bpo-25083: Python can sometimes create incorrect .pyc files (GH-8449)

Python 2 never checked for I/O error when reading .py files and
thus could mistake an I/O error for EOF and create incorrect .pyc
files.
This adds an check for this and aborts on an error.

Include/errcode.h

@@ -29,6 +29,7 @@ extern "C" {
 #define E_EOFS		23	/* EOF in triple-quoted string */
 #define E_EOLS		24	/* EOL in single-quoted string */
 #define E_LINECONT	25	/* Unexpected characters after a line continuation */
+#define E_IO    	26	/* I/O error */
 
 #ifdef __cplusplus
 }

Misc/NEWS.d/next/Core and Builtins/2018-07-25-22-47-19.bpo-25083.HT_hXh.rst

@@ -0,0 +1,2 @@
+Adding I/O error checking when reading .py files and aborting importing on
+error.

Parser/tokenizer.c

@@ -1681,6 +1681,11 @@ int
 PyTokenizer_Get(struct tok_state *tok, char **p_start, char **p_end)
 {
     int result = tok_get(tok, p_start, p_end);
+    if (tok->fp && ferror(tok->fp)) {
+        clearerr(tok->fp);
+        result = ERRORTOKEN;
+        tok->done = E_IO;
+    }
     if (tok->decoding_erred) {
         result = ERRORTOKEN;
         tok->done = E_DECODE;

Python/pythonrun.c

@@ -1654,6 +1654,9 @@ err_input(perrdetail *err)
         Py_XDECREF(tb);
         break;
     }
+    case E_IO:
+        msg = "I/O error while reading";
+        break;
     case E_LINECONT:
         msg = "unexpected character after line continuation character";
         break;