Reachability bitmaps do not work with shallow operations,
because they cache a view of the object reachability that
represents the true objects. Whereas a shallow repository
(or a shallow operation in a repository) is inherently
cutting off the object graph with a graft.
We explicitly disallow the use of bitmaps in shallow
repositories by checking is_repository_shallow(), and we
should continue to do that. However, we also want to
disallow bitmaps when we are serving a fetch to a shallow
client, since we momentarily take on their grafted view of
It used to be enough to call is_repository_shallow at the
start of pack-objects. Upload-pack wrote the other side's
shallow state to a temporary file and pointed the whole
pack-objects process at this state with "git --shallow-file",
and from the perspective of pack-objects, we really were
in a shallow repo. But since b790e0f (upload-pack: send
shallow info over stdin to pack-objects, 2014-03-11), we do
it differently: we send --shallow lines to pack-objects over
stdin, and it registers them itself.
This means that our is_repository_shallow check is way too
early (we have not been told about the shallowness yet), and
that it is insufficient (calling is_repository_shallow is
not enough, as the shallow grafts we register do not change
its return value). Instead, we can just turn off bitmaps
explicitly when we see these lines.
Signed-off-by: Jeff King <p...@peff.net>
Sorry not to catch this earlier. The bug is in v2.0, and I only noticed
the regression because a very small percentage of shallow fetches from
GitHub started failing after we deployed v2.0 a few weeks ago. It took
me a while to figure out the reproduction recipe below. :)
Arguably is_repository_shallow should return 1 if anybody has registered
a shallow graft, but that wouldn't be enough to fix this (we'd still
need to check it again _after_ reading the --shallow lines). So I think
this fix is fine here. I don't know if any other parts of the code would
builtin/pack-objects.c | 1 +
t/t5311-pack-bitmaps-shallow.sh | 39 +++++++++++++++++++++++++++++++++++++++
2 files changed, 40 insertions(+)
create mode 100755 t/t5311-pack-bitmaps-shallow.sh
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 238b502..b59f5d8 100644
@@ -2494,6 +2494,7 @@ static void get_object_list(int ac, const char **av)
if (get_sha1_hex(line + 10, sha1))
die("not an SHA-1 '%s'", line + 10);
+ use_bitmap_index = 0;
die("not a rev '%s'", line);
diff --git a/t/t5311-pack-bitmaps-shallow.sh b/t/t5311-pack-bitmaps-shallow.sh
new file mode 100755
@@ -0,0 +1,39 @@
+test_description='check bitmap operation with shallow repositories'
+# We want to create a situation where the shallow, grafted
+# view of reachability does not match reality in a way that
+# might cause us to send insufficient objects.
+# We do this with a history that repeats a state, like:
+# A -- B -- C
+# file=1 file=2 file=1
+# and then create a shallow clone to the second commit, B.
+# In a non-shallow clone, that would mean we already have
+# the tree for A. But in a shallow one, we've grafted away
+# A, and fetching A to B requires that the other side send
+# us the tree for file=1.
+test_expect_success 'setup shallow repo' '
+ echo 1 >file &&
+ git add file &&
+ git commit -m orig &&
+ echo 2 >file &&
+ git commit -a -m update &&
+ git clone --no-local --bare --depth=1 . shallow.git &&
+ echo 1 >file &&
+ git commit -a -m repeat
+test_expect_success 'turn on bitmaps in the parent' '
+ git repack -adb
+test_expect_success 'shallow fetch from bitmapped repo' '
+ (cd shallow.git && git fetch)
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