gzip: cannot create file if mtime > `2106-02-07T06:28:15`

[huwcbjones]

Bug report

Bug description:

We had a VM where ntp went very wonky and ended up thinking it was the year 2141!
Trying to write a gzip file with the system clock greater than 2106-02-07T06:28:15 results in a struct error because we tried to cram a 64-bit int into a 32 bit field (which won't work).

I wouldn't expect the gzip module to explode in this case, I'd expect it to set the MTIME to 0.
RFC 1952 (if that's at all relevant these days) states

MTIME = 0 means no time stamp is available

MWE

Only tested this on macOS on 3.13.2, but from a cursory glance, the mtime handling hasn't changed in over a decade.

>>> import gzip
>>> gzip.GzipFile("/dev/null", "w", mtime=2**32+1)
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    gzip.GzipFile("/dev/null", "w", mtime=2**32+1)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/gzip.py", line 237, in __init__
    self._write_gzip_header(compresslevel)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/gzip.py", line 281, in _write_gzip_header
    write32u(self.fileobj, int(mtime))
    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/gzip.py", line 77, in write32u
    output.write(struct.pack("<L", value))
                 ~~~~~~~~~~~^^^^^^^^^^^^^
struct.error: 'L' format requires 0 <= number <= 4294967295

Proposed Patch

I'd naïvely fix it with this patch

diff --git a/Lib/gzip.py b/Lib/gzip.py
index c00f51858de..29345de4659 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -297,6 +297,8 @@ def _write_gzip_header(self, compresslevel):
         mtime = self._write_mtime
         if mtime is None:
             mtime = time.time()
+        if mtime > 4294967295:
+            mtime = 0
         write32u(self.fileobj, int(mtime))
         if compresslevel == _COMPRESS_LEVEL_BEST:
             xfl = b'\002'

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-133998: Fix gzip file creation when time is out of range #134278
• [3.15] gh-133998: Fix gzip file creation when time is out of range (GH-134278) #150221
• gh-133998: Fix tarfile error on out-of-range mtime in streaming gzip mode #151828

[StanFromIreland]

It is consistent with the specification, I am +1 for setting it to 0. This will have to be documented.

[adang1345]

I'm at the PyCon US sprints right now, and I decided to give this issue a shot. I have made a pull request.

[StanFromIreland]

@adang1345 In the future please ask before using someones idea, they may want to open the pr themselves.

[huwcbjones]

@adang1345 , in this case I'd be 108 before this issue could cause any noticeable problems.
Whether I make it that far or not, I think my care levels would be quite low, so feel free to fix this!

[serhiy-storchaka]

From the changelog for the GNU gzip 1.9 (2018-01-07):

When converting from system-dependent time_t format to the 32-bit
unsigned MTIME format used in gzip files, if a timestamp does not
fit gzip now substitutes zero instead of the timestamp's low-order
32 bits, as per Internet RFC 1952. When converting from MTIME to
time_t format, if a timestamp does not fit gzip now warns and
substitutes the nearest in-range value instead of crashing or
silently substituting an implementation-defined value (typically,
the timestamp's low-order bits). This affects timestamps before
1970 and after 2106, and timestamps after 2038 on platforms with
32-bit signed time_t. [bug present since the beginning]

So replacing the mtime outside of the valid range with 0 looks the most preferable solution.

[serhiy-storchaka]

There is also a similar issue in tarfile (see _init_write_gz). @adang1345, are you interesting to fix it there?