Re: [PATCH v4 08/10] commit-graph: use generation v2 only if entire chain does

From: Abhishek Kumar <abhishekkumar8222@gmail.com>
Date: Nov 12, 2020, 10:01 UTC
Message-ID: <20201112100133.GA2691@Abhishek-Arch>
In-Reply-To: <85zh41sxow.fsf@gmail.com>

On Sun, Nov 01, 2020 at 01:55:11AM +0100, Jakub Narębski wrote:

Show 37 quoted lines

> Hi Abhishek,
> 
> "Abhishek Kumar via GitGitGadget" <gitgitgadget@gmail.com> writes:
> > From: Abhishek Kumar <abhishekkumar8222@gmail.com>
> >
> > Since there are released versions of Git that understand generation
> > numbers in the commit-graph's CDAT chunk but do not understand the GDAT
> > chunk, the following scenario is possible:
> >
> > 1. "New" Git writes a commit-graph with the GDAT chunk.
> > 2. "Old" Git writes a split commit-graph on top without a GDAT chunk.
> 
> All right.
> 
> >
> > Because of the current use of inspecting the current layer for a
> > chunk_generation_data pointer, the commits in the lower layer will be
> > interpreted as having very large generation values (commit date plus
> > offset) compared to the generation numbers in the top layer (topological
> > level). This violates the expectation that the generation of a parent is
> > strictly smaller than the generation of a child.
> 
> I think this paragraphs tries too much to be concise, with the result it
> is less clear than it could be.  Perhaps it would be better to separate
> "what-if" from the current behavior.
> 
>   If each layer of split commit-graph is treated independently, as it
>   were the case before this commit, with Git inspecting only the current
>   layer for chunk_generation_data pointer, commits in the lower layer
>   (one with GDAT) would have corrected commit date as their generation
>   number, while commits in the upper layer would have topological levels
>   as their generation.  Corrected commit dates have usually much larger
>   values than topological levels.  This means that if we take two
>   commits, one from the upper layer, and one reachable from it in the
>   lower layer, then the expectation that the generation of a parent is
>   smaller than the generation of a child would be violated.
>

Thanks, that's better.

Show 59 quoted lines

> >
> > It is difficult to expose this issue in a test. Since we _start_ with
> > artificially low generation numbers, any commit walk that prioritizes
> > generation numbers will walk all of the commits with high generation
> > number before walking the commits with low generation number. In all the
> > cases I tried, the commit-graph layers themselves "protect" any
> > incorrect behavior since none of the commits in the lower layer can
> > reach the commits in the upper layer.
> 
> I don't quite understand the issue here. Unless none of the following
> query commands short-circuit and all walk the commit graph regardless of
> what generation numbers tell them, they should give different results
> with and without the commit graph, if we take two commits one from lower
> layer of split commit graph with GDAT, and one commit from the higher
> layer without GDAT, one lower reachable from the other higher.
> 
> We have the following query commands that we can check:
>   $ git merge-base --is-ancestor <lower> <higher>
>   $ git merge-base --independent <lower> <higher>
>   
>   $ git tag --contains <tag-to-lower>
>   $ git tag --merged <tag-to-higher>
>   $ git branch --contains <branch-to-lower>
>   $ git branch --merged <branch-to-higher>
> 
> The second set of queries require for those commits to be tagged, or
> have branch pointing at them, respectively.
> 
> Also, shouldn't `git commit-graph verify` fail with split commit graph
> where the top layer is created with GIT_TEST_COMMIT_GRAPH_NO_GDAT=1?
> 
> Let's assume that we have the following history, with newer commits
> shown on top like in `git log --graph --oneline --all`:
> 
>           topological     corrected         generation
>           level           commit date       number^*
> 
>       d    3                                3
>       |
>    c  |    3                                3
>    |  |                                                 without GDAT
>  ..|..|.....[layer.boundary]........................................
>    |  |                                                    with GDAT
>    |  b    2              1112912113        1112912113
>    |  |
>    a  |    2              1112912053        1112912053
>    | /
>    |/
>    r       1              1112911993        1112911993
> 
> *) each layer inspected individually.
> 
> With such history, we can for example reach 'a' from 'c', thus
> `git merge-base --is-ancestor a b` should return true value, but
> without this commit gen(a) > gen(c), instead of gen(a) <= gen(c);
> I use here weaker reachability condition, but the one that works
> also for commits outside the commit-graph (and those for which
> generation numbers overflows).
>

The original explanation was given by Dr. Stolee and he might not have thought exhaustively about the issue.

In any case, your explanation and the history make sense to me. I will try to add test and report back to the mailing list if something goes wrong.

Thank you for clarifying in such detail.

