feat(WASI): Support itk_wasm_delayed_start

Build as a Reactor but add a default export, defined by the empty
string, that behaves like _start did before for command line execution
via wasmtime.

  https://docs.rs/wasmtime/0.17.0/src/wasmtime/linker.rs.html#685

src/docker/itk-wasm/ITKWebAssemblyInterface.cmake

@@ -55,15 +55,17 @@ function(add_executable target)
     set_property(TARGET ${wasm_target} PROPERTY LINK_FLAGS_DEBUG " -fno-lto -s SAFE_HEAP=1 -s DISABLE_EXCEPTION_CATCHING=0 --bind ${_link_flags_debug}")
     set_property(TARGET ${umd_target} PROPERTY LINK_FLAGS_DEBUG " -fno-lto -s SAFE_HEAP=1 -s DISABLE_EXCEPTION_CATCHING=0 --bind ${_link_flags_debug}")
   else()
+    # WASI
     set_property(TARGET ${wasm_target} PROPERTY SUFFIX ".wasi.wasm")
-    if (NOT TARGET wasi-exception-shim AND DEFINED CMAKE_CXX_COMPILE_OBJECT)
-      add_library(wasi-exception-shim STATIC /ITKWebAssemblyInterface/src/exceptionShim.cxx)
+    if (NOT TARGET wasi-itk-extras AND DEFINED CMAKE_CXX_COMPILE_OBJECT)
+      add_library(wasi-itk-extras STATIC /ITKWebAssemblyInterface/src/exceptionShim.cxx /ITKWebAssemblyInterface/src/initialization.cxx)
     endif()
     get_property(_is_imported TARGET ${wasm_target} PROPERTY IMPORTED)
     if (NOT ${_is_imported})
-      _target_link_libraries(${target} PRIVATE $<$<LINK_LANGUAGE:CXX>:wasi-exception-shim>)
+      _target_link_libraries(${target} PRIVATE $<$<LINK_LANGUAGE:CXX>:wasi-itk-extras>)
       get_property(_link_flags TARGET ${wasm_target} PROPERTY LINK_FLAGS)
-      set_property(TARGET ${wasm_target} PROPERTY LINK_FLAGS "-Wl,--export-if-defined=main -Wl,--export-if-defined=itk_wasm_input_array_alloc -Wl,--export-if-defined=itk_wasm_input_json_alloc -Wl,--export-if-defined=itk_wasm_output_json_address -Wl,--export-if-defined=itk_wasm_output_json_size -Wl,--export-if-defined=itk_wasm_output_array_address -Wl,--export-if-defined=itk_wasm_output_array_size -Wl,--export-if-defined=itk_wasm_free_all ${_link_flags}")
+      set_property(TARGET ${wasm_target} PROPERTY LINK_FLAGS
+        "-mexec-model=reactor -Wl,--export-if-defined=itk_wasm_input_array_alloc -Wl,--export-if-defined=itk_wasm_input_json_alloc -Wl,--export-if-defined=itk_wasm_output_json_address -Wl,--export-if-defined=itk_wasm_output_json_size -Wl,--export-if-defined=itk_wasm_output_array_address -Wl,--export-if-defined=itk_wasm_output_array_size -Wl,--export-if-defined=itk_wasm_free_all -Wl,--export-if-defined=_start -Wl,--export-if-defined=itk_wasm_delayed_start -Wl,--export-if-defined=itk_wasm_delayed_exit ${_link_flags}")
       if(NOT ITK_WASM_NO_INTERFACE_LINK)
         if(NOT TARGET WebAssemblyInterface)
           find_package(ITK QUIET COMPONENTS WebAssemblyInterface)

src/initialization.cxx

@@ -0,0 +1,69 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         https://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+// itk-wasm reactor-like initialization to lower values before _start
+// Based on wasi-libc.
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+#include <wasi/api.h>
+extern void __wasm_call_ctors(void);
+extern int __main_void(void);
+extern void __wasm_call_dtors(void);
+extern void _initialize(void);
+
+__attribute__((export_name("itk_wasm_delayed_exit")))
+void itk_wasm_delayed_exit(int returnCode)
+{
+  // Call atexit functions, destructors, stdio cleanup, etc.
+  __wasm_call_dtors();
+
+  // If main exited successfully, just return, otherwise call
+  // `__wasi_proc_exit`.
+  if (returnCode != 0) {
+    __wasi_proc_exit(returnCode);
+  }
+}
+
+__attribute__((export_name("itk_wasm_delayed_start")))
+int itk_wasm_delayed_start(void)
+{
+  // Call `__main_void` which will either be the application's zero-argument
+  // `__main_void` function or a libc routine which obtains the command-line
+  // arguments and calls `__main_argv_argc`.
+  const int r = __main_void();
+
+  return r;
+}
+
+__attribute__((export_name("")))
+void _start(void)
+{
+  _initialize();
+
+  const int returnCode = itk_wasm_delayed_start();
+
+  itk_wasm_delayed_exit(returnCode);
+}
+
+
+#ifdef __cplusplus
+}
+#endif // __cplusplus

src/python/itkwasm/itkwasm/pipeline.py

@@ -33,5 +33,11 @@ def run(self, args: List[str], outputs=[], inputs=[], preopen_directories=[], ma
 
         instance = Instance(self.module, import_object)
 
-        start = instance.exports._start
-        start()
+        _initialize = instance.exports._initialize
+        _initialize()
+
+        delayed_start = instance.exports.itk_wasm_delayed_start
+        return_code = delayed_start()
+
+        delayed_exit = instance.exports.itk_wasm_delayed_exit
+        delayed_exit(return_code)