Add support for the _initialize() crt1 startup function. (#72)

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

Author: PiotrSikora

include/proxy-wasm/wasm.h

@@ -172,7 +172,8 @@ class WasmBase : public std::enable_shared_from_this<WasmBase> {
   std::unique_ptr<ShutdownHandle> shutdown_handle_;
   std::unordered_set<ContextBase *> pending_done_; // Root contexts not done during shutdown.
 
-  WasmCallVoid<0> _start_; /* Emscripten v1.39.0+ */
+  WasmCallVoid<0> _initialize_; /* Emscripten v1.39.17+ */
+  WasmCallVoid<0> _start_;      /* Emscripten v1.39.0+ */
   WasmCallVoid<0> __wasm_call_ctors_;
 
   WasmCallWord<1> malloc_;

src/null/null_plugin.cc

@@ -33,7 +33,9 @@
 namespace proxy_wasm {
 
 void NullPlugin::getFunction(std::string_view function_name, WasmCallVoid<0> *f) {
-  if (function_name == "_start") {
+  if (function_name == "_initialize") {
+    *f = nullptr;
+  } else if (function_name == "_start") {
     *f = nullptr;
   } else if (function_name == "__wasm_call_ctors") {
     *f = nullptr;

src/wasm.cc

@@ -193,6 +193,7 @@ void WasmBase::registerCallbacks() {
 
 void WasmBase::getFunctions() {
 #define _GET(_fn) wasm_vm_->getFunction(#_fn, &_fn##_);
+  _GET(_initialize);
   _GET(_start);
   _GET(__wasm_call_ctors);
 
@@ -324,8 +325,9 @@ ContextBase *WasmBase::getOrCreateRootContext(const std::shared_ptr<PluginBase>
 }
 
 void WasmBase::startVm(ContextBase *root_context) {
-  /* Call "_start" function, and fallback to "__wasm_call_ctors" if the former is not available. */
-  if (_start_) {
+  if (_initialize_) {
+    _initialize_(root_context);
+  } else if (_start_) {
     _start_(root_context);
   } else if (__wasm_call_ctors_) {
     __wasm_call_ctors_(root_context);