[PATCH v4 0/3] rev-list: add support for commits in `--missing`

The `--missing` option in git-rev-list(1) was introduced intitally to deal with missing blobs in the context of promissory notes. Eventually the option was extended to also support tree objects in 7c0fe330d5 (rev-list: handle missing tree objects properly,2018-10-05).

This patch series extends the `--missing` option to also add support for commit objects. We do this by introducing a new flag `MISSING` which is added whenever we encounter a missing commit during traversal. Then in `builtin/rev-list` we check for this flag and take the appropriate action based on the `--missing=*` option used.

This series is an alternate to the patch series I had posted earlier: https://lore.kernel.org/git/20230908174208.249184-1-karthik.188@gmail.com/. In that patch, we introduced an option `--ignore-missing-links` which was added to expose the `ignore_missing_links` bit to the user. The issue in that patch was that, the option `--ignore-missing-links` didn't play well the pre-existing `--missing` option. This series avoids that route and just extends the `--missing` option for commits to solve the same problem.

V3 of the series can be found here: https://lore.kernel.org/git/20231019121024.194317-1-karthik.188@gmail.com/T/#mf6a442e06f323a78a45af086ddd353998bab0052

Changes from v3:
- Instead of using flags on the object level as in the previous vesrions, we
add and use a missing_objects oidset in rev_info. This avoids flag bit-collision
since we were using an existing bit and doesn't require extending the flag size
as done in v3. 

Changelog vs v3:

1:  8c469cf479 = 1:  8c469cf479 revision: rename bit to `do_not_die_on_missing_objects`
2:  76ce43d973 = 2:  76ce43d973 rev-list: move `show_commit()` to the bottom
3:  4c640f9ab4 ! 3:  d892f0b82d rev-list: add commit object support in `--missing` option
    @@ Commit message
         a fatal error.
     
         Let's extend the functionality of `--missing` option to also support
    -    commit objects. This is done by adding a new `MISSING` flag that the
    -    revision walker sets whenever it encounters a missing tree/commit. The
    -    revision walker will now continue the traversal and call `show_commit()`
    -    even for missing commits. In rev-list we can then check for this flag
    -    and call the existing code for parsing `--missing` objects.
    +    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
    @@ Commit message
         alternate object directory allows us to find the boundary objects
         between the main and alternate object directory.
     
    +    Helped-by: Patrick Steinhardt <ps@pks.im>
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
      ## builtin/rev-list.c ##
    @@ builtin/rev-list.c: static void show_commit(struct commit *commit, void *data)
      	display_progress(progress, ++progress_counter);
      
     +	if (revs->do_not_die_on_missing_objects &&
    -+	    commit->object.flags & MISSING) {
    ++	    oidset_contains(&revs->missing_objects, &commit->object.oid)) {
     +		finish_object__ma(&commit->object);
     +		return;
     +	}
    @@ builtin/rev-list.c: static void show_commit(struct commit *commit, void *data)
     
      ## list-objects.c ##
     @@ list-objects.c: static void do_traverse(struct traversal_context *ctx)
    - 		 * an uninteresting boundary commit may not have its tree
    - 		 * parsed yet, but we are not going to show them anyway
      		 */
    --		if (!ctx->revs->tree_objects)
    -+		if (!ctx->revs->tree_objects || commit->object.flags & MISSING)
    + 		if (!ctx->revs->tree_objects)
      			; /* do not bother loading tree */
    ++		else if (ctx->revs->do_not_die_on_missing_objects &&
    ++			 oidset_contains(&ctx->revs->missing_objects, &commit->object.oid))
    ++			;
      		else if (repo_get_commit_tree(the_repository, commit)) {
      			struct tree *tree = repo_get_commit_tree(the_repository,
    -
    - ## object.h ##
    -@@ object.h: void object_array_init(struct object_array *array);
    - 
    - /*
    -  * object flag allocation:
    -- * revision.h:               0---------10         15             23------27
    -+ * revision.h:               0---------10         15             22------28
    -  * fetch-pack.c:             01    67
    -  * negotiator/default.c:       2--5
    -  * walker.c:                 0-2
    -@@ object.h: void object_array_init(struct object_array *array);
    -  * builtin/show-branch.c:    0-------------------------------------------26
    -  * builtin/unpack-objects.c:                                 2021
    -  */
    --#define FLAG_BITS  28
    -+#define FLAG_BITS  29
    - 
    - #define TYPE_BITS 3
    - 
    + 								 commit);
     
