py/stream: Remove mp_stream_errno and use system errno instead.

This change is made for two reasons:

1. A 3rd-party library (eg berkeley-db-1.xx, axtls) may use the system
   provided errno for certain errors, and yet MicroPython stream objects
   that it calls will be using the internal mp_stream_errno.  So if the
   library returns an error it is not known whether the corresponding errno
   code is stored in the system errno or mp_stream_errno.  Using the system
   errno in all cases (eg in the mp_stream_posix_XXX wrappers) fixes this
   ambiguity.

2. For systems that have threading the system-provided errno should always
   be used because the errno value is thread-local.

For systems that do not have an errno, the new lib/embed/__errno.c file is
provided.

extmod/extmod.mk

@@ -47,7 +47,7 @@ CFLAGS_MOD += -DMICROPY_PY_USSL=1
 ifeq ($(MICROPY_SSL_AXTLS),1)
 CFLAGS_MOD += -DMICROPY_SSL_AXTLS=1 -I$(TOP)/lib/axtls/ssl -I$(TOP)/lib/axtls/crypto -I$(TOP)/extmod/axtls-include
 AXTLS_DIR = lib/axtls
-$(BUILD)/$(AXTLS_DIR)/%.o: CFLAGS += -Wno-all -Wno-unused-parameter -Wno-uninitialized -Wno-sign-compare -Wno-old-style-definition $(AXTLS_DEFS_EXTRA)
+$(BUILD)/$(AXTLS_DIR)/%.o: CFLAGS += -Wno-all -Wno-unused-parameter -Wno-uninitialized -Wno-sign-compare -Wno-old-style-definition -Dmp_stream_errno=errno $(AXTLS_DEFS_EXTRA)
 SRC_MOD += $(addprefix $(AXTLS_DIR)/,\
 	ssl/asn1.c \
 	ssl/loader.c \

lib/embed/__errno.c

@@ -0,0 +1,13 @@
+// This file provides a version of __errno() for embedded systems that do not have one.
+// This function is needed for expressions of the form: &errno
+
+static int embed_errno;
+
+#if defined(__linux__)
+int *__errno_location(void)
+#else
+int *__errno(void)
+#endif
+{
+    return &embed_errno;
+}

ports/esp32/mphalport.c

@@ -195,13 +195,6 @@ void mp_hal_delay_us(uint32_t us) {
     }
 }
 
-/*
-extern int mp_stream_errno;
-int *__errno() {
-    return &mp_stream_errno;
-}
-*/
-
 // Wake up the main task if it is sleeping
 void mp_hal_wake_main_task_from_isr(void) {
     BaseType_t xHigherPriorityTaskWoken = pdFALSE;

ports/esp8266/Makefile

@@ -120,6 +120,7 @@ EXTMOD_SRC_C = $(addprefix extmod/,\
         )
 
 LIB_SRC_C = $(addprefix lib/,\
+	embed/__errno.c \
 	libc/string0.c \
 	libm/math.c \
 	libm/fmodf.c \

ports/esp8266/esp_mphal.c

@@ -206,8 +206,3 @@ int ets_esf_free_bufs(int idx) {
     }
     return cnt;
 }
-
-extern int mp_stream_errno;
-int *__errno() {
-    return &mp_stream_errno;
-}

py/stream.c

@@ -503,14 +503,12 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj, 2, 3, stream_ioctl);
  * POSIX-compatible software to work with MicroPython streams.
  */
 
-// errno-like variable. If any of the functions below returned with error
-// status, this variable will contain error no.
-int mp_stream_errno;
+#include <errno.h>
 
 ssize_t mp_stream_posix_write(void *stream, const void *buf, size_t len) {
     mp_obj_base_t *o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t out_sz = stream_p->write(MP_OBJ_FROM_PTR(stream), buf, len, &mp_stream_errno);
+    mp_uint_t out_sz = stream_p->write(MP_OBJ_FROM_PTR(stream), buf, len, &errno);
     if (out_sz == MP_STREAM_ERROR) {
         return -1;
     } else {
@@ -521,7 +519,7 @@ ssize_t mp_stream_posix_write(void *stream, const void *buf, size_t len) {
 ssize_t mp_stream_posix_read(void *stream, void *buf, size_t len) {
     mp_obj_base_t *o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t out_sz = stream_p->read(MP_OBJ_FROM_PTR(stream), buf, len, &mp_stream_errno);
+    mp_uint_t out_sz = stream_p->read(MP_OBJ_FROM_PTR(stream), buf, len, &errno);
     if (out_sz == MP_STREAM_ERROR) {
         return -1;
     } else {
@@ -535,7 +533,7 @@ off_t mp_stream_posix_lseek(void *stream, off_t offset, int whence) {
     struct mp_stream_seek_t seek_s;
     seek_s.offset = offset;
     seek_s.whence = whence;
-    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_SEEK, (mp_uint_t)(uintptr_t)&seek_s, &mp_stream_errno);
+    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_SEEK, (mp_uint_t)(uintptr_t)&seek_s, &errno);
     if (res == MP_STREAM_ERROR) {
         return -1;
     }
@@ -545,7 +543,7 @@ off_t mp_stream_posix_lseek(void *stream, off_t offset, int whence) {
 int mp_stream_posix_fsync(void *stream) {
     mp_obj_base_t *o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_FLUSH, 0, &mp_stream_errno);
+    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_FLUSH, 0, &errno);
     if (res == MP_STREAM_ERROR) {
         return -1;
     }