Merge pull request #1253 from TitanNano/jovan/android_main_thread

Godot can use a different "main thread" for the main loop

Author: Bromeon

godot-core/src/init/mod.rs

@@ -158,6 +158,10 @@ unsafe fn gdext_on_level_init(level: InitLevel) {
     unsafe { sys::load_class_method_table(level) };
 
     match level {
+        InitLevel::Servers => {
+            // SAFETY: called from the main thread, sys::initialized has already been called.
+            unsafe { sys::discover_main_thread() };
+        }
         InitLevel::Scene => {
             // SAFETY: On the main thread, api initialized, `Scene` was initialized above.
             unsafe { ensure_godot_features_compatible() };

godot-ffi/src/binding/single_threaded.rs

@@ -123,8 +123,7 @@ impl BindingStorage {
 
         // We only check if we are in the main thread in debug builds if we aren't building for a non-threaded Godot build,
         // since we could otherwise assume there won't be multi-threading.
-        // TODO: figure out why the panic happens on Android, and how to resolve it. See https://github.com/godot-rust/gdext/pull/780.
-        #[cfg(all(debug_assertions, not(wasm_nothreads), not(target_os = "android")))]
+        #[cfg(all(debug_assertions, not(wasm_nothreads)))]
         {
             if !crate::is_main_thread() {
                 // If a binding is accessed the first time, this will panic and start unwinding. It can then happen that during unwinding,

godot-ffi/src/lib.rs

@@ -447,6 +447,32 @@ pub fn is_main_thread() -> bool {
     }
 }
 
+/// Assign the current thread id to be the main thread.
+///
+/// This is required for platforms on which Godot runs the main loop on a different thread than the thread the library was loaded on.
+/// Android is one such platform.
+///
+/// # Safety
+///
+/// - must only be called after [`initialize`] has been called.
+pub unsafe fn discover_main_thread() {
+    #[cfg(not(wasm_nothreads))]
+    {
+        if is_main_thread() {
+            // we don't have to do anything if the current thread is already the main thread.
+            return;
+        }
+
+        let thread_id = std::thread::current().id();
+
+        // SAFETY: initialize must have already been called before this function is called. By clearing and setting the cell again we can reinitialize it.
+        unsafe {
+            MAIN_THREAD_ID.clear();
+            MAIN_THREAD_ID.set(thread_id);
+        }
+    }
+}
+
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Macros to access low-level function bindings