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name-rev: avoid cutoff timestamp underflow

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When 'git name-rev' is invoked with commit-ish parameters, it tries to
save some work, and doesn't visit commits older than the committer
date of the oldest given commit minus a one day worth of slop.  Since
our 'timestamp_t' is an unsigned type, this leads to a timestamp
underflow when the committer date of the oldest given commit is within
a day of the UNIX epoch.  As a result the cutoff timestamp ends up
far-far in the future, and 'git name-rev' doesn't visit any commits,
and names each given commit as 'undefined'.

Check whether subtracting the slop from the oldest committer date
would lead to an underflow, and use no cutoff in that case.  We don't
have a TIME_MIN constant, dddbad728c (timestamp_t: a new data type for
timestamps, 2017-04-26) didn't add one, so do it now.

Note that the type of the cutoff timestamp variable used to be signed
before 5589e87fd8 (name-rev: change a "long" variable to timestamp_t,
2017-05-20).  The behavior was still the same even back then, but the
underflow didn't happen when substracting the slop from the oldest
committer date, but when comparing the signed cutoff timestamp with
unsigned committer dates in name_rev().  IOW, this underflow bug is as
old as 'git name-rev' itself.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
SZEDER Gábor
2019-09-24 09:32:13 +02:00
committed by Junio C Hamano
parent 5fa0f5238b
commit 2e09c01232
3 changed files with 28 additions and 3 deletions
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15
t/t6120-describe.sh
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@@ -424,4 +424,19 @@ test_expect_success 'describe complains about missing object' '
test_must_fail git describe $ZERO_OID
'
test_expect_success 'name-rev a rev shortly after epoch' '
test_when_finished "git checkout master" &&
git checkout --orphan no-timestamp-underflow &&
# Any date closer to epoch than the CUTOFF_DATE_SLOP constant
# in builtin/name-rev.c.
GIT_COMMITTER_DATE="@1234 +0000" \
git commit -m "committer date shortly after epoch" &&
old_commit_oid=$(git rev-parse HEAD) &&
echo "$old_commit_oid no-timestamp-underflow" >expect &&
git name-rev $old_commit_oid >actual &&
test_cmp expect actual
'
test_done