When we receive a pack and the shallow points from another repository,
we may want to add more shallow points to current repo to make sure no
commits point to nowhere. However we do not want to add unnecessary
shallow points and cut our history short because the other end is a
shallow version of this repo. The output shallow points may or may not
be added to .git/shallow, depending on whether they are actually
reachable in the new pack.

This function filters such shallow points out, leaving ones that might
potentially be added. A simple has_sha1_file won't do because we may
have incomplete object islands (i.e. not connected to any refs) and
the shallow points are on these islands. In that case we want to keep
the shallow points as candidates until we figure out if the new pack
connects to such object islands.

Typical cases that use remove_reachable_shallow_points() are:

 - fetch from a repo that has longer history: in this case all remote
   shallow roots do not exist in the client repo, this function will
   be cheap as it just does a few lookup_commit_graft and

 - fetch from a repo that has exactly the same shallow root set
   (e.g. a clone from a shallow repo): this case may trigger
   in_merge_bases_many all the way to roots. An exception is made to
   avoid such costly path with a faith that .git/shallow does not
   usually points to unreachable commit islands.

 - push from a shallow repo that has shorter history than the
   server's: in_merge_bases_many() is unavoidable, so the longer
   history the client has the higher the server cost is. The cost may
   be reduced with the help of pack bitmaps, or commit cache, though.

 - push from a shallow clone that has exactly the same shallow root
   set: the same as the second fetch case above, .i.e. cheap by

The function must be called before the new pack is installed, or we
won't know which objects are ours, which theirs.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclo...@gmail.com>
 commit.h  |  3 +++
 connect.c |  2 +-
 remote.h  |  1 +
 shallow.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/commit.h b/commit.h
index a879526..98044e6 100644
--- a/commit.h
+++ b/commit.h
@@ -193,6 +193,7 @@ extern struct commit_list *get_octopus_merge_bases(struct 
commit_list *in);
 /* largest positive number a signed 32-bit integer can contain */
 #define INFINITE_DEPTH 0x7fffffff
+struct extra_have_objects;
 extern int register_shallow(const unsigned char *sha1);
 extern int unregister_shallow(const unsigned char *sha1);
 extern int for_each_commit_graft(each_commit_graft_fn, void *);
@@ -206,6 +207,8 @@ extern void setup_alternate_shallow(struct lock_file 
                                    const char **alternate_shallow_file);
 extern char *setup_temporary_shallow(void);
 extern void advertise_shallow_grafts(int);
+extern void remove_reachable_shallow_points(struct extra_have_objects *out,
+                                           const struct extra_have_objects 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/connect.c b/connect.c
index d0602b0..80e4360 100644
--- a/connect.c
+++ b/connect.c
@@ -45,7 +45,7 @@ int check_ref_type(const struct ref *ref, int flags)
        return check_ref(ref->name, strlen(ref->name), flags);
-static void add_extra_have(struct extra_have_objects *extra, unsigned char 
+void add_extra_have(struct extra_have_objects *extra, unsigned char *sha1)
        ALLOC_GROW(extra->array, extra->nr + 1, extra->alloc);
        hashcpy(&(extra->array[extra->nr][0]), sha1);
diff --git a/remote.h b/remote.h
index 773faa9..ff604ff 100644
--- a/remote.h
+++ b/remote.h
@@ -145,6 +145,7 @@ extern struct ref **get_remote_heads(int in, char *src_buf, 
size_t src_len,
                                     struct ref **list, unsigned int flags,
                                     struct extra_have_objects *have,
                                     struct extra_have_objects *shallow);
+extern void add_extra_have(struct extra_have_objects *extra, unsigned char 
 int resolve_remote_symref(struct ref *ref, struct ref *list);
 int ref_newer(const unsigned char *new_sha1, const unsigned char *old_sha1);
diff --git a/shallow.c b/shallow.c
index 9552512..a974d2d 100644
--- a/shallow.c
+++ b/shallow.c
@@ -2,6 +2,8 @@
 #include "commit.h"
 #include "tag.h"
 #include "pkt-line.h"
+#include "remote.h"
+#include "refs.h"
 static int is_shallow = -1;
 static struct stat shallow_stat;
@@ -235,3 +237,46 @@ void advertise_shallow_grafts(int fd)
        for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
+struct commit_array {
+       struct commit **commits;
+       int nr, alloc;
+static int add_ref(const char *refname,
+                  const unsigned char *sha1, int flags, void *cb_data)
+       struct commit_array *ca = cb_data;
+       ALLOC_GROW(ca->commits, ca->nr + 1, ca->alloc);
+       ca->commits[ca->nr++] = lookup_commit(sha1);
+       return 0;
+void remove_reachable_shallow_points(struct extra_have_objects *out,
+                                    const struct extra_have_objects *in)
+       struct commit_array ca;
+       int i;
+       memset(&ca, 0, sizeof(ca));
+       head_ref(add_ref, &ca);
+       for_each_ref(add_ref, &ca);
+       for (i = 0; i < in->nr; i++) {
+               struct commit_graft *graft = lookup_commit_graft(in->array[i]);
+               /*
+                * For a clone from a shallow upstream, the clone has
+                * the same shallow roots as upstream and it will
+                * trigger in_merge_bases_many() all the way to roots.
+                * Avoid that costly path and assume .git/shallow is
+                * good most of the time.
+                */
+               if (graft && graft->nr_parent < 0)
+                       continue;
+               if (has_sha1_file(in->array[i]) &&
+                   in_merge_bases_many(lookup_commit(in->array[i]),
+                                       ca.nr, ca.commits))
+                       continue;
+               add_extra_have(out, in->array[i]);
+       }
+       free(ca.commits);

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