Show 47 quoted lines

> >
> > This issue would manifest itself as a performance problem in this case,
> > especially with something like "git log --graph" since the low
> > generation numbers would cause the in-degree queue to walk all of the
> > commits in the lower layer before allowing the topo-order queue to write
> > anything to output (depending on the size of the upper layer).
> 
> All right, that's good explanation.
> 
> ...
>
> > @@ -2030,6 +2047,9 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
> >  		}
> >  	}
> >  
> > +	if (!ctx->write_generation_data && g->chunk_generation_data)
> > +		ctx->write_generation_data = 1;
> > +
> 
> This needs more careful examination, and looking at larger context of
> those lines.
> 
> At this point, unless `--split=replace` option is used, 'g' points to
> the bottom layer out of all topmost layers being merged. We know that if
> there are GDAT-less layers then these must be top layers, so this means
> that we can write GDAT chunk in the result of the merge -- because we
> would be replacing all possible GDAT-less layers (and maybe some with
> GDAT) with a single layer with the GDAT chunk.
> 
> The ctx->write_generation_data is set to true unless environment
> variable GIT_TEST_COMMIT_GRAPH_NO_GDAT is true, and that in
> write_commit_graph() it would be set to false if topmost layer doesn't
> have GDAT chunk, and to true if `--split=replace` option is used; see
> below.
> 
> Looks good to me.
> 
> 
> NOTE that this means that GIT_TEST_COMMIT_GRAPH_NO_GDAT prevents from
> writing GDAT chunk with generation data v2 unless we are merging layers,
> or replacing all of them with a single layer: then it is _ignored_.
> 
> Should we clarify this fact in the description of GIT_TEST_COMMIT_GRAPH_NO_GDAT
> in t/README?  Currently it reads:
> 
>   GIT_TEST_COMMIT_GRAPH_NO_GDAT=<boolean>, when true, forces the
>   commit-graph to be written without generation data chunk.

I think it's better to *not* write generation data chunk if GIT_TEST_COMMIT_GRAPH_NO_GDAT is set even though all GDAT-less layers are merged, that is:

  if (!ctx->write_generation_data &&
      g->chunk_generation_data &&
     !git_env_bool(GIT_TEST_COMMIT_GRAPH_NO_GDAT, 0))
    ctx->write_generation_data = 1;

With this change, we would have a method to force-write commit-graph without generation data chunk regardless of the shape of split commit-graph files.

Show 22 quoted lines

> 
> ...
>
> > diff --git a/commit-graph.h b/commit-graph.h
> > index 19a02001fd..ad52130883 100644
> > --- a/commit-graph.h
> > +++ b/commit-graph.h
> > @@ -64,6 +64,7 @@ struct commit_graph {
> >  	struct object_directory *odb;
> >  
> >  	uint32_t num_commits_in_base;
> > +	unsigned int read_generation_data;
> >  	struct commit_graph *base_graph;
> 
> All right, this new field is here to propagate to each layer the
> information whether we can read from the generation number v2 data
> chunk.
> 
> Though I am not sure whether this field should be added here, and
> whether it should be `unsigned int` (we don't have to be that careful
> about saving space for this type).
>

I cannot think of a more appropriate struct than `struct commit_graph`. Any particular suggestions?

Show 24 quoted lines

> > 
> >  	const uint32_t *chunk_oid_fanout;
> > diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
> > index 651df89ab2..d0949a9eb8 100755
> > --- a/t/t5324-split-commit-graph.sh
> > +++ b/t/t5324-split-commit-graph.sh
> > @@ -440,4 +440,90 @@ test_expect_success '--split=replace with partial Bloom data' '
> >  	verify_chain_files_exist $graphdir
> >  '
> >  
> > +test_expect_success 'setup repo for mixed generation commit-graph-chain' '
> > +	mkdir mixed &&
> 
> This should probably go just before cd-ing into just created
> subdirectory.
> 
> > +	graphdir=".git/objects/info/commit-graphs" &&
> > +	test_oid_cache <<-EOM &&
> > +	oid_version sha1:1
> > +	oid_version sha256:2
> > +	EOM
> 
> Minor nitpick: Why use "EOM", which is used only twice in Git the test
> suite, and not the conventional "EOF" (used at least 4000 times)?

Right, both instances of "EOM" are actually my own. I looked up some test script for oid cache that did use EOM when I first wrote the tests but it's changed now. Will replace.

Show 12 quoted lines

> 
> > +	cd "$TRASH_DIRECTORY/mixed" &&
> 
> The t/README says:
> 
>    - Don't chdir around in tests.  It is not sufficient to chdir to
>      somewhere and then chdir back to the original location later in
>      the test, as any intermediate step can fail and abort the test,
>      causing the next test to start in an unexpected directory.  Do so
>      inside a subshell if necessary.
> 
> Though I am not sure if it should apply also to this situation.

