rev-list: add commit object support in `--missing` option

The `--missing` object option in rev-list currently works only with
missing blobs/trees. For missing commits the revision walker fails with
a fatal error.

Let's extend the functionality of `--missing` option to also support
commit objects. This is done by adding a `missing_objects` field to
`rev_info`. This field is an `oidset` to which we'll add the missing
commits as we encounter them. The revision walker will now continue the
traversal and call `show_commit()` even for missing commits. In rev-list
we can then check if the commit is a missing commit and call the
existing code for parsing `--missing` objects.

A scenario where this option would be used is to find the boundary
objects between different object directories. Consider a repository with
a main object directory (GIT_OBJECT_DIRECTORY) and one or more alternate
object directories (GIT_ALTERNATE_OBJECT_DIRECTORIES). In such a
repository, using the `--missing=print` option while disabling the
alternate object directory allows us to find the boundary objects
between the main and alternate object directory.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/revision.h b/revision.h
index c73c92ef40..94c43138bc 100644
--- a/revision.h
+++ b/revision.h
@@ -4,6 +4,7 @@
 #include "commit.h"
 #include "grep.h"
 #include "notes.h"
+#include "oidset.h"
 #include "pretty.h"
 #include "diff.h"
 #include "commit-slab-decl.h"
@@ -373,6 +374,9 @@ struct rev_info {
 
 	/* Location where temporary objects for remerge-diff are written. */
 	struct tmp_objdir *remerge_objdir;
+
+	/* Missing commits to be tracked without failing traversal. */
+	struct oidset missing_commits;
 };
 
 /**