[PATCH v3 0/2] no lazy fetch in lookup_commit_in_graph()

This patch fixes the following issue: When we found the commit in the graph in lookup_commit_in_graph(), but the commit is missing from the repository, we will try promisor_remote_get_direct() and then enter another loop.

Then we will go into an endless loop:
  git fetch -> deref_without_lazy_fetch() ->
    lookup_commit_in_graph() -> repo_has_object_file() ->
      promisor_remote_get_direct() -> fetch_objects() ->
        git fetch (a new loop round)

Changes since v2:

* Remove test_have_prereq() from ULIMIT_PROCESSES as
  "run_with_limited_processses true" is enough.

* Teach run_with_limited_processses() to support dash and zsh.

* Skip the whole test file if ulimit is not avaliable.

* Minor grammar/comment etc. fixes throughout.

Han Xin (2):
  test-lib.sh: add limited processes to test-lib
  commit-graph.c: no lazy fetch in lookup_commit_in_graph()

 commit-graph.c                             |  2 +-
 t/t5329-no-lazy-fetch-with-commit-graph.sh | 53 ++++++++++++++++++++++
 t/test-lib.sh                              | 16 +++++++
 3 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100755 t/t5329-no-lazy-fetch-with-commit-graph.sh

Range-diff against v2:
1:  442a4c351d ! 1:  ad0a539759 test-lib.sh: add limited processes to test-lib
    @@ t/test-lib.sh: test_lazy_prereq ULIMIT_FILE_DESCRIPTORS '
      '
      
     +run_with_limited_processses () {
    -+	(ulimit -u 512 && "$@")
    ++	# bash and ksh use "ulimit -u", dash uses "ulimit -p"
    ++	if test -n "$BASH_VERSION"
    ++	then
    ++		ulimit_max_process="-u"
    ++	elif test -n "$KSH_VERSION"
    ++	then
    ++		ulimit_max_process="-u"
    ++	fi
    ++	(ulimit ${ulimit_max_process-"-p"} 512 && "$@")
     +}
     +
     +test_lazy_prereq ULIMIT_PROCESSES '
    -+	test_have_prereq !HPPA,!MINGW,!CYGWIN &&
     +	run_with_limited_processses true
     +'
     +
2:  a7d456db9b ! 2:  3cdb1abd43 commit-graph.c: no lazy fetch in lookup_commit_in_graph()
    @@ Metadata
      ## Commit message ##
         commit-graph.c: no lazy fetch in lookup_commit_in_graph()
     
    -    If a commit is in the commit graph, we would expect the commit to also
    -    be present. So we should use has_object() instead of
    -    repo_has_object_file(), which will help us avoid getting into an endless
    -    loop of lazy fetch.
    +    The commit-graph is used to opportunistically optimize accesses to
    +    certain pieces of information on commit objects, and
    +    lookup_commit_in_graph() tries to say "no" when the requested commit
    +    does not locally exist by returning NULL, in which case the caller
    +    can ask for (which may result in on-demand fetching from a promisor
    +    remote) and parse the commit object itself.
     
    -    When we found the commit in the graph in lookup_commit_in_graph(), but
    -    the commit is missing from the repository, we will try
    -    promisor_remote_get_direct() and then enter another loop. While
    -    sometimes it will finally succeed because it cannot fork subprocess,
    -    it has exhausted the local process resources and can be harmful to the
    -    remote service.
    +    However, it uses a wrong helper, repo_has_object_file(), to do so.
    +    This helper not only checks if an object is mmediately available in
    +    the local object store, but also tries to fetch from a promisor remote.
    +    But the fetch machinery calls lookup_commit_in_graph(), thus causing an
    +    infinite loop.
    +
    +    We should make lookup_commit_in_graph() expect that a commit given to it
    +    can be legitimately missing from the local object store, by using the
    +    has_object_file() helper instead.
     
         Signed-off-by: Han Xin <hanxin.hx@bytedance.com>
     
    @@ t/t5329-no-lazy-fetch-with-commit-graph.sh (new)
     +
     +. ./test-lib.sh
     +
    ++if ! test_have_prereq ULIMIT_PROCESSES
    ++then
    ++	skip_all='skipping tests for no lazy fetch with the commit-graph, ulimit processes not available'
    ++	test_done
    ++fi
    ++
     +test_expect_success 'setup: prepare a repository with a commit' '
     +	git init with-commit &&
     +	test_commit -C with-commit the-commit &&
    @@ t/t5329-no-lazy-fetch-with-commit-graph.sh (new)
     +	# create a ref that points to the commit in alternates
     +	git -C with-commit-graph update-ref refs/ref_to_the_commit "$oid" &&
     +	# prepare some other objects to commit-graph
    -+	test_commit -C with-commit-graph somthing &&
    ++	test_commit -C with-commit-graph something &&
     +	git -c gc.writeCommitGraph=true -C with-commit-graph gc &&
     +	test_path_is_file with-commit-graph/.git/objects/info/commit-graph
     +'
    @@ t/t5329-no-lazy-fetch-with-commit-graph.sh (new)
     +	test_must_fail git -C with-commit-graph cat-file -e $oid
     +'
     +
    -+test_expect_success 'setup: prepare another commit to fetch' '
    -+	test_commit -C with-commit another-commit &&
    ++test_expect_success 'setup: prepare any commit to fetch' '
    ++	test_commit -C with-commit any-commit &&
     +	anycommit=$(git -C with-commit rev-parse HEAD)
     +'
     +
    -+test_expect_success ULIMIT_PROCESSES 'fetch any commit from promisor with the usage of the commit graph' '
    ++test_expect_success 'fetch any commit from promisor with the usage of the commit graph' '
     +	git -C with-commit-graph remote add origin "$(pwd)/with-commit" &&
     +	git -C with-commit-graph config remote.origin.promisor true &&
     +	git -C with-commit-graph config remote.origin.partialclonefilter blob:none &&