While I cannot avoid changing directory, using a subshell would be best to avoid causing the later tests to start in unexpected directories.

Show 28 quoted lines

> 
> > +	git init &&
> > +	git config core.commitGraph true &&
> > +	git config gc.writeCommitGraph false &&
> 
> All right.
> 
> > +	for i in $(test_seq 3)
> > +	do
> > +		test_commit $i &&
> > +		git branch commits/$i || return 1
> > +	done &&
> > +	git reset --hard commits/1 &&
> > +	for i in $(test_seq 4 5)
> > +	do
> > +		test_commit $i &&
> > +		git branch commits/$i || return 1
> > +	done &&
> > +	git reset --hard commits/2 &&
> > +	for i in $(test_seq 6 10)
> > +	do
> > +		test_commit $i &&
> > +		git branch commits/$i || return 1
> > +	done &&
> > +	git commit-graph write --reachable --split &&
> 
> Is there a reason why we do not check just written commit-graph file
> with `test-tool read-graph >output-layer-1`?

We could check the written commit-graph file at this point but it's same as existing tests as above.

Show 6 quoted lines

> 
> > +	git reset --hard commits/2 &&
> > +	git merge commits/4 &&
> 
> Shouldn't we use `test_merge` instead of `git merge`; I am not sure when
> to use one or the other?

`test_merge` is used in 26 places whereas `git merge` is used in over a thousand places. `test_merge` is just not widely adopted and this lack of adoption prevents further use.

Show 46 quoted lines

> 
> > +	git branch merge/1 &&
> > +	git reset --hard commits/4 &&
> > +	git merge commits/6 &&
> > +	git branch merge/2 &&
> 
> It would be nice to have ASCII-art of the history (of the graph of
> revisions) created here for subsequent tests:
> 
>                                         
>            /- 6 <-- 7 <-- 8 <-- 9 <-- 10*
>           /    \-\
>          /        \
>   1 <-- 2 <-- 3*   \--\
>   |      \             \ 
>   |       \-----\       \
>    \             \       \
>     \-- 4*<------ M/1     M/2
>         |\               /  
>         | \-- 5*        /
>         \              /
>          \------------/
> 
>   * - 1st layer  
> 
> Though I am not sure if what I have created is readable; I think a
> better way to draw this graph is possible, for example:
> 
>                /- 3*
>               /
>              /
>   1 <------ 2 <---- 6 <-- 7 <-- 8 <-- 9 <-- 10*
>    \         \       \
>     \         \       \
>      \         \       \
>       \- 4* <-- M/1     \    
>          |\              \
>          | \------------- M/2
>          \
>           \---- 5*
> 
> Edit: as I see the history gets even more complicated, so perhaps
> ASCII-art diagram of the history with layers marked would be too
> complicated, and wouldn't bring much.
> 
> Why do we need such shape of the history in the repository?

We don't need such a complicated shape. Any commit-graph file with 2-3 layers regardless of how commits are related should suffice. Will simplify.

Show 23 quoted lines

> 
> > +	GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable --split=no-merge &&
> > +	test-tool read-graph >output &&
> > +	cat >expect <<-EOF &&
> > +	header: 43475048 1 $(test_oid oid_version) 4 1
> > +	num_commits: 2
> > +	chunks: oid_fanout oid_lookup commit_metadata
> > +	EOF
> > +	test_cmp expect output &&
> 
> All right, we check that we have 2 commits, and that there is no GDAT
> chunk.
> 
> > +	git commit-graph verify
> 
> All right, we verify commit-graph as a whole (both layers).
> 
> > +'
> > +
> > +test_expect_success 'does not write generation data chunk if not present on existing tip' '
> 
> Hmmm... I wonder if we can come up with a better name for this test;
> for example should it be "does not write" or "do not write"?

That's better.

