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From: "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Derrick Stolee <dstolee@microsoft.com>,
	Junio C Hamano <gitster@pobox.com>,
	Derrick Stolee <dstolee@microsoft.com>
Subject: [PATCH 1/1] commit-graph: fix writing first commit-graph during fetch
Date: Tue, 22 Oct 2019 17:28:55 +0000	[thread overview]
Message-ID: <a1e5280d4b61a972426063574f1ea890a7dab73b.1571765336.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.415.git.1571765335.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

While dogfooding, Johannes found a bug in the fetch.writeCommitGraph
config behavior. While his example initially happened during a clone
with --recurse-submodules, we found that it is actually a problem with
the first fetch after a new clone and no existing commit-graph file:

	$ 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)

In the repo I had cloned, there were really 60 commits to scan, but
only 12 were in the list to write when calling
compute_generation_numbers(). A commit in the list expects to see a
parent, but that parent is not in the list.

As an initial test, 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
t5510-fetch.sh uses local filesystem clones, which somehow avoids this
logic.

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.

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.

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

I have failed to produce a test using the file:// protocol that
demonstrates this bug.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 commit-graph.c | 11 +++++++----
 commit-reach.c |  1 -
 object.h       |  3 ++-
 3 files changed, 9 insertions(+), 6 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 @@
 #define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
 			+ 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->oids.nr + 1, ctx->oids.alloc);
 			oidcpy(&ctx->oids.list[ctx->oids.nr], &(parent->item->object.oid));
 			ctx->oids.nr++;
-			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;
 	}
 	stop_progress(&ctx->progress);
 
@@ -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;
 	}
 	stop_progress(&ctx->progress);
 }
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
-- 
gitgitgadget

  reply	other threads:[~2019-10-22 17:29 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-22 17:28 [PATCH 0/1] [v2.24.0-rc0 BUG] fetch.writeCommitGraph fails on first fetch Derrick Stolee via GitGitGadget
2019-10-22 17:28 ` Derrick Stolee via GitGitGadget [this message]
2019-10-22 20:33   ` [PATCH 1/1] commit-graph: fix writing first commit-graph during fetch Jeff King
2019-10-22 21:45     ` Jeff King
2019-10-22 23:35       ` SZEDER Gábor
2019-10-23  0:35         ` Derrick Stolee
2019-10-23  0:48           ` Jeff King
2019-10-23  1:22             ` Jeff King
2019-10-23 13:01 ` [PATCH v2 0/2] [v2.24.0-rc0 BUG] fetch.writeCommitGraph fails on first fetch Derrick Stolee via GitGitGadget
2019-10-23 13:01   ` [PATCH v2 1/2] t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug Derrick Stolee via GitGitGadget
2019-10-23 14:18     ` SZEDER Gábor
2019-10-23 20:46       ` Derrick Stolee
2019-10-24 12:18     ` SZEDER Gábor
2019-10-23 13:01   ` [PATCH v2 2/2] commit-graph: fix writing first commit-graph during fetch Derrick Stolee via GitGitGadget
2019-10-23 15:04     ` SZEDER Gábor
2019-10-24 10:39       ` Derrick Stolee
2019-10-30 14:31         ` SZEDER Gábor
2019-10-24 12:18   ` [PATCH v3 0/2] [v2.24.0-rc0 BUG] fetch.writeCommitGraph fails on first fetch Derrick Stolee via GitGitGadget
2019-10-24 12:18     ` [PATCH v3 1/2] t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug Derrick Stolee via GitGitGadget
2019-10-24 12:18     ` [PATCH v3 2/2] commit-graph: fix writing first commit-graph during fetch Derrick Stolee via GitGitGadget
2019-10-24 13:40     ` [PATCH v4 0/2] [v2.24.0-rc0 BUG] fetch.writeCommitGraph fails on first fetch Derrick Stolee via GitGitGadget
2019-10-24 13:40       ` [PATCH v4 1/2] t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug Derrick Stolee via GitGitGadget
2019-10-24 13:40       ` [PATCH v4 2/2] commit-graph: fix writing first commit-graph during fetch Derrick Stolee via GitGitGadget

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