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[PR #13170/2b906869 backport][3.14] Fix StreamResponse.last_modified rounding inconsistency between datetime and timestamp (#13192)
**This is a backport of PR #13170 as merged into master (2b90686).** Co-authored-by: agu2347 <94227848+agu2347@users.noreply.github.com>
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‎CHANGES/5303.bugfix.rst‎

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Fixed :py:attr:`~aiohttp.web.StreamResponse.last_modified` rounding a
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:class:`datetime.datetime` with a fractional second down while rounding an
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equivalent :class:`float` timestamp up, so the two produced different
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``Last-Modified`` headers for the same instant. Both now round up to the
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next whole second, consistent with how :py:class:`~aiohttp.web.FileResponse`
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relies on this rounding to avoid falsely reporting a resource as
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unmodified.

‎aiohttp/web_response.py‎

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@@ -371,6 +371,8 @@ def last_modified(
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"%a, %d %b %Y %H:%M:%S GMT", time.gmtime(math.ceil(value))
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)
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elif isinstance(value, datetime.datetime):
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if value.microsecond:
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value = value.replace(microsecond=0) + datetime.timedelta(seconds=1)
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self._headers[hdrs.LAST_MODIFIED] = time.strftime(
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"%a, %d %b %Y %H:%M:%S GMT", value.utctimetuple()
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)

‎tests/test_web_response.py‎

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@@ -339,6 +339,23 @@ def test_last_modified_datetime() -> None:
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assert resp.last_modified == dt
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def test_last_modified_datetime_and_timestamp_round_consistently() -> None:
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dt = datetime.datetime(2020, 12, 2, 9, 51, 2, 500000, datetime.timezone.utc)
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resp_dt = web.StreamResponse()
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resp_dt.last_modified = dt
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resp_ts = web.StreamResponse()
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resp_ts.last_modified = dt.timestamp()
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assert resp_dt.headers["Last-Modified"] == resp_ts.headers["Last-Modified"]
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# Both should round up to the next whole second, so that Last-Modified
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# is never earlier than the file's true st_mtime, invalidating the caching.
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assert resp_dt.last_modified == datetime.datetime(
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2020, 12, 2, 9, 51, 3, 0, datetime.timezone.utc
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)
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def test_last_modified_reset() -> None:
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resp = StreamResponse()
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