Global main thread ID

godot-core/build.rs

@@ -17,4 +17,5 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
     godot_bindings::emit_godot_version_cfg();
+    godot_bindings::emit_wasm_nothreads_cfg();
 }

godot-core/src/init/mod.rs

@@ -16,6 +16,9 @@ use crate::out;
 
 pub use sys::GdextBuild;
 
+#[cfg(not(wasm_nothreads))]
+pub use sys::{is_main_thread, main_thread_id};
+
 #[doc(hidden)]
 #[deny(unsafe_op_in_unsafe_fn)]
 pub unsafe fn __gdext_load_library<E: ExtensionLibrary>(

godot-ffi/src/binding/single_threaded.rs

@@ -11,21 +11,13 @@
 
 use std::cell::Cell;
 
-#[cfg(not(wasm_nothreads))]
-use std::thread::ThreadId;
-
 use super::GodotBinding;
 use crate::ManualInitCell;
 
 pub(super) struct BindingStorage {
     // No threading when linking against Godot with a nothreads Wasm build.
     // Therefore, we just need to check if the bindings were initialized, as all accesses are from the main thread.
-    #[cfg(wasm_nothreads)]
     initialized: Cell<bool>,
-
-    // Is used in to check that we've been called from the right thread, so must be thread-safe to access.
-    #[cfg(not(wasm_nothreads))]
-    main_thread_id: Cell<Option<ThreadId>>,
     binding: ManualInitCell<GodotBinding>,
 }
 
@@ -38,11 +30,7 @@ impl BindingStorage {
     #[inline(always)]
     unsafe fn storage() -> &'static Self {
         static BINDING: BindingStorage = BindingStorage {
-            #[cfg(wasm_nothreads)]
             initialized: Cell::new(false),
-
-            #[cfg(not(wasm_nothreads))]
-            main_thread_id: Cell::new(None),
             binding: ManualInitCell::new(),
         };
 
@@ -53,11 +41,7 @@ impl BindingStorage {
     ///
     /// It is recommended to use this function for that purpose as the field to check varies depending on the compilation target.
     fn initialized(&self) -> bool {
-        #[cfg(wasm_nothreads)]
-        return self.initialized.get();
-
-        #[cfg(not(wasm_nothreads))]
-        self.main_thread_id.get().is_some()
+        self.initialized.get()
     }
 
     /// Marks the binding storage as initialized or deinitialized.
@@ -78,17 +62,7 @@ impl BindingStorage {
             }
         }
 
-        // 'std::thread::current()' fails when linking to a Godot web build without threads. When compiling to wasm-nothreads,
-        // we assume it is impossible to have multi-threading, so checking if we are in the main thread is not needed.
-        // Therefore, we don't store the thread ID, but rather just whether initialization already occurred.
-        #[cfg(wasm_nothreads)]
         self.initialized.set(initialized);
-
-        #[cfg(not(wasm_nothreads))]
-        {
-            let thread_id = initialized.then(|| std::thread::current().id());
-            self.main_thread_id.set(thread_id);
-        }
     }
 
     /// Initialize the binding storage, this must be called before any other public functions.
@@ -152,11 +126,7 @@ impl BindingStorage {
         // 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")))]
         {
-            let main_thread_id = storage.main_thread_id.get().expect(
-                "Godot engine not available; make sure you are not calling it from unit/doc tests",
-            );
-
-            if main_thread_id != std::thread::current().id() {
+            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,
                 // another FFI call happens (e.g. Godot destructor), which would cause immediate abort, swallowing the error message.
                 // Thus check if a panic is already in progress.

godot-ffi/src/lib.rs

@@ -84,6 +84,9 @@ use binding::{
     initialize_class_scene_method_table, initialize_class_server_method_table, runtime_metadata,
 };
 
+#[cfg(not(wasm_nothreads))]
+static MAIN_THREAD_ID: ManualInitCell<std::thread::ThreadId> = ManualInitCell::new();
+
 /// Stage of the Godot initialization process.
 ///
 /// Godot's initialization and deinitialization processes are split into multiple stages, like a stack. At each level,
@@ -167,6 +170,14 @@ pub unsafe fn initialize(
         GdextBuild::godot_static_version_string()
     );
 
+    // We want to initialize the main thread ID as early as possible.
+    //
+    // SAFETY: We set the main thread ID exactly once here and never again.
+    #[cfg(not(wasm_nothreads))]
+    unsafe {
+        MAIN_THREAD_ID.set(std::thread::current().id())
+    };
+
     // Before anything else: if we run into a Godot binary that's compiled differently from gdext, proceeding would be UB -> panic.
     interface_init::ensure_static_runtime_compatibility(get_proc_address);
 
@@ -380,6 +391,32 @@ pub unsafe fn godot_has_feature(
     return_ptr
 }
 
+/// Get the [`ThreadId`](std::thread::ThreadId) of the main thread.
+///
+/// # Panics
+/// - If it is called before the engine bindings have been initialized.
+#[cfg(not(wasm_nothreads))]
+pub fn main_thread_id() -> std::thread::ThreadId {
+    assert!(
+        MAIN_THREAD_ID.is_initialized(),
+        "Godot engine not available; make sure you are not calling it from unit/doc tests"
+    );
+
+    // SAFETY: We initialized the cell during library initialization, before any other code is executed.
+    let thread_id = unsafe { MAIN_THREAD_ID.get_unchecked() };
+
+    *thread_id
+}
+
+/// Check if the current thread is the main thread.
+///
+/// # Panics
+/// - If it is called before the engine bindings have been initialized.
+#[cfg(not(wasm_nothreads))]
+pub fn is_main_thread() -> bool {
+    std::thread::current().id() == main_thread_id()
+}
+
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Macros to access low-level function bindings