Delete test/js/bun/spawn/spawn-flush-stdio-on-exit.test.ts

Move from test/regression/issue/ (which expects a real issue number) to
test/js/bun/spawn/ where other spawn/exit tests live.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Global.zig

@@ -93,6 +93,10 @@ pub fn runExitCallbacks() void {
     on_exit_callbacks.items.len = 0;
 }
 
+/// Flush the C runtime stdout/stderr buffers. Must be called before
+/// TerminateProcess on Windows because it skips CRT teardown.
+pub extern "c" fn Bun__flushCStdio() void;
+
 var is_exiting = std.atomic.Value(bool).init(false);
 export fn bun_is_exiting() c_int {
     return @intFromBool(isExiting());
@@ -130,6 +134,10 @@ pub fn exit(code: u32) noreturn {
             // TerminateProcess skips DLL cleanup entirely, matching the
             // behavior on Linux where quick_exit() also skips library
             // teardown. Bun's own cleanup has already run via Bun__onExit().
+            //
+            // TerminateProcess does not run CRT teardown, so we must
+            // explicitly flush the C stdio buffers to avoid losing output.
+            Bun__flushCStdio();
             const rc = std.os.windows.kernel32.TerminateProcess(
                 std.os.windows.kernel32.GetCurrentProcess(),
                 code,

src/bun.js/bindings/c-bindings.cpp

@@ -384,6 +384,18 @@ extern "C" ssize_t pwritev2(int fd, const struct iovec* iov, int iovcnt,
 #endif
 
 extern "C" void Bun__onExit();
+
+#if OS(WINDOWS)
+/// Flush the C runtime stdout/stderr buffers. Called before TerminateProcess
+/// which, unlike ExitProcess, does not run CRT teardown and therefore would
+/// otherwise lose any buffered output.
+extern "C" void Bun__flushCStdio()
+{
+    fflush(stdout);
+    fflush(stderr);
+}
+#endif
+
 extern "C" int32_t bun_stdio_tty[3];
 #if !OS(WINDOWS)
 static termios termios_to_restore_later[3];

src/bun.zig

@@ -1506,6 +1506,9 @@ pub fn reloadProcess(
     if (comptime Environment.isWindows) {
         // on windows we assume that we have a parent process that is monitoring us and will restart us if we exit with a magic exit code
         // see becomeWatcherManager
+        //
+        // Flush C stdio buffers before TerminateProcess since it skips CRT teardown.
+        Global.Bun__flushCStdio();
         const rc = c.TerminateProcess(c.GetCurrentProcess(), windows.watcher_reload_exit);
         if (rc == 0) {
             const err = bun.windows.GetLastError();