add support for nothreads build to single threaded init

Author: PgBiel

godot-bindings/Cargo.toml

@@ -15,6 +15,8 @@ homepage = "https://godot-rust.github.io"
 # requiring no-default-features), so we unfortunately still need to depend on prebuilt and just ignore it.
 # The artifact generator explicitly excludes that though (to avoid a quasi-circular dependency back to its repo).
 [features]
+experimental-wasm-nothreads = []
+
 # [version-sync] [[
 #  [line] api-$kebabVersion = []
 api-4-0 = []

godot-bindings/src/lib.rs

@@ -179,6 +179,22 @@ pub fn emit_godot_version_cfg() {
     }
 }
 
+/// Emit `#[cfg(wasm_nothreads)]` flag when compiling to Wasm with the "experimental-wasm-nothreads" feature.
+pub fn emit_wasm_nothreads_cfg() {
+    println!(r#"cargo:rustc-check-cfg=cfg(wasm_nothreads, values(none()))"#);
+
+    // The environment variable for target family has a list of applicable families separated by commas.
+    // For Emscripten in particular, this can be "unix,wasm". Therefore, to check for the Wasm target, we must check each item in the list.
+    #[cfg(feature = "experimental-wasm-nothreads")]
+    if std::env::var("CARGO_CFG_TARGET_FAMILY")
+        .expect("target family environment variable")
+        .split(',')
+        .any(|family| family == "wasm")
+    {
+        println!(r#"cargo:rustc-cfg=wasm_nothreads"#);
+    }
+}
+
 // Function for safely removal of build directory. Workaround for errors happening during CI builds:
 // https://github.com/godot-rust/gdext/issues/616
 pub fn remove_dir_all_reliable(path: &Path) {

godot-core/Cargo.toml

@@ -21,6 +21,7 @@ codegen-lazy-fptrs = [
 double-precision = ["godot-codegen/double-precision"]
 experimental-godot-api = ["godot-codegen/experimental-godot-api"]
 experimental-threads = ["godot-ffi/experimental-threads"]
+experimental-wasm-nothreads = ["godot-ffi/experimental-wasm-nothreads"]
 debug-log = ["godot-ffi/debug-log"]
 trace = []
 

godot-ffi/Cargo.toml

@@ -15,6 +15,7 @@ codegen-rustfmt = ["godot-codegen/codegen-rustfmt"]
 codegen-lazy-fptrs = ["godot-codegen/codegen-lazy-fptrs"]
 experimental-godot-api = ["godot-codegen/experimental-godot-api"]
 experimental-threads = []
+experimental-wasm-nothreads = ["godot-bindings/experimental-wasm-nothreads"]
 debug-log = []
 
 api-custom = ["godot-bindings/api-custom"]

godot-ffi/build.rs

@@ -28,4 +28,5 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
     godot_bindings::emit_godot_version_cfg();
+    godot_bindings::emit_wasm_nothreads_cfg();
 }

godot-ffi/src/binding/single_threaded.rs

@@ -10,13 +10,21 @@
 //! If used from different threads then there will be runtime errors in debug mode and UB in release mode.
 
 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>,
 }
@@ -30,13 +38,59 @@ 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(),
         };
 
         &BINDING
     }
 
+    /// Returns whether the binding storage has already been initialized.
+    ///
+    /// 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()
+    }
+
+    /// Marks the binding storage as initialized or deinitialized.
+    /// We store the thread ID to ensure future accesses to the binding only come from the main thread.
+    ///
+    /// # Safety
+    /// Must be called from the main thread. Additionally, the binding storage must be initialized immediately
+    /// after this function if `initialized` is `true`, or deinitialized if it is `false`.
+    ///
+    /// # Panics
+    /// If attempting to deinitialize before initializing, or vice-versa.
+    unsafe fn set_initialized(&self, initialized: bool) {
+        if initialized == self.initialized() {
+            if initialized {
+                panic!("already initialized");
+            } else {
+                panic!("deinitialize without prior initialize");
+            }
+        }
+
+        // '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.
     ///
     /// # Safety
@@ -49,9 +103,10 @@ impl BindingStorage {
         // in which case we can tell that the storage has been initialized, and we don't access `binding`.
         let storage = unsafe { Self::storage() };
 
