Experimental Reactor support.

This adds support for a new experimental "Reactor" executable model.

The "Commands" and "Reactors" concepts are introduced here:
WebAssembly/WASI#13

A companion Clang patch, which just consists of using the new
reactor-crt1.o and Reactor-specific entry point name, is here:
https://reviews.llvm.org/D62922

Instead of an entrypoint named "_start", which calls "main", which
then scopes the lifetime of the program, Reactors have a
"__wasi_unstable_reactor_start" function, which calls "reactor_setup".
When "reactor_setup" exits, the intention is that the program should
persist and be available for calling.

At present, the main anticipated use for this is in environments like
Node, where WASI-using modules can be imported and don't necessarily
want the semantics of a "main" function.

The "unstable" in "__wasi_unstable_reactor_start" reflects that this
Reactor concept is not yet stable, and likely to evolve.

Makefile

@@ -33,7 +33,7 @@ endif
 # directories in the source tree.
 BASICS_DIR = $(CURDIR)/basics
 BASICS_INC = $(BASICS_DIR)/include
-BASICS_CRT_SOURCES = $(wildcard $(BASICS_DIR)/crt/*.c)
+BASICS_CRT1_SOURCE = $(BASICS_DIR)/crt/crt1.c
 BASICS_SOURCES = $(wildcard $(BASICS_DIR)/sources/*.c)
 DLMALLOC_DIR = $(CURDIR)/dlmalloc
 DLMALLOC_SRC_DIR = $(DLMALLOC_DIR)/src
@@ -54,7 +54,7 @@ LIBC_BOTTOM_HALF_ALL_SOURCES = \
     $(shell find $(LIBC_BOTTOM_HALF_SOURCES) -name \*.c)
 LIBWASI_EMULATED_MMAN_SOURCES = \
     $(shell find $(LIBC_BOTTOM_HALF_DIR)/mman -name \*.c)
-LIBC_BOTTOM_HALF_CRT_SOURCES = $(wildcard $(LIBC_BOTTOM_HALF_DIR)/crt/*.c)
+LIBC_BOTTOM_HALF_CRT1_SOURCE = $(LIBC_BOTTOM_HALF_DIR)/crt/crt1.c
 LIBC_TOP_HALF_DIR = $(CURDIR)/libc-top-half
 LIBC_TOP_HALF_MUSL_DIR = $(LIBC_TOP_HALF_DIR)/musl
 LIBC_TOP_HALF_MUSL_SRC_DIR = $(LIBC_TOP_HALF_MUSL_DIR)/src
@@ -407,9 +407,9 @@ ifeq ($(THREAD_MODEL), single)
 endif
 
 ifeq ($(BUILD_LIBC_BOTTOM_HALF),no)
-override CRT_SOURCES = $(BASICS_CRT_SOURCES)
+override CRT1_SOURCE = $(BASICS_CRT1_SOURCE)
 else
-override CRT_SOURCES = $(LIBC_BOTTOM_HALF_CRT_SOURCES)
+override CRT1_SOURCE = $(LIBC_BOTTOM_HALF_CRT1_SOURCE)
 endif
 
 startup_files: include_dirs
@@ -418,7 +418,8 @@ startup_files: include_dirs
 	#
 	@mkdir -p "$(OBJDIR)"
 	cd "$(OBJDIR)" && \
-	"$(WASM_CC)" $(WASM_CFLAGS) -c $(CRT_SOURCES) -MD -MP && \
+	"$(WASM_CC)" $(WASM_CFLAGS) -c $(CRT1_SOURCE) -MD -MP && \
+	"$(WASM_CC)" $(WASM_CFLAGS) -c $(CRT1_SOURCE) -MD -MP -DREACTOR_RUNTIME -o reactor-crt1.o && \
 	mkdir -p "$(SYSROOT_LIB)" && \
 	mv *.o "$(SYSROOT_LIB)"
 

basics/crt/crt1.c

@@ -1,12 +1,25 @@
 extern void __wasm_call_ctors(void);
+void _Exit(int) __attribute__((noreturn));
+
+#ifdef REACTOR_RUNTIME
+extern void reactor_setup(int, char *[]);
+#else
 extern int main(int, char *[]);
 extern void __prepare_for_exit(void);
-void _Exit(int) __attribute__((noreturn));
+#endif
 
+#ifdef REACTOR_RUNTIME
+void __wasi_unstable_reactor_start(void) {
+#else
 void _start(void) {
+#endif
     /* The linker synthesizes this to call constructors. */
     __wasm_call_ctors();
 
+#ifdef REACTOR_RUNTIME
+    /* Call reactor_setup with the arguments. */
+    reactor_setup(argc, argv);
+#else
     /* Call main with no arguments. */
     int r = main(0, 0);
 
@@ -17,4 +30,5 @@ void _start(void) {
     if (r != 0) {
         _Exit(r);
     }
+#endif
 }

expected/wasm32-wasi/defined-symbols.txt

@@ -231,6 +231,7 @@ __uflow
 __unlist_locked_file
 __uselocale
 __utc
+__wasi_unstable_reactor_start
 __wasilibc_fd_renumber
 __wasilibc_find_relpath
 __wasilibc_init_preopen

expected/wasm32-wasi/undefined-symbols.txt

@@ -68,3 +68,4 @@ __wasi_sock_send
 __wasi_sock_shutdown
 __wasm_call_ctors
 main
+reactor_setup

libc-bottom-half/crt/crt1.c

@@ -5,9 +5,14 @@
 
 extern char **__environ;
 extern void __wasm_call_ctors(void);
+void _Exit(int) __attribute__((noreturn));
+
+#ifdef REACTOR_RUNTIME
+extern void reactor_setup(int, char *[]);
+#else
 extern int main(int, char *[]);
 extern void __prepare_for_exit(void);
-void _Exit(int) __attribute__((noreturn));
+#endif
 
 static __wasi_errno_t populate_args(size_t *argc, char ***argv) {
     __wasi_errno_t err;
@@ -104,7 +109,11 @@ static __wasi_errno_t populate_libpreopen(void) {
     return __WASI_ESUCCESS;
 }
 
+#ifdef REACTOR_RUNTIME
+void __wasi_unstable_reactor_start(void) {
+#else
 void _start(void) {
+#endif
     /* Record the preopened resources. */
     if (populate_libpreopen() != __WASI_ESUCCESS) {
         _Exit(EX_OSERR);
@@ -125,6 +134,10 @@ void _start(void) {
     /* The linker synthesizes this to call constructors. */
     __wasm_call_ctors();
 
+#ifdef REACTOR_RUNTIME
+    /* Call reactor_setup with the arguments. */
+    reactor_setup(argc, argv);
+#else
     /* Call main with the arguments. */
     int r = main(argc, argv);
 
@@ -135,4 +148,5 @@ void _start(void) {
     if (r != 0) {
         _Exit(r);
     }
+#endif
 }