 +6758,18 @@ class CGRegisterProxyHandlersMethod(CGAbstractMethod):
 class CGRegisterProxyHandlers(CGThing):
     def __init__(self, config):
         descriptors = config.getDescriptors(proxy=True)
-        length = len(descriptors)
         self.root = CGList([
-            CGGeneric("pub static mut PROXY_HANDLERS: [*const libc::c_void; %d] = [0 as *const libc::c_void; %d];"
-                      % (length, length)),
+            CGGeneric(
+                "#[allow(non_upper_case_globals)]\n" +
+                "pub mod proxy_handlers {\n" +
+                "".join(
+                    "    pub static %s: std::sync::atomic::AtomicPtr<libc::c_void> =\n"
+                    "        std::sync::atomic::AtomicPtr::new(std::ptr::null_mut());\n"
+                    % desc.name
+                    for desc in descriptors
+                ) +
+                "}\n"
+            ),
             CGRegisterProxyHandlersMethod(descriptors),
         ], "\n")
 
@@ -7606,8 +7619,6 @@ class GlobalGenRoots():
                                for d in config.getDescriptors(hasInterfaceObject=True)
                                if d.shouldHaveGetConstructorObjectMethod()])
 
-        proxies = [d.name for d in config.getDescriptors(proxy=True)]
-
         return CGList([
             CGGeneric(AUTOGENERATED_WARNING_COMMENT),
             CGGeneric("pub const PROTO_OR_IFACE_LENGTH: usize = %d;\n" % (len(protos) + len(constructors))),
@@ -7624,7 +7635,6 @@ class GlobalGenRoots():
                       "    debug_assert!(proto_id < ID::Last as u16);\n"
                       "    INTERFACES[proto_id as usize]\n"
                       "}\n\n"),
-            CGNonNamespacedEnum('Proxies', proxies, 0, deriving="PartialEq, Copy, Clone"),
         ])
 
     @staticmethod
@@ -7636,8 +7646,6 @@ class GlobalGenRoots():
 
         return CGImports(code, descriptors=[], callbacks=[], dictionaries=[], enums=[], typedefs=[], imports=[
             'crate::dom::bindings::codegen::Bindings',
-            'crate::dom::bindings::codegen::PrototypeList::Proxies',
-            'libc',
         ], config=config, ignored_warnings=[])
 
     @staticmethod

components/script/dom/eventtarget.rs

@@ -515,8 +515,8 @@ impl EventTarget {
         let name = CString::new(format!("on{}", &**ty)).unwrap();
 
         // Step 3.9, subsection ParameterList
-        static mut ARG_NAMES: [*const c_char; 1] = [b"event\0" as *const u8 as *const c_char];
-        static mut ERROR_ARG_NAMES: [*const c_char; 5] = [
+        const ARG_NAMES: &[*const c_char] = &[b"event\0" as *const u8 as *const c_char];
+        const ERROR_ARG_NAMES: &[*const c_char] = &[
             b"event\0" as *const u8 as *const c_char,
             b"source\0" as *const u8 as *const c_char,
             b"lineno\0" as *const u8 as *const c_char,
@@ -524,13 +524,7 @@ impl EventTarget {
             b"error\0" as *const u8 as *const c_char,
         ];
         let is_error = ty == &atom!("error") && self.is::<Window>();
-        let args = unsafe {
-            if is_error {
-                &ERROR_ARG_NAMES[..]
-            } else {
-                &ARG_NAMES[..]
-            }
-        };
+        let args = if is_error { ERROR_ARG_NAMES } else { ARG_NAMES };
 
         let cx = window.get_cx();
         let options = unsafe {