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[v2,00/10] Change midx.c and midx-write.c to not use global variables

Message ID 20241119-374-refactor-midx-c-and-midx-write-c-to-not-depend-on-global-state-v2-0-e2f607174efc@gmail.com (mailing list archive)
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Series Change midx.c and midx-write.c to not use global variables | expand

Message

karthik nayak Nov. 19, 2024, 3:36 p.m. UTC
Similar to the earlier patch series on cleaning up packfile.c and
removing usage of global variables [1], we change the midx.c and
midx-write.c files to no longer use global variables.

This is done by the following:
  - Usage of repository variable already available in existing structs.
  - Passing down repository variable from other subsystems.
  - Modifying all subcommands to obtain repository variable from the
  command in `builtins/` and passing down the variable from there.

The biggest change is in the first commit, wherein we modify all
subcommands to add the repository variable. Since the subcommand
definition are not often changed, it shouldn't cause too many conflicts
with in flight topics.

Since the `packfile.c` cleanup is still in flight, this series is based
on top of master: b31fb630c0 (Merge https://github.com/j6t/git-gui,
2024-11-11) with those patches merged in. This applies cleanly on top 
of next, but conflicts with `bf/set-head-symref` in seen, the conflict
is mostly straight forward. I'll merge the topic in if it is merged into
next soon.

[1]: https://lore.kernel.org/git/cover.1729504640.git.karthik.188@gmail.com/

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
Changes in v2:
- Split commit modifying static functions in `midx-write.c` to multiple
commits, which makes it easier to review.
- Remove usage of `the_repository` in `test-parse-options.c` and use
NULL instead.
- Fix the commit messages to be imperative.
- Fix error in commit sign off.
- Link to v1: https://lore.kernel.org/r/20241115-374-refactor-midx-c-and-midx-write-c-to-not-depend-on-global-state-v1-0-761f8a2c7775@gmail.com

---
Karthik Nayak (10):
      builtin: pass repository to sub commands
      midx-write: pass down repository to static functions
      midx-write: use `revs->repo` inside `read_refs_snapshot`
      write-midx: add repository field to `write_midx_context`
      midx-write: pass down repository to `write_midx_file[_only]`
      midx: cleanup internal usage of `the_repository` and `the_hash_algo`
      midx: pass `repository` to `load_multi_pack_index`
      midx: pass down `hash_algo` to `get_midx_filename[_ext]`
      midx: pass down `hash_algo` to `get_split_midx_filename_ext`
      midx: inline the `MIDX_MIN_SIZE` definition

 builtin/bisect.c              |  32 +++++++----
 builtin/bundle.c              |  16 ++++--
 builtin/commit-graph.c        |  10 ++--
 builtin/config.c              |  25 +++++---
 builtin/gc.c                  |  21 ++++---
 builtin/hook.c                |   7 ++-
 builtin/multi-pack-index.c    |  20 ++++---
 builtin/notes.c               |  36 +++++++-----
 builtin/reflog.c              |  17 ++++--
 builtin/refs.c                |  10 ++--
 builtin/remote.c              |  34 +++++++----
 builtin/repack.c              |   2 +-
 builtin/sparse-checkout.c     |  25 +++++---
 builtin/stash.c               |  39 ++++++++-----
 builtin/submodule--helper.c   |  46 +++++++++------
 builtin/worktree.c            |  28 +++++----
 midx-write.c                  | 130 ++++++++++++++++++++++--------------------
 midx.c                        |  88 +++++++++++++++-------------
 midx.h                        |  24 ++++----
 pack-bitmap.c                 |   6 +-
 pack-revindex.c               |   2 +-
 parse-options.h               |   4 +-
 t/helper/test-parse-options.c |   8 ++-
 t/helper/test-read-midx.c     |   8 +--
 24 files changed, 378 insertions(+), 260 deletions(-)
---

Range-diff versus v1:

 1:  277db8a45a <  -:  ---------- packfile: add repository to struct `packed_git`
 2:  10ff177a40 <  -:  ---------- packfile: use `repository` from `packed_git` directly
 3:  c531a865d7 <  -:  ---------- packfile: pass `repository` to static function in the file
 4:  36cda6c7e9 <  -:  ---------- packfile: pass down repository to `odb_pack_name`
 5:  dbe2d93cbe <  -:  ---------- packfile: pass down repository to `has_object[_kept]_pack`
 6:  5b030a81c4 <  -:  ---------- packfile: pass down repository to `for_each_packed_object`
 7:  b4d6dda97c <  -:  ---------- config: make `delta_base_cache_limit` a non-global variable
 8:  bf13b948d4 <  -:  ---------- config: make `packed_git_(limit|window_size)` non-global variables
 9:  3ec55c737b <  -:  ---------- midx: add repository to `multi_pack_index` struct
10:  4df5b4fb30 !  1:  a423d7785c builtin: pass repository to sub commands
    @@ Commit message
         builtin: pass repository to sub commands
     
         In 9b1cb5070f (builtin: add a repository parameter for builtin
    -    functions, 2024-09-13) we passed down the repository to all builtin
    -    commands. This allowed us to pass down the repository to lower layers
    +    functions, 2024-09-13) the repository was passed down to all builtin
    +    commands. This allowed the repository to be passed down to lower layers
         without depending on the global `the_repository` variable.
     
    -    To continue this work, we also pass down the repository parameter from
    -    the command to sub-commands. In the upcoming commit, we'll utilize this
    -    to remove global variable usage in `midx-write.c`.
    +    Continue this work by also passing down the repository parameter from
    +    the command to sub-commands. In the upcoming commit, utilize this to
    +    remove global variable usage in `midx-write.c`.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
    @@ parse-options.h: typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_
       * `type`::
     
      ## t/helper/test-parse-options.c ##
    -@@
    -+#define USE_THE_REPOSITORY_VARIABLE
    -+
    - #include "test-tool.h"
    - #include "parse-options.h"
    - #include "strbuf.h"
    - #include "string-list.h"
    - #include "trace2.h"
    -+#include "repository.h"
    - 
    - static int boolean = 0;
    - static int integer = 0;
     @@ t/helper/test-parse-options.c: int cmd__parse_options_flags(int argc, const char **argv)
      	return parse_options_flags__cmd(argc, argv, test_flags);
      }
    @@ t/helper/test-parse-options.c: static int parse_subcommand__cmd(int argc, const
      	printf("opt: %d\n", opt);
      
     -	return fn(argc, argv, NULL);
    -+	return fn(argc, argv, NULL, the_repository);
    ++	return fn(argc, argv, NULL, NULL);
      }
      
      int cmd__parse_subcommand(int argc, const char **argv)
11:  3cfab22dc1 !  2:  3f39af60b4 midx-write: add repository field to `write_midx_context`
    @@ Metadata
     Author: Karthik Nayak <karthik.188@gmail.com>
     
      ## Commit message ##
    -    midx-write: add repository field to `write_midx_context`
    +    midx-write: pass down repository to static functions
     
    -    The struct `write_midx_context` is used to pass context for creating
    -    MIDX files. Adding the repository field here ensures that most functions
    -    within `midx-write.c` have access to the field and can use that instead
    -    of the global `repository` variable. This involves passing the
    -    `repository` field to `write_midx_internal`. To do this, we add
    -    `the_repository` usage to two non-static functions, which we'll remove
    -    in upcoming commits.
    +    In 'midx-write.c' there are a lot of static functions which use global
    +    variables `the_repository` or `the_hash_algo`. In a follow up commit,
    +    the repository variable will be added to `write_midx_context`, which
    +    some of the functions can use. But for functions which do not have
    +    access to this struct, pass down the required information from
    +    non-static functions `write_midx_file` and `write_midx_file_only`.
     
    -    With this, modify the static functions in `midx-write.c` to not use
    -    global variables. This means, either we use existing alternatives (like
    -    `repository` struct), or we pass down required fields from other
    -    functions.
    +    This ensures that the usage of global variables is limited to these
    +    non-static functions, which will be cleaned up in a follow up commits.
     
