unix: fix several instances of lost errno

This commit preserves errno properly in the following cases:

- new_socket() was previously discarding errno in some cases,
  and replacing it with the result of getsockname().
- uv__close() was not preserving errno when __MVS__ is defined.

Fixes: https://github.com/libuv/libuv/issues/1756
PR-URL: https://github.com/libuv/libuv/pull/1763
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/core.c

@@ -536,7 +536,7 @@ int uv__close_nocheckstdio(int fd) {
 int uv__close(int fd) {
   assert(fd > STDERR_FILENO);  /* Catch stdio close bugs. */
 #if defined(__MVS__)
-  epoll_file_close(fd);
+  SAVE_ERRNO(epoll_file_close(fd));
 #endif
   return uv__close_nocheckstdio(fd);
 }

src/unix/internal.h

@@ -190,7 +190,7 @@ int uv__cloexec_ioctl(int fd, int set);
 int uv__cloexec_fcntl(int fd, int set);
 int uv__nonblock_ioctl(int fd, int set);
 int uv__nonblock_fcntl(int fd, int set);
-int uv__close(int fd);
+int uv__close(int fd); /* preserves errno */
 int uv__close_nocheckstdio(int fd);
 int uv__socket(int domain, int type, int protocol);
 int uv__dup(int fd);

src/unix/tcp.c

@@ -49,16 +49,14 @@ static int new_socket(uv_tcp_t* handle, int domain, unsigned long flags) {
     /* Bind this new socket to an arbitrary port */
     slen = sizeof(saddr);
     memset(&saddr, 0, sizeof(saddr));
-    err = getsockname(uv__stream_fd(handle), (struct sockaddr*) &saddr, &slen);
-    if (err) {
+    if (getsockname(uv__stream_fd(handle), (struct sockaddr*) &saddr, &slen)) {
       uv__close(sockfd);
-      return err;
+      return UV__ERR(errno);
     }
 
-    err = bind(uv__stream_fd(handle), (struct sockaddr*) &saddr, slen);
-    if (err) {
+    if (bind(uv__stream_fd(handle), (struct sockaddr*) &saddr, slen)) {
       uv__close(sockfd);
-      return err;
+      return UV__ERR(errno);
     }
   }