commit-graph: fix writing generations with dates exceeding 34 bits

The `timestamp_t` type is declared as `uintmax_t` and thus typically has
64 bits of precision. Usually, the full precision of such dates is not
required: it would be comforting to know that Git is still around in
millions of years, but all in all the chance is rather low.

We abuse this fact in the commit-graph: instead of storing the full 64
bits of precision, committer dates only store 34 bits. This is still
plenty of headroom, as it means that we can represent dates until year
2514. Commits which are dated beyond that year will simply get a date
whose remaining bits are masked.

The result of this is somewhat curious: the committer date will be
different depending on whether a commit gets parsed via the commit-graph
or via the object database. This isn't really too much of an issue in
general though, as we don't typically use the date parsed from the
commit-graph in user-facing output.

But with 024b4c9697 (commit: make `repo_parse_commit_no_graph()` more
robust, 2026-02-16) it started to become a problem when writing the
commit-graph itself. This commit changed `repo_parse_commit_no_graph()`
so that we re-parse the commit via the object database in case it was
already parsed beforehand via the commit-graph.

The consequence is that we may now act with two different commit dates
at different stages:

  - Initially, we use the 34-bit precision timestamp when writing the
    chunk generation data. We thus correctly compute the offsets
    relative to the on-disk timestamp here.

  - Later, when writing the overflow data, we may end up with the
    full-precision timestamp. When the date is larger than 34 bits the
    result of this is an underflow when computing the offset.

This causes a mismatch in the number of generation data overflow records
we want to write, and that ultimately causes Git to die.

Introduce a new helper function that computes the generation offset for
a commit while correctly masking the date to 34 bits. This makes the
previously-implicit assumptions about the commit date precision explicit
and thus hopefully less fragile going forward.

Adapt sites that compute the offset to use the function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions