unix: fix swapping fds order when spawning processes

Alternative implementation (and test) to
libuv#226

Fixes: joyent/libuv#1084

src/unix/process.c

@@ -280,6 +280,17 @@ static void uv__process_child_init(const uv_process_options_t* options,
   if (options->flags & UV_PROCESS_DETACHED)
     setsid();
 
+  /* First duplicate low numbered fds, since it's not safe to duplicate them,
+   * they could get replaced. Example: swapping stdout and stderr; without
+   * this fd 2 (stderr) would be duplicated into fd 1, thus making both
+   * stdout and stderr go to the same fd, which was not the intention. */
+  for (fd = 0; fd < stdio_count; fd++) {
+    use_fd = pipes[fd][1];
+    if (use_fd >= fd)
+      continue;
+    pipes[fd][1] = fcntl(use_fd, F_DUPFD, stdio_count);
+  }
+
   for (fd = 0; fd < stdio_count; fd++) {
     close_fd = pipes[fd][0];
     use_fd = pipes[fd][1];

test/test-list.h

@@ -215,6 +215,7 @@ TEST_DECLARE   (spawn_stdout_to_file)
 TEST_DECLARE   (spawn_stdout_and_stderr_to_file)
 #ifndef _WIN32
 TEST_DECLARE   (spawn_stdout_and_stderr_to_file2)
+TEST_DECLARE   (spawn_stdout_and_stderr_to_file_swap)
 #endif
 TEST_DECLARE   (spawn_auto_unref)
 TEST_DECLARE   (spawn_closed_process_io)
@@ -578,6 +579,7 @@ TASK_LIST_START
   TEST_ENTRY  (spawn_stdout_and_stderr_to_file)
 #ifndef _WIN32
   TEST_ENTRY  (spawn_stdout_and_stderr_to_file2)
+  TEST_ENTRY  (spawn_stdout_and_stderr_to_file_swap)
 #endif
   TEST_ENTRY  (spawn_auto_unref)
   TEST_ENTRY  (spawn_closed_process_io)

test/test-spawn.c

@@ -431,6 +431,90 @@ TEST_IMPL(spawn_stdout_and_stderr_to_file2) {
   MAKE_VALGRIND_HAPPY();
   return 0;
 }
+
+
+TEST_IMPL(spawn_stdout_and_stderr_to_file_swap) {
+  int r;
+  uv_file stdout_file;
+  uv_file stderr_file;
+  uv_fs_t fs_req;
+  uv_stdio_container_t stdio[3];
+  uv_buf_t buf;
+
+  /* Setup. */
+  unlink("stdout_file");
+  unlink("stderr_file");
+
+  init_process_options("spawn_helper6", exit_cb);
+
+  /* open 'stdout_file' and replace STDOUT_FILENO with it */
+  r = uv_fs_open(uv_default_loop(), &fs_req, "stdout_file", O_CREAT | O_RDWR,
+      S_IRUSR | S_IWUSR, NULL);
+  ASSERT(r != -1);
+  uv_fs_req_cleanup(&fs_req);
+  stdout_file = dup2(r, STDOUT_FILENO);
+  ASSERT(stdout_file != -1);
+
+  /* open 'stderr_file' and replace STDERR_FILENO with it */
+  r = uv_fs_open(uv_default_loop(), &fs_req, "stderr_file", O_CREAT | O_RDWR,
+      S_IRUSR | S_IWUSR, NULL);
+  ASSERT(r != -1);
+  uv_fs_req_cleanup(&fs_req);
+  stderr_file = dup2(r, STDERR_FILENO);
+  ASSERT(stderr_file != -1);
+
+  /* now we're going to swap them: the child process' stdout will be our
+   * stderr_file and vice versa */
+  options.stdio = stdio;
+  options.stdio[0].flags = UV_IGNORE;
+  options.stdio[1].flags = UV_INHERIT_FD;
+  options.stdio[1].data.fd = stderr_file;
+  options.stdio[2].flags = UV_INHERIT_FD;
+  options.stdio[2].data.fd = stdout_file;
+  options.stdio_count = 3;
+
+  r = uv_spawn(uv_default_loop(), &process, &options);
+  ASSERT(r == 0);
+
+  r = uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+  ASSERT(r == 0);
+
+  ASSERT(exit_cb_called == 1);
+  ASSERT(close_cb_called == 1);
+
+  buf = uv_buf_init(output, sizeof(output));
+
+  /* check the content of stdout_file */
+  r = uv_fs_read(uv_default_loop(), &fs_req, stdout_file, &buf, 1, 0, NULL);
+  ASSERT(r >= 15);
+  uv_fs_req_cleanup(&fs_req);
+
+  r = uv_fs_close(uv_default_loop(), &fs_req, stdout_file, NULL);
+  ASSERT(r == 0);
+  uv_fs_req_cleanup(&fs_req);
+
+  printf("output is: %s", output);
+  ASSERT(strncmp("hello errworld\n", output, 15) == 0);
+
+  /* check the content of stderr_file */
+  r = uv_fs_read(uv_default_loop(), &fs_req, stderr_file, &buf, 1, 0, NULL);
+  ASSERT(r >= 12);
+  uv_fs_req_cleanup(&fs_req);
+
+  r = uv_fs_close(uv_default_loop(), &fs_req, stderr_file, NULL);
+  ASSERT(r == 0);
+  uv_fs_req_cleanup(&fs_req);
+
+  printf("output is: %s", output);
+  ASSERT(strncmp("hello world\n", output, 12) == 0);
+
+  /* Cleanup. */
+  unlink("stdout_file");
+  unlink("stderr_file");
+
+  MAKE_VALGRIND_HAPPY();
+  return 0;
+}
 #endif