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gh-156780: Emscripten: move main() wrapping out of libpython
Running main() under WebAssembly.promising only makes sense when Python is the program. Move the wrapper into Programs/emscripten_beforemain.c, linked into the interpreter but not into libpython, and let the runtime shut down normally afterwards so atexit handlers run and buffered output is flushed. FS.createAsyncInputDevice() stays in libpython. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
1 parent 0a2c8f4 commit 76d45d9

5 files changed

Lines changed: 68 additions & 45 deletions

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‎Makefile.pre.in‎

Lines changed: 11 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -970,9 +970,12 @@ clinic: check-clean-src
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clinic-tests: check-clean-src $(srcdir)/Lib/test/clinic.test.c
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$(PYTHON_FOR_REGEN) $(srcdir)/Tools/clinic/clinic.py -f $(srcdir)/Lib/test/clinic.test.c
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# Objects linked into the interpreter only, not into libpython.
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PLATFORM_MAIN_OBJS= @PLATFORM_MAIN_OBJS@
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# Build the interpreter
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$(BUILDPYTHON): Programs/python.o $(LINK_PYTHON_DEPS)
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$(LINKCC) $(PY_CORE_EXE_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/python.o $(LINK_PYTHON_OBJS) $(LIBS) $(MODLIBS) $(SYSLIBS)
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$(BUILDPYTHON): Programs/python.o $(PLATFORM_MAIN_OBJS) $(LINK_PYTHON_DEPS)
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$(LINKCC) $(PY_CORE_EXE_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/python.o $(PLATFORM_MAIN_OBJS) $(LINK_PYTHON_OBJS) $(LIBS) $(MODLIBS) $(SYSLIBS)
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platform: $(PYTHON_FOR_BUILD_DEPS) pybuilddir.txt
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$(RUNSHARED) $(PYTHON_FOR_BUILD) -c 'import sys ; from sysconfig import get_platform ; print("%s-%d.%d" % (get_platform(), *sys.version_info[:2]))' >platform
@@ -1614,8 +1617,8 @@ regen-re: $(BUILDPYTHON)
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# using Tools/build/generate_re_casefix.py
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$(RUNSHARED) ./$(BUILDPYTHON) $(srcdir)/Tools/build/generate_re_casefix.py $(srcdir)/Lib/re/_casefix.py
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1617-
Programs/_testembed: Programs/_testembed.o $(LINK_PYTHON_DEPS)
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$(LINKCC) $(PY_CORE_EXE_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/_testembed.o $(LINK_PYTHON_OBJS) $(LIBS) $(MODLIBS) $(SYSLIBS)
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Programs/_testembed: Programs/_testembed.o $(PLATFORM_MAIN_OBJS) $(LINK_PYTHON_DEPS)
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$(LINKCC) $(PY_CORE_EXE_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/_testembed.o $(PLATFORM_MAIN_OBJS) $(LINK_PYTHON_OBJS) $(LIBS) $(MODLIBS) $(SYSLIBS)
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############################################################################
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# "Bootstrap Python" used to run Programs/_freeze_module.py
@@ -2235,7 +2238,7 @@ regen-slots: Python/slots.toml
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$(PYTHON_FOR_REGEN) $(srcdir)/Tools/build/generate_slots.py \
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--generate-all
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$(LIBRARY_OBJS) $(MODOBJS) Programs/python.o: $(PYTHON_HEADERS)
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$(LIBRARY_OBJS) $(MODOBJS) Programs/python.o $(PLATFORM_MAIN_OBJS): $(PYTHON_HEADERS)
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######################################################################
@@ -2961,6 +2964,9 @@ libainstall: all scripts
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fi; \
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fi; \
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$(INSTALL_DATA) Programs/python.o $(DESTDIR)$(LIBPL)/python.o; \
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for i in $(PLATFORM_MAIN_OBJS); do \
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$(INSTALL_DATA) $$i $(DESTDIR)$(LIBPL); \
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done; \
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fi
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$(INSTALL_DATA) Modules/config.c $(DESTDIR)$(LIBPL)/config.c
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$(INSTALL_DATA) $(srcdir)/Modules/config.c.in $(DESTDIR)$(LIBPL)/config.c.in

‎Programs/emscripten_beforemain.c‎

Lines changed: 40 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,40 @@
1+
/* Emscripten setup for when Python is the program. Linked into the
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* interpreter only, never into libpython.
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*
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* Runs main() under WebAssembly.promising so that libpython can suspend the
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* wasm stack in a syscall (see Python/emscripten_syscalls.c).
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*/
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#include <emscripten.h>
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EM_JS(void, _PyEmscripten_BeforeMain_js, (void), {
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if (!WebAssembly.promising) {
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// No stack switching support =(
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return;
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}
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if (ENVIRONMENT_IS_NODE && !Module.onExit) {
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Module.onExit = (code) => process.exit(code);
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}
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// promising() needs the raw export; _main may be a JS wrapper around it.
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const main = WebAssembly.promising(wasmExports.__main_argc_argv);
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_main = (...args) => {
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// Exit the way callMain() would have, once main() is actually done.
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main(...args).then((ret) => exitJS(ret, true)).catch(handleException);
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// Unwind to callMain() without letting it exit: main() is still
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// running on the promising stack.
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throw "unwind";
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};
27+
// callMain() takes the entry point from _main, or from wasmImports.main
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// when linked with -sMAIN_MODULE.
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if ("main" in wasmImports) {
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wasmImports.main = _main;
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}
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})
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EM_JS_DEPS(_PyEmscripten_BeforeMain, "$exitJS,$handleException");
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__attribute__((constructor)) void
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_PyEmscripten_BeforeMain(void)
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{
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_PyEmscripten_BeforeMain_js();
40+
}

