[WasmFS] Fix standalone wasm printing, newline instead of 0 (#16076)

0 works in some places but at least in wasmtime it does not.

Also rename the method to make that more obvious.

Author: kripken

.circleci/config.yml

@@ -384,7 +384,7 @@ jobs:
       - run-tests:
           # also add a little select testing for wasm2js in -O3
           # also add a little select wasmfs testing
-          test_targets: "core3 wasm2js3.test_memorygrowth_2 wasmfs.test_hello_world wasmfs.test_hello_world_standalone wasmfs.test_unistd_links*"
+          test_targets: "core3 wasm2js3.test_memorygrowth_2 wasmfs.test_hello_world wasmfs.test_hello_world_standalone wasmfs.test_unistd_links* wasmfs.test_atexit_standalone"
   test-wasm2js1:
     executor: bionic
     steps:

system/lib/standalone/standalone.c

@@ -180,14 +180,17 @@ imported__wasi_fd_write(__wasi_fd_t fd,
                         size_t iovs_len,
                         __wasi_size_t* nwritten);
 
-static void wasi_write(__wasi_fd_t fd, char* buffer) {
-  struct __wasi_ciovec_t iov;
-  iov.buf = (uint8_t*)buffer;
-  iov.buf_len = strlen(buffer) + 1;
+// Write a buffer + a newline.
+static void wasi_writeln(__wasi_fd_t fd, char* buffer) {
+  struct __wasi_ciovec_t iovs[2];
+  iovs[0].buf = (uint8_t*)buffer;
+  iovs[0].buf_len = strlen(buffer);
+  iovs[1].buf = (uint8_t*)"\n";
+  iovs[1].buf_len = 1;
   __wasi_size_t nwritten;
-  imported__wasi_fd_write(fd, &iov, 1, &nwritten);
+  imported__wasi_fd_write(fd, iovs, 2, &nwritten);
 }
 
-void _emscripten_out(char* text) { wasi_write(1, text); }
+void _emscripten_out(char* text) { wasi_writeln(1, text); }
 
-void _emscripten_err(char* text) { wasi_write(2, text); }
+void _emscripten_err(char* text) { wasi_writeln(2, text); }