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Strange reversed dict behavior #82706

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Strange reversed dict behavior#82706
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3.8 (EOL)end of life3.9 (EOL)end of lifeinterpreter-core(Objects, Python, Grammar, and Parser dirs)type-crashA hard crash of the interpreter, possibly with a core dump

Description

@ivb
ivb
mannequin
opened on Oct 19, 2019
BPO 38525
Nosy @ned-deily, @methane, @serhiy-storchaka, @corona10, @pablogsal, @miss-islington, @Harmon758, @remilapeyre, @tirkarthi
PRs
  • bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict #16846
  • bpo-38525: Fix segfault when using reverse iterators of empty dict literals #16847
  • [3.8] bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict (GH-16846) #16853
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = <Date 2019-10-19.20:23:30.477>
created_at = <Date 2019-10-19.08:00:52.660>
labels = ['interpreter-core', '3.8', '3.9', 'type-crash']
title = 'Strange reversed dict behavior'
updated_at = <Date 2019-11-14.10:48:55.574>
user = 'https://bugs.python.org/ivb'

    bugs.python.org fields:

    activity = <Date 2019-11-14.10:48:55.574>
actor = 'Harmon758'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-19.20:23:30.477>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2019-10-19.08:00:52.660>
creator = 'ivb'
dependencies = []
files = []
hgrepos = []
issue_num = 38525
keywords = ['patch']
message_count = 15.0
messages = ['354931', '354932', '354933', '354938', '354939', '354940', '354944', '354945', '354946', '354947', '354948', '354949', '354961', '354962', '354963']
nosy_count = 10.0
nosy_names = ['ned.deily', 'methane', 'ivb', 'serhiy.storchaka', 'corona10', 'pablogsal', 'miss-islington', 'Harmon758', 'remi.lapeyre', 'xtreak']
pr_nums = ['16846', '16847', '16853']
priority = 'critical'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue38525'
versions = ['Python 3.8', 'Python 3.9']

    Activity

    1. ivb commented on Oct 19, 2019

      @ivb
      ivbmannequin
      on Oct 19, 2019
      MannequinAuthor
      Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> list(reversed({1: 1}))
[1]
>>> list(reversed({}))

      ================================ RESTART: Shell ================================

      >>

    2. ivb
      added
      type-bugAn unexpected behavior, bug, or error
      3.8 (EOL)end of life
      interpreter-core(Objects, Python, Grammar, and Parser dirs)
      on Oct 19, 2019

    3. ned-deily commented on Oct 19, 2019

      @ned-deily
      ned-deily
      on Oct 19, 2019
      Member

      Outside of IDLE, the example causes a segfault. With debug build of current master HEAD:

      Assertion failed: (value != NULL), function dictreviter_iternext, file ../../source/Objects/dictobject.c, line 3834.

          if (d->ma_values) {
        if (i < 0) {
            goto fail;
        }
        key = DK_ENTRIES(k)[i].me_key;
        value = d->ma_values[i];
        assert (value != NULL);
    }
    4. ned-deily
      added
      3.9 (EOL)end of life
      type-crashA hard crash of the interpreter, possibly with a core dump
      and removed
      type-bugAn unexpected behavior, bug, or error
      on Oct 19, 2019

    5. ned-deily commented on Oct 19, 2019

      @ned-deily
      ned-deily
      on Oct 19, 2019
      Member

      Thanks for the report, by the way!

    6. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member

      This issue is related to reversed dict iter.

      >>> a = reversed({})
>>> next(a)
Assertion failed: (value != NULL), function dictreviter_iternext, file Objects/dictobject.c, line 3834.
[1]    1366 abort      ./python.exe

    7. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member
      >>> reversed({}.items())
<dict_reverseitemiterator object at 0x106a2bc50>
>>> a = reversed({}.items())
>>> next(a)
Assertion failed: (value != NULL), function dictreviter_iternext, file Objects/dictobject.c, line 3834.
[1]    4106 abort      ./python.exe

    8. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member

      Can I take look at this issue?

    9. pablogsal commented on Oct 19, 2019

      @pablogsal
      pablogsal
      on Oct 19, 2019
      Member

      Wops, we made a PR at the same time, Dong-hee Na.

      I will close mine :)

    10. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member

      Wops, we made a PR at the same time, Dong-hee Na.

      lol

      I will close mine :)

      Oh.. thank you for giving me a chance.

    11. serhiy-storchaka commented on Oct 19, 2019

      @serhiy-storchaka
      serhiy-storchaka
      on Oct 19, 2019
      Member

      The proposed fix fixes a crash, but there is other bug in iterating shared dicts.

      class A:
    def __init__(self, x, y):
        if x: self.x = x
        if y: self.y = y

a = A(1, 2)
print(list(iter(a.__dict__)))
print(list(reversed(a.__dict__)))
b = A(1, 0)
print(list(iter(b.__dict__)))
print(list(reversed(b.__dict__)))

      With PR 16846 the last print outputs [] instead of expected ['x']. It crashes without PR 16846, so this issue is not only about empty dicts.

    12. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member

      The proposed fix fixes a crash, but there is other bug in iterating shared dicts.

      Yes, you are right.
      But I can not work on this issue today.
      (I have to finalize my paperwork by today :-()
      since priority is critical if other core developers finalize this issue today. It will be okay for me.

    13. corona10 commented on Oct 19, 2019

      @corona10
      corona10
      on Oct 19, 2019
      Member

      FYI

      c = A(0, 1)
print(type(c.__dict__))
print(list(iter(c.__dict__)))
print(list(reversed(c.__dict__)))

      works as we expected.

    14. methane commented on Oct 19, 2019

      @methane
      methane
      on Oct 19, 2019
      Member

      When dict is empty, di_pos of reverse iterator must be -1, not 0.
      Additionally, di_pos must be initialized from ma_used when dict is key sharing dict.

      diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 64876e0519..6c4b41700b 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -3452,10 +3452,15 @@ dictiter_new(PyDictObject *dict, PyTypeObject *itertype)
     di->di_dict = dict;
     di->di_used = dict->ma_used;
     di->len = dict->ma_used;
-    if ((itertype == &PyDictRevIterKey_Type ||
-         itertype == &PyDictRevIterItem_Type ||
-         itertype == &PyDictRevIterValue_Type) && dict->ma_used) {
+    if (itertype == &PyDictRevIterKey_Type ||
+            itertype == &PyDictRevIterItem_Type ||
+            itertype == &PyDictRevIterValue_Type) {
+        if (dict->ma_values) {
+            di->di_pos = dict->ma_used - 1;
+        }
+        else {
             di->di_pos = dict->ma_keys->dk_nentries - 1;
+        }
     }
     else {

    15. pablogsal commented on Oct 19, 2019

      @pablogsal
      pablogsal
      on Oct 19, 2019
      Member

      New changeset 24dc2f8 by Pablo Galindo (Dong-hee Na) in branch 'master':
      bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict (GH-16846)
      24dc2f8

    16. miss-islington commented on Oct 19, 2019

      @miss-islington
      miss-islington
      on Oct 19, 2019
      Contributor

      New changeset d73205d by Miss Islington (bot) in branch '3.8':
      bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict (GH-16846)
      d73205d

    17. pablogsal commented on Oct 19, 2019

      @pablogsal
      pablogsal
      on Oct 19, 2019
      Member

      Closing this.

      Thanks, Dong-hee Na and everyone involved in the issue!

    18. pablogsal
      closed this as completedon Oct 19, 2019
    19. ezio-melotti
      transferred this issue fromon Apr 10, 2022
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