wasm: call __stdio_exit on exit

This flushes stdio, so that functions like puts and printf write out all
buffered data even if the output isn't connected to a terminal.

For discussion, see:
https://github.com/bytecodealliance/wasmtime/issues/7833

src/runtime/runtime_tinygowasm.go

@@ -21,6 +21,11 @@ func fd_write(id uint32, iovs *__wasi_iovec_t, iovs_len uint, nwritten *uint) (e
 //go:wasmimport wasi_snapshot_preview1 proc_exit
 func proc_exit(exitcode uint32)
 
+// Flush stdio on exit.
+//
+//export __stdio_exit
+func __stdio_exit()
+
 const (
 	putcharBufferSize = 120
 	stdout            = 1
@@ -72,6 +77,9 @@ func abort() {
 
 //go:linkname syscall_Exit syscall.Exit
 func syscall_Exit(code int) {
+	// TODO: should we call __stdio_exit here?
+	// It's a low-level exit (syscall.Exit) so doing any libc stuff seems
+	// unexpected, but then where else should stdio buffers be flushed?
 	proc_exit(uint32(code))
 }
 

src/runtime/runtime_wasip1.go

@@ -19,6 +19,7 @@ func _start() {
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
 	run()
+	__stdio_exit()
 }
 
 // Read the command line arguments from WASI.

src/runtime/runtime_wasm_js.go

@@ -18,6 +18,7 @@ func _start() {
 
 	wasmNested = true
 	run()
+	__stdio_exit()
 	wasmNested = false
 }