‎Python/emscripten_syscalls.c‎

Lines changed: 10 additions & 40 deletions
Original file line numberDiff line numberDiff line change
@@ -54,13 +54,13 @@ __attribute__((constructor)) void __syscall_init_umask(void) {
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#define EM_JS_MACROS(ret, func_name, args, body...) \
5555
EM_JS(ret, func_name, args, body)
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57-
EM_JS_MACROS(void, _emscripten_promising_main_js, (void), {
58-
// Define FS.createAsyncInputDevice(), This is quite similar to
59-
// FS.createDevice() defined here:
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// https://github.com/emscripten-core/emscripten/blob/4.0.11/src/lib/libfs.js?plain=1#L1642
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// but instead of returning one byte at a time, the input() function should
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// return a Uint8Array. This makes the handler code simpler, the
63-
// `createAsyncInputDevice` simpler, and everything faster.
57+
// Define FS.createAsyncInputDevice(), This is quite similar to
58+
// FS.createDevice() defined here:
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// https://github.com/emscripten-core/emscripten/blob/4.0.11/src/lib/libfs.js?plain=1#L1642
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// but instead of returning one byte at a time, the input() function should
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// return a Uint8Array. This makes the handler code simpler, the
62+
// `createAsyncInputDevice` simpler, and everything faster.
63+
EM_JS_MACROS(void, __fs_init_async_input_device_js, (void), {
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FS.createAsyncInputDevice = function(parent, name, input) {
6565
parent = typeof parent == 'string' ? parent : FS.getPath(parent);
6666
var path = PATH.join2(parent, name);
@@ -103,44 +103,14 @@ EM_JS_MACROS(void, _emscripten_promising_main_js, (void), {
103103
FS.registerDevice(dev, ops);
104104
return FS.mkdev(path, mode, dev);
105105
};
106-
if (!WebAssembly.promising) {
107-
// No stack switching support =(
108-
return;
109-
}
110-
const origResolveGlobalSymbol = resolveGlobalSymbol;
111-
if (ENVIRONMENT_IS_NODE && !Module.onExit) {
112-
Module.onExit = (code) => process.exit(code);
113-
}
114-
// * wrap the main symbol with WebAssembly.promising,
115-
// * call exit_with_live_runtime() to prevent emscripten from shutting down
116-
// the runtime before the promise resolves,
117-
// * call onExit / process.exit ourselves, since exit_with_live_runtime()
118-
// prevented Emscripten from calling it normally.
119-
resolveGlobalSymbol = function (name, direct = false) {
120-
const orig = origResolveGlobalSymbol(name, direct);
121-
if (name === "main") {
122-
const main = WebAssembly.promising(orig.sym);
123-
orig.sym = (...args) => {
124-
(async () => {
125-
const ret = await main(...args);
126-
Module.onExit?.(ret);
127-
})();
128-
_emscripten_exit_with_live_runtime();
129-
};
130-
}
131-
return orig;
132-
};
133106
})
134107

135-
EM_JS_DEPS(_emscripten_promising_main,
136-
"$FS,$PATH,$FS_getMode,$resolveGlobalSymbol,"
137-
"emscripten_exit_with_live_runtime");
108+
EM_JS_DEPS(__fs_init_async_input_device, "$FS,$PATH,$FS_getMode");
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139-
__attribute__((constructor)) void _emscripten_promising_main(void) {
140-
_emscripten_promising_main_js();
110+
__attribute__((constructor)) void __fs_init_async_input_device(void) {
111+
__fs_init_async_input_device_js();
141112
}
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143-
144114
#define IOVEC_T_BUF_OFFSET 0
145115
#define IOVEC_T_BUF_LEN_OFFSET 4
146116
#define IOVEC_T_SIZE 8

‎configure‎

Lines changed: 4 additions & 0 deletions
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‎configure.ac‎

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Original file line numberDiff line numberDiff line change
@@ -5427,10 +5427,12 @@ fi
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dnl Platform-specific C and header files.
54285428
PLATFORM_HEADERS=
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PLATFORM_OBJS=
5430+
PLATFORM_MAIN_OBJS=
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54315432
AS_CASE([$ac_sys_system],
54325433
[Emscripten], [
54335434
AS_VAR_APPEND([PLATFORM_OBJS], [' Python/emscripten_signal.o Python/emscripten_trampoline.o Python/emscripten_trampoline_wasm.o'])
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AS_VAR_APPEND([PLATFORM_MAIN_OBJS], [' Programs/emscripten_beforemain.o'])
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AS_VAR_IF([enable_emscripten_syscalls], [yes], [
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AS_VAR_APPEND([PLATFORM_OBJS], [' Python/emscripten_syscalls.o'])
54365438
])
@@ -5439,6 +5441,7 @@ AS_CASE([$ac_sys_system],
54395441
)
54405442
AC_SUBST([PLATFORM_HEADERS])
54415443
AC_SUBST([PLATFORM_OBJS])
5444+
AC_SUBST([PLATFORM_MAIN_OBJS])
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54435446
# -I${DLINCLDIR} is added to the compile rule for importdl.o
54445447
AC_SUBST([DLINCLDIR])

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