pythongh-141314: Fix TextIOWrapper.tell() assertion failure with standalone carriage return

When TextIOWrapper.tell() is called after reading a line that ends with a
standalone carriage return (\r), the tell optimization algorithm incorrectly
assumes there is buffered data to search through. This causes an assertion
failure when skip_back=1 exceeds the empty buffer size.

The fix detects when next_input is empty and skips the optimization phase,
falling back to the byte-by-byte decoding method which always works correctly.
This properly handles the architectural constraint that buffer optimization
cannot function without buffered data.

Author: mohsinm-dev

Lib/test/test_io/test_textio.py

@@ -686,6 +686,24 @@ def test_multibyte_seek_and_tell(self):
         self.assertEqual(f.tell(), p1)
         f.close()
 
+    def test_tell_after_readline_with_cr(self):
+        # Test for gh-141314: TextIOWrapper.tell() assertion failure
+        # when dealing with standalone carriage returns
+        data = b'line1=1\r'
+        with self.open(os_helper.TESTFN, "wb") as f:
+            f.write(data)
+
+        with self.open(os_helper.TESTFN, "r") as f:
+            # Read line that ends with \r
+            line = f.readline()
+            self.assertEqual(line, "line1=1\n")
+            # This should not cause an assertion failure
+            pos = f.tell()
+            # Verify we can seek back to this position
+            f.seek(pos)
+            remaining = f.read()
+            self.assertEqual(remaining, "")
+
     def test_seek_with_encoder_state(self):
         f = self.open(os_helper.TESTFN, "w", encoding="euc_jis_2004")
         f.write("\u00e6\u0300")

Modules/_io/textio.c

@@ -2844,10 +2844,16 @@ _io_TextIOWrapper_tell_impl(textio *self)
     /* Fast search for an acceptable start point, close to our
        current pos */
     skip_bytes = (Py_ssize_t) (self->b2cratio * chars_to_skip);
-    skip_back = 1;
-    assert(skip_back <= PyBytes_GET_SIZE(next_input));
-    input = PyBytes_AS_STRING(next_input);
-    while (skip_bytes > 0) {
+    
+    /* Skip the optimization if next_input is empty */
+    if (PyBytes_GET_SIZE(next_input) == 0) {
+        skip_bytes = 0;
+    }
+    else {
+        skip_back = 1;
+        assert(skip_back <= PyBytes_GET_SIZE(next_input));
+        input = PyBytes_AS_STRING(next_input);
+        while (skip_bytes > 0) {
         /* Decode up to temptative start point */
         if (_textiowrapper_decoder_setstate(self, &cookie) < 0)
             goto fail;
@@ -2870,6 +2876,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
             skip_back *= 2;
         }
     }
+    }
     if (skip_bytes <= 0) {
         skip_bytes = 0;
         if (_textiowrapper_decoder_setstate(self, &cookie) < 0)