Segfault in master due to 4fbcca4eff

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Fri, Sep 21 2018, Junio C Hamano wrote:

> * ds/reachable (2018-08-28) 19 commits
>   (merged to 'next' on 2018-08-28 at b1634b371d)
>  + commit-reach: correct accidental #include of C file
>   (merged to 'next' on 2018-08-22 at 17f3275afb)
>  + commit-reach: use can_all_from_reach
>  + commit-reach: make can_all_from_reach... linear
>  + commit-reach: replace ref_newer logic
>  + test-reach: test commit_contains
>  + test-reach: test can_all_from_reach_with_flags
>  + test-reach: test reduce_heads
>  + test-reach: test get_merge_bases_many
>  + test-reach: test is_descendant_of
>  + test-reach: test in_merge_bases
>  + test-reach: create new test tool for ref_newer
>  + commit-reach: move can_all_from_reach_with_flags
>  + upload-pack: generalize commit date cutoff
>  + upload-pack: refactor ok_to_give_up()
>  + upload-pack: make reachable() more generic
>  + commit-reach: move commit_contains from ref-filter
>  + commit-reach: move ref_newer from remote.c
>  + commit.h: remove method declarations
>  + commit-reach: move walk methods from commit.c
>
>  The code for computing history reachability has been shuffled,
>  obtained a bunch of new tests to cover them, and then being
>  improved.

There's a segfault now in master when fetching because of 4fbcca4eff
("commit-reach: make can_all_from_reach... linear", 2018-07-20). I have
not had time to debug this, or found an easy isolated test case, but
this script will reliably make it segfault for me:

    #!/bin/sh

    git_dot_git=/home/avar/g/git

    old=v0.99
    new=v0.99.1

    rm -rf /tmp/srv
    rm -rf /tmp/cln
    git init --bare /tmp/srv
    git init --bare /tmp/cln
    $git_dot_git/git --exec-path=$git_dot_git push file:///tmp/srv $old:refs/whatever/ref
    $git_dot_git/git --exec-path=$git_dot_git -C /tmp/cln fetch file:///tmp/srv 'refs/*:refs/*'
    $git_dot_git/git --exec-path=$git_dot_git push file:///tmp/srv $new:refs/whatever/ref
    if GIT_TRACE=1 $git_dot_git/git --exec-path=$git_dot_git -C /tmp/cln fetch file:///tmp/srv 'refs/*:refs/*'
    then
        exit 0
    else
        exit 1
    fi

I.e. you need to push the v0.99 tag to its own repo, fetch that from
another one, then push v0.99.1, fetch everything, and you'll get a
segfault:

    remote: Resolving deltas: 100% (187/187), completed with 48 local objects.
    To file:///tmp/srv
       d6602ec519..f25a265a34  v0.99.1 -> refs/whatever/ref
    14:26:44.505787 git.c:415               trace: built-in: git fetch file:///tmp/srv 'refs/*:refs/*'
    14:26:44.506708 run-command.c:637       trace: run_command: unset GIT_DIR GIT_IMPLICIT_WORK_TREE GIT_PREFIX; 'git-upload-pack '\''/tmp/srv'\'''
    14:26:44.508831 git.c:415               trace: built-in: git upload-pack /tmp/srv
    14:26:44.509953 run-command.c:637       trace: run_command: git rev-list --objects --stdin --not --all --quiet
    Segmentation fault
    fatal: The remote end hung up unexpectedly

Same with refs/tags/ref b.t.w., not just refs/whatever/ref, I just was
initially testing this for some follow-up work on my series for checking
how fetching to various non-heads/tags namespaces works.