    -    Signed-off-by: default avatarKarthik Nayak <karthik.188@gmail.com>
    +    Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
      ## midx-write.c ##
     @@ midx-write.c: extern void clear_incremental_midx_files_ext(const char *object_dir,
    @@ midx-write.c: extern void clear_incremental_midx_files_ext(const char *object_di
      	hashwrite_u8(f, num_chunks);
      	hashwrite_u8(f, 0); /* unused */
      	hashwrite_be32(f, num_packs);
    -@@ midx-write.c: struct write_midx_context {
    - 	uint32_t num_multi_pack_indexes_before;
    - 
    - 	struct string_list *to_include;
    -+
    -+	struct repository *repo;
    - };
    - 
    - static int should_include_pack(const struct write_midx_context *ctx,
    -@@ midx-write.c: static void add_pack_to_midx(const char *full_path, size_t full_path_len,
    - 			return;
    - 
    - 		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
    --		p = add_packed_git(the_repository, full_path, full_path_len, 0);
    -+		p = add_packed_git(ctx->repo, full_path, full_path_len, 0);
    - 		if (!p) {
    - 			warning(_("failed to add packfile '%s'"),
    - 				full_path);
    -@@ midx-write.c: static int write_midx_oid_lookup(struct hashfile *f,
    - 				 void *data)
    - {
    - 	struct write_midx_context *ctx = data;
    --	unsigned char hash_len = the_hash_algo->rawsz;
    -+	unsigned char hash_len = ctx->repo->hash_algo->rawsz;
    - 	struct pack_midx_entry *list = ctx->entries;
    - 	uint32_t i;
    - 
    -@@ midx-write.c: static uint32_t *midx_pack_order(struct write_midx_context *ctx)
    - 	uint32_t *pack_order, base_objects = 0;
    - 	uint32_t i;
    - 
    --	trace2_region_enter("midx", "midx_pack_order", the_repository);
    -+	trace2_region_enter("midx", "midx_pack_order", ctx->repo);
    - 
    - 	if (ctx->incremental && ctx->base_midx)
    - 		base_objects = ctx->base_midx->num_objects +
    -@@ midx-write.c: static uint32_t *midx_pack_order(struct write_midx_context *ctx)
    - 	}
    - 	free(data);
    - 
    --	trace2_region_leave("midx", "midx_pack_order", the_repository);
    -+	trace2_region_leave("midx", "midx_pack_order", ctx->repo);
    - 
    - 	return pack_order;
    - }
    -@@ midx-write.c: static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
    - 	struct strbuf buf = STRBUF_INIT;
    - 	char *tmp_file;
    - 
    --	trace2_region_enter("midx", "write_midx_reverse_index", the_repository);
    -+	trace2_region_enter("midx", "write_midx_reverse_index", ctx->repo);
    - 
    --	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
    -+	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex_algop(midx_hash,
    -+								    ctx->repo->hash_algo));
    - 
    - 	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
    - 					midx_hash, WRITE_REV);
    -@@ midx-write.c: static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
    - 	strbuf_release(&buf);
    - 	free(tmp_file);
    - 
    --	trace2_region_leave("midx", "write_midx_reverse_index", the_repository);
    -+	trace2_region_leave("midx", "write_midx_reverse_index", ctx->repo);
    - }
    - 
    - static void prepare_midx_packing_data(struct packing_data *pdata,
    -@@ midx-write.c: static void prepare_midx_packing_data(struct packing_data *pdata,
    - {
    - 	uint32_t i;
    - 
    --	trace2_region_enter("midx", "prepare_midx_packing_data", the_repository);
    -+	trace2_region_enter("midx", "prepare_midx_packing_data", ctx->repo);
    - 
    - 	memset(pdata, 0, sizeof(struct packing_data));
    --	prepare_packing_data(the_repository, pdata);
    -+	prepare_packing_data(ctx->repo, pdata);
    - 
    - 	for (i = 0; i < ctx->entries_nr; i++) {
    - 		uint32_t pos = ctx->pack_order[i];
    -@@ midx-write.c: static void prepare_midx_packing_data(struct packing_data *pdata,
    - 			       ctx->info[ctx->pack_perm[from->pack_int_id]].p);
    - 	}
    - 
    --	trace2_region_leave("midx", "prepare_midx_packing_data", the_repository);
    -+	trace2_region_leave("midx", "prepare_midx_packing_data", ctx->repo);
    - }
    - 
    - static int add_ref_to_pending(const char *refname, const char *referent UNUSED,
     @@ midx-write.c: static int add_ref_to_pending(const char *refname, const char *referent UNUSED,
      		return 0;
      	}
    @@ midx-write.c: static int add_ref_to_pending(const char *refname, const char *ref
      		oid = &peeled;
      
