From: Derrick Stolee <>

The previous commit includes a failing test for an issue around
fetch.writeCommitGraph and fetching in a repo with a submodule. Here, we
fix that bug and set the test to "test_expect_success".

The prolem arises with this set of commands when the remote repo at
<url> has a submodule. Note that --recurse-submodules is not needed to
demonstrate the bug.

        $ git clone <url> test
        $ cd test
        $ git -c fetch.writeCommitGraph=true fetch origin
        Computing commit graph generation numbers: 100% (12/12), done.
        BUG: commit-graph.c:886: missing parent <hash1> for commit <hash2>
        Aborted (core dumped)

As an initial fix, I converted the code in builtin/fetch.c that calls
write_commit_graph_reachable() to instead launch a "git commit-graph
write --reachable --split" process. That code worked, but is not how we
want the feature to work long-term.

That test did demonstrate that the issue must be something to do with
internal state of the 'git fetch' process.

The write_commit_graph() method in commit-graph.c ensures the commits we
plan to write are "closed under reachability" using close_reachable().
This method walks from the input commits, and uses the UNINTERESTING
flag to mark which commits have already been visited. This allows the
walk to take O(N) time, where N is the number of commits, instead of
O(P) time, where P is the number of paths. (The number of paths can be
exponential in the number of commits.)

However, the UNINTERESTING flag is used in lots of places in the
codebase. This flag usually means some barrier to stop a commit walk,
such as in revision-walking to compare histories. It is not often
cleared after the walk completes because the starting points of those
walks do not have the UNINTERESTING flag, and clear_commit_marks() would
stop immediately.

This is happening during a 'git fetch' call with a remote. The fetch
negotiation is comparing the remote refs with the local refs and marking
some commits as UNINTERESTING.

You may ask: did this feature ever work at all? Yes, it did, as long as
you had a commit-graph covering all of your local refs. My testing was
unfortunately limited to this scenario. The UNINTERESTING commits are
always part of the "old" commit-graph, and when we add new commits to a
top layer of the commit-graph chain those are not needed. If we happen
to merge layers of the chain, then the commits are added as a list, not
using a commit walk. Further, the test added for this config option in uses local filesystem clones, which somehow avoids this

I tested running clear_commit_marks_many() to clear the UNINTERESTING
flag inside close_reachable(), but the tips did not have the flag, so
that did nothing.

It turns out that the calculate_changed_submodule_paths() method is at
fault. Thanks, Peff, for pointing out this detail! More specifically,
for each submodule, the collect_changed_submodules() runs a revision
walk to essentially do file-history on the list of submodules. That
revision walk marks commits UNININTERESTING if they are simiplified away
by not changing the submodule.

Instead, I finally arrived on the conclusion that I should use a flag
that is not used in any other part of the code. In commit-reach.c, a
number of flags were defined for commit walk algorithms. The REACHABLE
flag seemed like it made the most sense, and it seems it was not
actually used in the file. The REACHABLE flag was used in early versions
of commit-reach.c, but was removed by 4fbcca4 (commit-reach: make
can_all_from_reach... linear, 2018-07-20).

Add the REACHABLE flag to commit-graph.c and use it instead of
UNINTERESTING in close_reachable(). This fixes the bug in manual

Reported-by: Johannes Schindelin <>
Helped-by: Jeff King <>
Helped-by: Szeder Gábor <>
Signed-off-by: Derrick Stolee <>
 commit-graph.c   | 11 +++++++----
 commit-reach.c   |  1 -
 object.h         |  3 ++-
 t/ |  2 +-
 4 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index fc4a43b8d6..0aea7b2ae5 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -41,6 +41,9 @@
                        + GRAPH_FANOUT_SIZE + the_hash_algo->rawsz)
+/* Remember to update object flag allocation in object.h */
+#define REACHABLE       (1u<<15)
 char *get_commit_graph_filename(const char *obj_dir)
        char *filename = xstrfmt("%s/info/commit-graph", obj_dir);
@@ -1030,11 +1033,11 @@ static void add_missing_parents(struct 
write_commit_graph_context *ctx, struct c
        struct commit_list *parent;
        for (parent = commit->parents; parent; parent = parent->next) {
-               if (!(parent->item->object.flags & UNINTERESTING)) {
+               if (!(parent->item->object.flags & REACHABLE)) {
                        ALLOC_GROW(ctx->oids.list, ctx-> + 1, 
-                       parent->item->object.flags |= UNINTERESTING;
+                       parent->item->object.flags |= REACHABLE;
@@ -1052,7 +1055,7 @@ static void close_reachable(struct 
write_commit_graph_context *ctx)
                display_progress(ctx->progress, i + 1);
                commit = lookup_commit(ctx->r, &ctx->oids.list[i]);
                if (commit)
-                       commit->object.flags |= UNINTERESTING;
+                       commit->object.flags |= REACHABLE;
@@ -1089,7 +1092,7 @@ static void close_reachable(struct 
write_commit_graph_context *ctx)
                commit = lookup_commit(ctx->r, &ctx->oids.list[i]);
                if (commit)
-                       commit->object.flags &= ~UNINTERESTING;
+                       commit->object.flags &= ~REACHABLE;
diff --git a/commit-reach.c b/commit-reach.c
index 3ea174788a..4ca7e706a1 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -10,7 +10,6 @@
 #include "commit-reach.h"
 /* Remember to update object flag allocation in object.h */
-#define REACHABLE       (1u<<15)
 #define PARENT1                (1u<<16)
 #define PARENT2                (1u<<17)
 #define STALE          (1u<<18)
diff --git a/object.h b/object.h
index 0120892bbd..25f5ab3d54 100644
--- a/object.h
+++ b/object.h
@@ -68,7 +68,8 @@ struct object_array {
  * bisect.c:                                        16
  * bundle.c:                                        16
  * http-push.c:                                     16-----19
- * commit-reach.c:                                15-------19
+ * commit-graph.c:                                15
+ * commit-reach.c:                                  16-----19
  * sha1-name.c:                                              20
  * list-objects-filter.c:                                      21
  * builtin/fsck.c:           0--3
diff --git a/t/ b/t/
index e8ae3af0b6..7bfbcc2036 100755
--- a/t/
+++ b/t/
@@ -583,7 +583,7 @@ test_expect_success 'fetch.writeCommitGraph' '
-test_expect_failure 'fetch.writeCommitGraph with submodules' '
+test_expect_success 'fetch.writeCommitGraph with submodules' '
        pwd="$(pwd)" &&
        git clone dups super &&

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