[Bug 50532] New: get_timezone_info doing binary search in the wrong way

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=50532

            Bug ID: 50532
           Summary: get_timezone_info doing binary search in the wrong way
           Product: Wine
           Version: 6.0-rc6
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ntdll
          Assignee: wine-bugs(a)winehq.org
          Reporter: gmascellani(a)codeweavers.com
      Distribution: ---

get_timezone_info in dlls/ntdll/unix/system.c appears to compute the daylight
saving time changes for a given year in a smelly if not buggy way. I don't have
any specific examples of mistaken computations, but the code seems to basically
work by chance.

More specifically, the indention of the code seems to be the following:
 * two time_t values are computed, corresponding to the beginning and the end
of the requested year (year_start and year_end);
 * a binary search procedure is used to narrow down that interval around a DST
change event.

However, this cannot work in general, because binary searches only work for
monotonic data, and DST is not monotonic (each year it switches forth and back,
assuming there is DST at all). The algorithm works "by chance", because it
first tests the beginning of the interval and then half of the interval, and
usually beginning of January and beginning of June are on different DST values;
then the algorithm recurses on the first half of the year, and there DST is
monotonic (again, usually; I don't know if this is always true).

Moreover, year_end is also set to an invalid value (0th day of 12th month,
which is a month past December). Doing the "arithmetic" this corresponds to
31st of December, but is that a supported way to specify it? Why not directly
setting 31st of December? Anyway, the core of this bug is the issue with binary
search.

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[Bug 50532] get_timezone_info doing binary search in the wrong way

[WineHQ Bugzilla]

http://bugs.winehq.org/show_bug.cgi?id=50532

Paul Gofman <[e-mail]> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Fixed by SHA1|                            |7d7df9e80c56065d95b7cea6ae9
                   |                            |9cbee7d77dd72
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
                 CC|                            |[e-mail]

--- Comment #1 from Paul Gofman <[e-mail]> ---
I came across this issue when working on broader timezone matching improvements
in https://gitlab.winehq.org/wine/wine/-/merge_requests/10134. At least in 2026
there is exactly one timezone affected by this bug, Africa/Casablanca, which
changed to STD on Feb15 and is  going to DST month later.

It should be hopefully fixed by 7d7df9e80c56065d95b7cea6ae99cbee7d77dd72 (or,
well, until there is timezone change with some 2 weeks interval).

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[Bug 50532] get_timezone_info doing binary search in the wrong way

[WineHQ Bugzilla]

http://bugs.winehq.org/show_bug.cgi?id=50532

Alexandre Julliard <[e-mail]> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #2 from Alexandre Julliard <[e-mail]> ---
Closing bugs fixed in 11.5.

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