      ## revision.c ##
    +@@
    + #include "object-name.h"
    + #include "object-file.h"
    + #include "object-store-ll.h"
    ++#include "oidset.h"
    + #include "tag.h"
    + #include "blob.h"
    + #include "tree.h"
     @@ revision.c: static int process_parents(struct rev_info *revs, struct commit *commit,
    - 	struct commit_list *parent = commit->parents;
    - 	unsigned pass_flags;
      
    --	if (commit->object.flags & ADDED)
    -+	if (commit->object.flags & (ADDED | MISSING))
    + 	if (commit->object.flags & ADDED)
      		return 0;
    ++	if (revs->do_not_die_on_missing_objects &&
    ++		oidset_contains(&revs->missing_objects, &commit->object.oid))
    ++		return 0;
      	commit->object.flags |= ADDED;
      
    + 	if (revs->include_check &&
     @@ revision.c: static int process_parents(struct rev_info *revs, struct commit *commit,
      	for (parent = commit->parents; parent; parent = parent->next) {
      		struct commit *p = parent->item;
    @@ revision.c: static int process_parents(struct rev_info *revs, struct commit *com
     +			if (!revs->do_not_die_on_missing_objects)
     +				return -1;
     +			else
    -+				p->object.flags |= MISSING;
    ++				oidset_insert(&revs->missing_objects, &p->object.oid);
      		}
      		if (revs->sources) {
      			char **slot = revision_sources_at(revs->sources, p);
    +@@ revision.c: int prepare_revision_walk(struct rev_info *revs)
    + 				       FOR_EACH_OBJECT_PROMISOR_ONLY);
    + 	}
    + 
    ++	if (revs->do_not_die_on_missing_objects)
    ++		oidset_init(&revs->missing_objects, 0);
    ++
    + 	if (!revs->reflog_info)
    + 		prepare_to_use_bloom_filter(revs);
    + 	if (!revs->unsorted_input)
     
      ## revision.h ##
     @@
    - #define ANCESTRY_PATH	(1u<<27)
    - #define ALL_REV_FLAGS	(((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR | PULL_MERGE)
    + #include "commit.h"
    + #include "grep.h"
    + #include "notes.h"
    ++#include "oidset.h"
    + #include "pretty.h"
    + #include "diff.h"
    + #include "commit-slab-decl.h"
    +@@ revision.h: struct rev_info {
      
    -+#define MISSING (1u<<28)
    + 	/* Location where temporary objects for remerge-diff are written. */
    + 	struct tmp_objdir *remerge_objdir;
     +
    - #define DECORATE_SHORT_REFS	1
    - #define DECORATE_FULL_REFS	2
    ++	/* Missing objects to be tracked without failing traversal. */
    ++	struct oidset missing_objects;
    + };
      
    + /**
     
      ## t/t6022-rev-list-missing.sh (new) ##
     @@

Karthik Nayak (3):
  revision: rename bit to `do_not_die_on_missing_objects`
  rev-list: move `show_commit()` to the bottom
  rev-list: add commit object support in `--missing` option

 builtin/reflog.c            |  2 +-
 builtin/rev-list.c          | 93 +++++++++++++++++++------------------
 list-objects.c              |  5 +-
 revision.c                  | 16 ++++++-
 revision.h                  | 21 +++++----
 t/t6022-rev-list-missing.sh | 74 +++++++++++++++++++++++++++++
 6 files changed, 155 insertions(+), 56 deletions(-)
 create mode 100755 t/t6022-rev-list-missing.sh