-        storage
-            .main_thread_id
-            .set(Some(std::thread::current().id()));
+        // SAFETY: We are about to initialize the binding below, so marking the binding as initialized is correct.
+        // If we can't initialize the binding at this point, we get a panic before changing the status, thus the
+        // binding won't be set.
+        unsafe { storage.set_initialized(true) };
 
         // SAFETY: We are the first thread to set this binding (possibly after deinitialize), as otherwise the above set() would fail and
         // return early. We also know initialize() is not called concurrently with anything else that can call another method on the binding,
@@ -70,12 +125,10 @@ impl BindingStorage {
         // SAFETY: We only call this once no other operations happen anymore, i.e. no other access to the binding.
         let storage = unsafe { Self::storage() };
 
-        storage
-            .main_thread_id
-            .get()
-            .expect("deinitialize without prior initialize");
-
-        storage.main_thread_id.set(None);
+        // SAFETY: We are about to deinitialize the binding below, so marking the binding as deinitialized is correct.
+        // If we can't deinitialize the binding at this point, we get a panic before changing the status, thus the
+        // binding won't be deinitialized.
+        unsafe { storage.set_initialized(false) };
 
         // SAFETY: We are the only thread that can access the binding, and we know that it's initialized.
         unsafe {
@@ -92,7 +145,10 @@ impl BindingStorage {
     pub unsafe fn get_binding_unchecked() -> &'static GodotBinding {
         let storage = Self::storage();
 
-        if cfg!(debug_assertions) {
+        // 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.
+        #[cfg(all(debug_assertions, not(wasm_nothreads)))]
+        {
             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",
             );
@@ -111,7 +167,8 @@ impl BindingStorage {
     pub fn is_initialized() -> bool {
         // SAFETY: We don't access the binding.
         let storage = unsafe { Self::storage() };
-        storage.main_thread_id.get().is_some()
+
+        storage.initialized()
     }
 }
 

godot/Cargo.toml

@@ -18,6 +18,7 @@ double-precision = ["godot-core/double-precision"]
 experimental-godot-api = ["godot-core/experimental-godot-api"]
 experimental-threads = ["godot-core/experimental-threads"]
 experimental-wasm = []
+experimental-wasm-nothreads = ["godot-core/experimental-wasm-nothreads"]
 codegen-rustfmt = ["godot-core/codegen-rustfmt"]
 lazy-function-tables = ["godot-core/codegen-lazy-fptrs"]
 serde = ["godot-core/serde"]

godot/src/lib.rs

@@ -68,7 +68,7 @@
 //! * **`api-custom`**
 //!
 //!   Sets the [**API level**](https://godot-rust.github.io/book/toolchain/godot-version.html) to the specified Godot version,
-//!   or a custom-built local binary.  
+//!   or a custom-built local binary.
 //!   You can use at most one `api-*` feature. If absent, the current Godot minor version is used, with patch level 0.<br><br>
 //!
 //! * **`double-precision`**
@@ -124,6 +124,12 @@ pub mod __docs;
 #[cfg(all(feature = "lazy-function-tables", feature = "experimental-threads"))]
 compile_error!("Thread safety for lazy function pointers is not yet implemented.");
 
+#[cfg(all(
+    feature = "experimental-wasm-nothreads",
+    feature = "experimental-threads"
+))]
+compile_error!("Cannot use 'experimental-threads' with a nothreads Wasm build yet.");
+
 #[cfg(all(target_family = "wasm", not(feature = "experimental-wasm")))]
 compile_error!("Must opt-in using `experimental-wasm` Cargo feature; keep in mind that this is work in progress");