bpo-40780: Fix failure of _Py_dg_dtoa to remove trailing zeros (GH-20435)

* Fix failure of _Py_dg_dtoa to remove trailing zeros

* Add regression test and news entry

* Add explanation about why it's safe to strip trailing zeros

* Make code safer, clean up comments, add change note at top of file

* Nitpick: avoid implicit int-to-float conversion in tests

1 files changed, 11 insertions, 0 deletions

diff --git a/Python/dtoa.c b/Python/dtoa.c
index 822adc6..e629b29 100644
--- a/Python/dtoa.c
+++ b/Python/dtoa.c
@@ -64,6 +64,9 @@
  *  7. _Py_dg_strtod has been modified so that it doesn't accept strings with
  *     leading whitespace.
  *
+ *  8. A corner case where _Py_dg_dtoa didn't strip trailing zeros has been
+ *     fixed. (bugs.python.org/issue40780)
+ *
  ***************************************************************/
 
 /* Please send bug reports for the original dtoa.c code to David M. Gay (dmg
@@ -2563,6 +2566,14 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
                         }
                     ++*s++;
                 }
+                else {
+                    /* Strip trailing zeros. This branch was missing from the
+                       original dtoa.c, leading to surplus trailing zeros in
+                       some cases. See bugs.python.org/issue40780. */
+                    while (s > s0 && s[-1] == '0') {
+                        --s;
+                    }
+                }
                 break;
             }
         }