bpo-40985: Show correct SyntaxError text when last line has a LINECONT (GH-20888)

When a file ends with a line that contains a line continuation character
the text of the emitted SyntaxError is empty, contrary to the old
parser, where the error text contained the text of the last line.
(cherry picked from commit 113e2b0a07)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Lib/test/test_eof.py

@@ -52,10 +52,14 @@ class EOFTestCase(unittest.TestCase):
             file_name = script_helper.make_script(temp_dir, 'foo', '\\')
             rc, out, err = script_helper.assert_python_failure(file_name)
             self.assertIn(b'unexpected EOF while parsing', err)
+            self.assertIn(b'line 2', err)
+            self.assertIn(b'\\', err)
 
             file_name = script_helper.make_script(temp_dir, 'foo', 'y = 6\\')
             rc, out, err = script_helper.assert_python_failure(file_name)
             self.assertIn(b'unexpected EOF while parsing', err)
+            self.assertIn(b'line 2', err)
+            self.assertIn(b'y = 6\\', err)
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2020-06-15-16-29-55.bpo-40985.IIN_xX.rst

@@ -0,0 +1 @@
+Fix a bug that caused the :exc:`SyntaxError` text to be empty when a file ends with a line ending in a line continuation character (i.e. backslash). The error text should contain the text of the last line.

Python/errors.c

@@ -1648,16 +1648,18 @@ err_programtext(PyThreadState *tstate, FILE *fp, int lineno)
 {
     int i;
     char linebuf[1000];
-
+    if (fp == NULL) {
-    if (fp == NULL)
         return NULL;
+    }
+
     for (i = 0; i < lineno; i++) {
         char *pLastChar = &linebuf[sizeof(linebuf) - 2];
         do {
             *pLastChar = '\0';
             if (Py_UniversalNewlineFgets(linebuf, sizeof linebuf,
-                                         fp, NULL) == NULL)
+                                         fp, NULL) == NULL) {
-                break;
+                goto after_loop;
+            }
             /* fgets read *something*; if it didn't get as
                far as pLastChar, it must have found a newline
                or hit the end of the file; if pLastChar is \n,
@@ -1665,6 +1667,8 @@ err_programtext(PyThreadState *tstate, FILE *fp, int lineno)
                yet seen a newline, so must continue */
         } while (*pLastChar != '\0' && *pLastChar != '\n');
     }
+
+after_loop:
     fclose(fp);
     if (i == lineno) {
         PyObject *res;