When reading data from a file into a str, check if it's utf-8

Otherwise, weird stuff can happen down the line when it is print()ed,
especially as it can break the webrepl of circuitpython.

py/stream.c

@@ -31,6 +31,7 @@
 #include "py/objstr.h"
 #include "py/stream.h"
 #include "py/runtime.h"
+#include "py/unicode.h"
 #include "supervisor/shared/translate/translate.h"
 
 // This file defines generic Python stream read/write methods which
@@ -43,6 +44,13 @@ STATIC mp_obj_t stream_readall(mp_obj_t self_in);
 
 #define STREAM_CONTENT_TYPE(stream) (((stream)->is_text) ? &mp_type_str : &mp_type_bytes)
 
+static mp_obj_t mp_obj_new_str_from_vstr_check(const mp_obj_type_t *type, vstr_t *vstr) {
+    if (type == &mp_type_str && !utf8_check((void *)vstr->buf, vstr->len)) {
+        mp_raise_msg(&mp_type_UnicodeError, NULL);
+    }
+    return mp_obj_new_str_from_vstr(type, vstr);
+}
+
 // Returns error condition in *errcode, if non-zero, return value is number of bytes written
 // before error condition occurred. If *errcode == 0, returns total bytes written (which will
 // be equal to input size).
@@ -201,8 +209,7 @@ STATIC mp_obj_t stream_read_generic(size_t n_args, const mp_obj_t *args, byte fl
                 }
             }
         }
-
-        return mp_obj_new_str_from_vstr(&mp_type_str, &vstr);
+        return mp_obj_new_str_from_vstr_check(&mp_type_str, &vstr);
     }
     #endif
 
@@ -223,7 +230,7 @@ STATIC mp_obj_t stream_read_generic(size_t n_args, const mp_obj_t *args, byte fl
         mp_raise_OSError(error);
     } else {
         vstr.len = out_sz;
-        return mp_obj_new_str_from_vstr(STREAM_CONTENT_TYPE(stream_p), &vstr);
+        return mp_obj_new_str_from_vstr_check(STREAM_CONTENT_TYPE(stream_p), &vstr);
     }
 }
 
@@ -364,7 +371,7 @@ STATIC mp_obj_t stream_readall(mp_obj_t self_in) {
     }
 
     vstr.len = total_size;
-    return mp_obj_new_str_from_vstr(STREAM_CONTENT_TYPE(stream_p), &vstr);
+    return mp_obj_new_str_from_vstr_check(STREAM_CONTENT_TYPE(stream_p), &vstr);
 }
 
 // Unbuffered, inefficient implementation of readline() for raw I/O files.
@@ -417,7 +424,7 @@ STATIC mp_obj_t stream_unbuffered_readline(size_t n_args, const mp_obj_t *args)
         }
     }
 
-    return mp_obj_new_str_from_vstr(STREAM_CONTENT_TYPE(stream_p), &vstr);
+    return mp_obj_new_str_from_vstr_check(STREAM_CONTENT_TYPE(stream_p), &vstr);
 }
 MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_unbuffered_readline_obj, 1, 2, stream_unbuffered_readline);