fix(msgpack): strptime() fails east of UTC+12 #41743

Problem:
msgpack#strptime() finds a timestamp by bisecting strftime() output, and
brackets the search with the extreme UTC offsets. The lower bound
subtracts 12 hours, but -12:00 is the westernmost offset, which produces
the *largest* timestamp for a given local time. The easternmost offset
is +14:00, so in any zone east of UTC+12 the search can start above the
target and the function throws:

    internal-start-string:Internal error: start > string

With TZ=GMT-14, four of the five timestamps in msgpack_spec.lua fail,
as do three tests in shada_spec.lua, which reaches the same function
through shada#strings_to_sd().

Solution:
Subtract 14 hours instead. The upper bound is already generous enough at
+14, and widening a bisection bracket downwards cannot change the result
in zones that worked before.

The test disables the python3 provider, because msgpack#strptime() only
uses the Vimscript implementation changed here when no provider answers,
and otherwise hands the work to datetime.strptime().

AI-assisted

runtime/autoload/msgpack.vim

@@ -132,11 +132,11 @@ function s:msgpack_init_python() abort
     let [year, month, day, hour, minute, second] = match[1:6]
     " Bisection start and end:
     "
-    " Start: 365 days in year, 28 days in month, -12 hours tz offset.
+    " Start: 365 days in year, 28 days in month, -14 hours tz offset.
     let bisect_ts_start = (((((year - 1970) * 365
                              \+ (month - 1) * 28
                              \+ (day - 1)) * 24
-                            \+ hour - 12) * 60
+                            \+ hour - 14) * 60
                            \+ minute) * 60
                           \+ second)
     if bisect_ts_start < 0

test/functional/plugin/msgpack_spec.lua

@@ -735,3 +735,25 @@ describe('autoload/msgpack.vim', function()
     end)
   end)
 end)
+
+describe('autoload/msgpack.vim in an eastern timezone', function()
+  setup(function()
+    clear({
+      -- msgpack#strptime() prefers a Python implementation and only falls back to the
+      -- Vimscript one, which is the one under test, when no provider answers.
+      args = { '-u', 'NORC', '--cmd', 'let g:loaded_python3_provider = 0' },
+      -- POSIX TZ signs are inverted: GMT-14 is UTC+14, the easternmost offset.
+      env = { TZ = 'GMT-14' },
+    })
+  end)
+
+  describe('function msgpack#strptime', function()
+    it('works east of UTC+12 #7625', function()
+      eq(0, nvim_eval('has("python3")'))
+      for _, v in ipairs({ 0, 10, 100000, 204, 1000000000 }) do
+        local time = nvim_eval(('strftime("%%Y-%%m-%%dT%%H:%%M:%%S", %d)'):format(v))
+        eq(v, nvim_eval(('msgpack#strptime("%%Y-%%m-%%dT%%H:%%M:%%S", "%s")'):format(time)))
+      end
+    end)
+  end)
+end)