Use __stdio_exit rather then fflush(0) on exit (#16130)

This is how musl internally closes down stdio after main returns
(and its how we do it in standalone mode).

Author: sbc100

emcc.py

@@ -1985,12 +1985,13 @@ def default_setting(name, new_default):
       ]
 
     if settings.FILESYSTEM and not settings.STANDALONE_WASM:
-      # to flush streams on FS exit, we need to be able to call fflush
-      # we only include it if the runtime is exitable, or when ASSERTIONS
+      # We use __stdio_exit to shut down musl's stdio subsystems and flush
+      # streams on exit.
+      # We only include it if the runtime is exitable, or when ASSERTIONS
       # (ASSERTIONS will check that streams do not need to be flushed,
       # helping people see when they should have enabled EXIT_RUNTIME)
       if settings.EXIT_RUNTIME or settings.ASSERTIONS:
-        settings.EXPORT_IF_DEFINED += ['fflush']
+        settings.EXPORT_IF_DEFINED += ['__stdio_exit']
 
     if settings.SUPPORT_ERRNO:
       # so setErrNo JS library function can report errno back to C

src/library_fs.js

@@ -1490,9 +1490,10 @@ FS.staticInit();` +
     },
     quit: () => {
       FS.init.initialized = false;
-      // force-flush all streams, so we get musl std streams printed out
-#if hasExportedFunction('_fflush')
-      _fflush(0);
+      // Call musl-internal function to close all stdio streams, so nothing is
+      // left in internal buffers.
+#if hasExportedFunction('___stdio_exit')
+      ___stdio_exit();
 #endif
       // close all of our streams
       for (var i = 0; i < FS.streams.length; i++) {

src/library_wasi.js

@@ -183,7 +183,9 @@ var WasiLibrary = {
 #if SYSCALLS_REQUIRE_FILESYSTEM == 0 && (!MINIMAL_RUNTIME || EXIT_RUNTIME)
   $flush_NO_FILESYSTEM: function() {
     // flush anything remaining in the buffers during shutdown
-    if (typeof _fflush !== 'undefined') _fflush({{{ pointerT(0) }}});
+#if hasExportedFunction('___stdio_exit')
+    ___stdio_exit();
+#endif
     var buffers = SYSCALLS.buffers;
     if (buffers[1].length) SYSCALLS.printChar(1, {{{ charCode("\n") }}});
     if (buffers[2].length) SYSCALLS.printChar(2, {{{ charCode("\n") }}});

src/postamble.js

@@ -364,8 +364,8 @@ function checkUnflushedContent() {
 #if SYSCALLS_REQUIRE_FILESYSTEM == 0
     var flush = {{{ '$flush_NO_FILESYSTEM' in addedLibraryItems ? 'flush_NO_FILESYSTEM' : 'null' }}};
     if (flush) flush();
-#elif hasExportedFunction('_fflush')
-    _fflush(0);
+#elif hasExportedFunction('___stdio_exit')
+    ___stdio_exit();
 #endif
 #if '$FS' in addedLibraryItems && '$TTY' in addedLibraryItems
     // also flush in the JS FS layer

src/postamble_minimal.js

@@ -31,12 +31,12 @@ function run() {
   PThread.terminateAllThreads();
 #endif
 
-#if IN_TEST_HARNESS
-  // fflush() filesystem stdio for test harness, since there are existing
+#if IN_TEST_HARNESS && hasExportedFunction('___stdio_exit')
+  // flush any stdio streams for test harness, since there are existing
   // tests that depend on this behavior.
   // For production use, instead print full lines to avoid this kind of lazy
   // behavior.
-  if (typeof _fflush !== 'undefined') _fflush();
+  ___stdio_exit();
 #endif
 
 #if ASSERTIONS

tests/other/metadce/hello_world.exports

@@ -1,11 +1,11 @@
 __errno_location
 __indirect_function_table
+__stdio_exit
 __wasm_call_ctors
 dynCall_jiji
 emscripten_stack_get_end
 emscripten_stack_get_free
 emscripten_stack_init
-fflush
 main
 memory
 stackAlloc

tests/other/metadce/hello_world.funcs

@@ -10,22 +10,21 @@ $__lockfile
 $__lshrti3
 $__memcpy
 $__ofl_lock
-$__ofl_unlock
 $__original_main
+$__stdio_exit
 $__stdio_write
 $__syscall_getpid
 $__towrite
 $__trunctfdf2
-$__unlock
 $__unlockfile
 $__vfprintf_internal
 $__wasi_syscall_ret
 $__wasm_call_ctors
+$close_file
 $dynCall_jiji
 $emscripten_stack_get_end
 $emscripten_stack_get_free
 $emscripten_stack_init
-$fflush
 $fmt_fp
 $fmt_o
 $fmt_u

tests/other/metadce/hello_world.jssize

@@ -1 +1 @@
-125367
+125354

tests/other/metadce/hello_world.size

@@ -1 +1 @@
-12066
+11910

tests/other/metadce/hello_world_O1.jssize

@@ -1 +1 @@
-58444
+58390