newlib: fix return value of no-VFS _read_r if nothing received

'read' function should return 0 when encountering an end of file. When
newlib calls read and sees EOF returned, it assumes that this
condition is permanent and never calls 'read' for this file again
(unless the read pointer is moved using fseek).
The correct behavior in case no characters were received over UART is
to return -1. In this case newlib will retry reading from file on next
call to fread, fgetc or another function which calls __srefill_r.

components/newlib/syscalls.c

@@ -1,16 +1,8 @@
-// Copyright 2015-2020 Espressif Systems (Shanghai) PTE LTD
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+/*
+ * SPDX-FileCopyrightText: 2015-2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
 
 #include <stdbool.h>
 #include <stdlib.h>
@@ -59,6 +51,10 @@ ssize_t _read_r_console(struct _reent *r, int fd, void * data, size_t size)
                 break;
             }
         }
+        if (received == 0) {
+            errno = EWOULDBLOCK;
+            return -1;
+        }
         return received;
     }
     __errno_r(r) = EBADF;

tools/ci/check_copyright_ignore.txt

@@ -1090,7 +1090,6 @@ components/newlib/poll.c
 components/newlib/priv_include/esp_time_impl.h
 components/newlib/pthread.c
 components/newlib/reent_init.c
-components/newlib/syscalls.c
 components/newlib/termios.c
 components/newlib/test/test_atomic.c
 components/newlib/test/test_locks.c