Show 56 quoted lines

> 
> > +	cd "$TRASH_DIRECTORY/mixed" &&
> > +	git reset --hard commits/3 &&
> > +	git merge merge/1 &&
> > +	git merge commits/5 &&
> > +	git merge merge/2 &&
> > +	git branch merge/3 &&
> 
> The commit graph gets complicated, so it would not be easy to visualize
> it with ASCII-art diagram without any crossed lines.  Maybe `git log
> --graph --oneline --all` would help:
> 
> *   (merge/3) Merge branch 'merge/2'
> |\
> | *   (merge/2) Merge branch 'commits/6'
> | |\
> * | \   Merge branch 'commits/5'
> |\ \ \
> | * | | (commits/5) 5
> | |/ /
> * | |   Merge branch 'merge/1'
> |\ \ \
> | * | | (merge/1) Merge branch 'commits/4'
> | |\| |
> | | * | (commits/4) 4
> * | | | (commits/3) 3
> |/ / /
> | | | * (commits/10) 10
> | | | * (commits/9) 9
> | | | * (commits/8) 8
> | | | * (commits/7) 7
> | | |/
> | | * (commits/6) 6
> | |/
> |/|
> * | (commits/2) 2
> |/
> * (commits/1) 1
> 
> 
> > +	git commit-graph write --reachable --split=no-merge &&
> > +	test-tool read-graph >output &&
> > +	cat >expect <<-EOF &&
> > +	header: 43475048 1 $(test_oid oid_version) 4 2
> > +	num_commits: 3
> > +	chunks: oid_fanout oid_lookup commit_metadata
> > +	EOF
> > +	test_cmp expect output &&
> > +	git commit-graph verify
> 
> All right, so here we check that we have layer without GDAT at the top,
> and we request not to merge layers thus new layer will be created, then
> the new layer also does not have GDAT chunk (and has 3 commits).
> 
> Minor nitpick: shouldn't those test be indented?
>

The tests look indented to me and `git diff HEAD^ --check` gives nothing.

Did you mean the lines enclosed by EOF delimiter?

Show 25 quoted lines

> > +'
> > +
> > +test_expect_success 'writes generation data chunk when commit-graph chain is replaced' '
> > +	cd "$TRASH_DIRECTORY/mixed" &&
> > +	git commit-graph write --reachable --split=replace &&
> > +	test_path_is_file $graphdir/commit-graph-chain &&
> > +	test_line_count = 1 $graphdir/commit-graph-chain &&
> > +	verify_chain_files_exist $graphdir &&
> 
> All right, this checks that we have split commit-graph chain that
> consist of a single layer, and that the commit-graph file for this
> single layer exists.
> 
> > +	graph_read_expect 15 &&
> 
> Shouldn't we use `test-tool read-graph` to check whether generation_data
> chunk is present... ah, sorry, I have realized that after previous
> patches `graph_read_expect 15` implicitly checks the latter, because in
> its' use of `test-tool read-graph` it does expect generation_data chunk.
> 
> So we use `test-tool read-graph` manually to check that generation_data
> chunk is absent, and we use graph_read_expect to check that it is
> present (and in both cases that the number of commits matches).  I
> wonder if it would be possible to simplify that...
>

The problem here is graph_read_expect() as defined in t5324-split-commit-graph takes two parameters - number of commits and number of base graphs. If the number of base graphs is not passed to the function call, it's assumed to be zero. Using a default parameter is tricky - I can fix it by manually adding a zero to each of graph_read_expect() in an additional preparatory patch.

Any other suggestions are welcome too.

Show 23 quoted lines

> 
> > +	git commit-graph verify
> 
> All right.
> 
> > +'
> > +
> > +test_expect_success 'add one commit, write a tip graph' '
> > +	cd "$TRASH_DIRECTORY/mixed" &&
> > +	test_commit 11 &&
> > +	git branch commits/11 &&
> > +	git commit-graph write --reachable --split &&
> > +	test_path_is_missing $infodir/commit-graph &&
> > +	test_path_is_file $graphdir/commit-graph-chain &&
> > +	ls $graphdir/graph-*.graph >graph-files &&
> > +	test_line_count = 2 graph-files &&
> > +	verify_chain_files_exist $graphdir
> > +'
> 
> What it is meant to test?  That adding single-commit to a 15 commit
> commit-graph file in split mode does not result in layers merging, and
> actually adds a new layer: we check that we have exactly two layers and
> that they are all OK.

This test is meant to check writing to a split graph in "normal" conditions (i.e. all existing layers have generation data chunk). The above tests are special cases as they involve merging layers with mixed generation number versions.

Show 14 quoted lines

> 
> We don't check here that the newly created top layer commit-graph does
> have GDAT chunk, as it should be if the top layer (in this case the only
> layer) has GDAT chunk.
> > +
> >  test_done
> 
> One test we are missing is testing that merging layers is done
> correctly, namely that if we are merging layers in split commit-graph
> file, and the layer below the ones we are merging lacks GDAT chunk, then
> the result of the merge should also be without GDAT chunk.  This would
> require at least two GDAT-less layers in a setup.
> 
> I'm not sure how difficult writing such test should be.

It wouldn't be too hard.

After the last test, I can write some more commits and write split commit-graph file without GDAT chunk. Then write some more commits and merge layers using `git commit-graph write --max-commits=<nr>`.

Thanks for pointing this out!

> 
> Best,
> -- 
> Jakub Narębski

Thanks
- Abhishek