Call __stdio_exit rather than fflush

See musl commit a71e0af25544fd2486e57ea51c6d05abdbf44c3e.

Author: kleisauke

emcc.py

@@ -1899,11 +1899,17 @@ 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
+      # To flush streams on FS exit, we need to be able to call __stdio_exit,
+      # this is a streamlined fflush variant that avoids performing any unnecessary
+      # operations and which never unlocks the files or open file list, so we can
+      # be sure no other threads write new data to a stream's buffer after it's
+      # already flushed. This function is only included if the runtime is exitable.
+      if settings.EXIT_RUNTIME:
+        settings.EXPORTED_FUNCTIONS += ['___stdio_exit']
+      # Include the fflush function when ASSERTIONS is enabled.
       # (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:
+      if settings.ASSERTIONS:
         settings.EXPORTED_FUNCTIONS += ['_fflush']
 
     if settings.SUPPORT_ERRNO:

src/library_fs.js

@@ -1514,9 +1514,10 @@ FS.staticInit();` +
     },
     quit: function() {
       FS.init.initialized = false;
-      // force-flush all streams, so we get musl std streams printed out
-      var fflush = Module['_fflush'];
-      if (fflush) fflush(0);
+      // ensure the musl std streams are printed out by
+      // calling the streamlined fflush variant
+      var stdio_exit = Module['___stdio_exit'];
+      if (stdio_exit) stdio_exit();
       // close all of our streams
       for (var i = 0; i < FS.streams.length; i++) {
         var stream = FS.streams[i];

src/library_wasi.js

@@ -182,8 +182,14 @@ var WasiLibrary = {
 
 #if SYSCALLS_REQUIRE_FILESYSTEM == 0 && (!MINIMAL_RUNTIME || EXIT_RUNTIME)
   $flush_NO_FILESYSTEM: function() {
-    // flush anything remaining in the buffers during shutdown
+    // flush anything remaining in the buffers during shutdown.
+    // Only call the streamlined fflush variant when ASSERTIONS are disabled
+    // to ensure that the warning for unflushed content is still displayed.
+#if ASSERTIONS
     if (typeof _fflush !== 'undefined') _fflush({{{ pointerT(0) }}});
+#else
+    if (typeof ___stdio_exit !== 'undefined') ___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") }}});