rust: export funcs explicitly invoke static constructors at most once (#533)

* wit-binden-rust: explicitly invoke ctors at start of each export func

* rust: wasm_call_ctors only exists in target_arch=wasm32

this is a problem when running `cargo test --workspace` which apparently
is building some of this code for native :/

* rust: run ctors at most once

* target arch guard

Author: pchickey

crates/guest-rust/src/lib.rs

@@ -11,6 +11,36 @@ pub use bitflags;
 
 #[doc(hidden)]
 pub mod rt {
+
+    /// Provide a hook for generated export functions to run static
+    /// constructors at most once. wit-bindgen-rust generates a call to this
+    /// function at the start of all component export functions. Importantly,
+    /// it is not called as part of `cabi_realloc`, which is a *core* export
+    /// func, but may not execute ctors, because the environment ctor in
+    /// wasi-libc (before rust 1.69.0) calls an import func, which is not
+    /// permitted by the Component Model when inside realloc.
+    ///
+    /// We intend to remove this once rust 1.69.0 stabilizes.
+    #[cfg(target_arch = "wasm32")]
+    pub fn run_ctors_once() {
+        static mut RUN: bool = false;
+        unsafe {
+            if !RUN {
+                // This function is synthesized by `wasm-ld` to run all static
+                // constructors. wasm-ld will either provide an implementation
+                // of this symbol, or synthesize a wrapper around each
+                // exported function to (unconditionally) run ctors. By using
+                // this function, the linked module is opting into "manually"
+                // running ctors.
+                extern "C" {
+                    fn __wasm_call_ctors();
+                }
+                __wasm_call_ctors();
+                RUN = true;
+            }
+        }
+    }
+
     use super::alloc::alloc::Layout;
 
     // Re-export things from liballoc for convenient use.

crates/rust/src/lib.rs

@@ -509,6 +509,21 @@ impl InterfaceGenerator<'_> {
             "
                 #[allow(unused_imports)]
                 use wit_bindgen::rt::{{alloc, vec::Vec, string::String}};
+
+                // Before executing any other code, use this function to run all static
+                // constructors, if they have not yet been run. This is a hack required
+                // to work around wasi-libc ctors calling import functions to initialize
+                // the environment.
+                //
+                // This functionality will be removed once rust 1.69.0 is stable, at which
+                // point wasi-libc will no longer have this behavior.
+                //
+                // See
+                // https://github.com/bytecodealliance/preview2-prototyping/issues/99
+                // for more details.
+                #[cfg(target_arch=\"wasm32\")]
+                wit_bindgen::rt::run_ctors_once();
+
             "
         );