1 files changed, 98 insertions, 0 deletions
diff --git a/servo/components/style/thread_state.rs b/servo/components/style/thread_state.rs
new file mode 100644
index 0000000000..cb7cfd0d64
--- /dev/null
+++ b/servo/components/style/thread_state.rs
@@ -0,0 +1,98 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Supports dynamic assertions in about what sort of thread is running and
+//! what state it's in.
+
+#![deny(missing_docs)]
+
+use std::cell::RefCell;
+
+bitflags! {
+    /// A thread state flag, used for multiple assertions.
+    #[derive(Clone, Copy, Eq, PartialEq)]
+    pub struct ThreadState: u32 {
+        /// Whether we're in a script thread.
+        const SCRIPT          = 0x01;
+        /// Whether we're in a layout thread.
+        const LAYOUT          = 0x02;
+
+        /// Whether we're in a script worker thread (actual web workers), or in
+        /// a layout worker thread.
+        const IN_WORKER       = 0x0100;
+
+        /// Whether the current thread is going through a GC.
+        const IN_GC           = 0x0200;
+    }
+}
+
+macro_rules! thread_types ( ( $( $fun:ident = $flag:path ; )* ) => (
+    impl ThreadState {
+        /// Whether the current thread is a worker thread.
+        pub fn is_worker(self) -> bool {
+            self.contains(ThreadState::IN_WORKER)
+        }
+
+        $(
+            #[allow(missing_docs)]
+            pub fn $fun(self) -> bool {
+                self.contains($flag)
+            }
+        )*
+    }
+));
+
+thread_types! {
+    is_script = ThreadState::SCRIPT;
+    is_layout = ThreadState::LAYOUT;
+}
+
+thread_local!(static STATE: RefCell<Option<ThreadState>> = RefCell::new(None));
+
+/// Initializes the current thread state.
+pub fn initialize(x: ThreadState) {
+    STATE.with(|ref k| {
+        if let Some(ref s) = *k.borrow() {
+            if x != *s {
+                panic!("Thread state already initialized as {:?}", s);
+            }
+        }
+        *k.borrow_mut() = Some(x);
+    });
+}
+
+/// Initializes the current thread as a layout worker thread.
+pub fn initialize_layout_worker_thread() {
+    initialize(ThreadState::LAYOUT | ThreadState::IN_WORKER);
+}
+
+/// Gets the current thread state.
+pub fn get() -> ThreadState {
+    let state = STATE.with(|ref k| {
+        match *k.borrow() {
+            None => ThreadState::empty(), // Unknown thread.
+            Some(s) => s,
+        }
+    });
+
+    state
+}
+
+/// Enters into a given temporary state. Panics if re-entring.
+pub fn enter(x: ThreadState) {
+    let state = get();
+    debug_assert!(!state.intersects(x));
+    STATE.with(|ref k| {
+        *k.borrow_mut() = Some(state | x);
+    })
+}
+
+/// Exits a given temporary state.
+pub fn exit(x: ThreadState) {
+    let state = get();
+    debug_assert!(state.contains(x));
+    STATE.with(|ref k| {
+        *k.borrow_mut() = Some(state & !x);
+    })
+}