      	object = parse_object_or_die(oid, refname);
    -@@ midx-write.c: static int read_refs_snapshot(const char *refs_snapshot,
    - 			hex = &buf.buf[1];
    - 		}
    - 
    --		if (parse_oid_hex(hex, &oid, &end) < 0)
    -+		if (parse_oid_hex_algop(hex, &oid, &end, revs->repo->hash_algo) < 0)
    - 			die(_("could not parse line: %s"), buf.buf);
    - 		if (*end)
    - 			die(_("malformed line: %s"), buf.buf);
    -@@ midx-write.c: static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr
    - 	struct rev_info revs;
    - 	struct bitmap_commit_cb cb = {0};
    - 
    --	trace2_region_enter("midx", "find_commits_for_midx_bitmap",
    --			    the_repository);
    -+	trace2_region_enter("midx", "find_commits_for_midx_bitmap", ctx->repo);
    - 
    - 	cb.ctx = ctx;
    - 
    --	repo_init_revisions(the_repository, &revs, NULL);
    -+	repo_init_revisions(ctx->repo, &revs, NULL);
    - 	if (refs_snapshot) {
    - 		read_refs_snapshot(refs_snapshot, &revs);
    - 	} else {
    - 		setup_revisions(0, NULL, &revs, NULL);
    --		refs_for_each_ref(get_main_ref_store(the_repository),
    -+		refs_for_each_ref(get_main_ref_store(ctx->repo),
    - 				  add_ref_to_pending, &revs);
    - 	}
    - 
     @@ midx-write.c: static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr
    - 
    - 	release_revisions(&revs);
    - 
    --	trace2_region_leave("midx", "find_commits_for_midx_bitmap",
    --			    the_repository);
    -+	trace2_region_leave("midx", "find_commits_for_midx_bitmap", ctx->repo);
    - 
      	return cb.commits;
      }
      
    @@ midx-write.c: static int write_midx_bitmap(const char *midx_name,
      
      	return ret;
      }
    -@@ midx-write.c: static int fill_packs_from_midx(struct write_midx_context *ctx,
    - 			 */
    - 			if (flags & MIDX_WRITE_REV_INDEX ||
    - 			    preferred_pack_name) {
    --				if (prepare_midx_pack(the_repository, m,
    -+				if (prepare_midx_pack(ctx->repo, m,
    - 						      m->num_packs_in_base + i)) {
    - 					error(_("could not load pack"));
    - 					return 1;
     @@ midx-write.c: static void clear_midx_files(const char *object_dir,
      	strbuf_release(&buf);
      }
    @@ midx-write.c: static int write_midx_internal(const char *object_dir,
     -	trace2_region_enter("midx", "write_midx_internal", the_repository);
     +	trace2_region_enter("midx", "write_midx_internal", r);
     +
    -+	ctx.repo = r;
      
      	ctx.incremental = !!(flags & MIDX_WRITE_INCREMENTAL);
      	if (ctx.incremental && (flags & MIDX_WRITE_BITMAP))
    @@ midx-write.c: static int write_midx_internal(const char *object_dir,
      	}
      
      	if (ctx.m || ctx.base_midx)
    --		close_object_store(the_repository->objects);
    -+		close_object_store(ctx.repo->objects);
    - 
    - 	if (commit_lock_file(&lk) < 0)
    - 		die_errno(_("could not write multi-pack-index"));
     @@ midx-write.c: static int write_midx_internal(const char *object_dir,
      	}
      	strbuf_release(&midx_name);
 -:  ---------- >  3:  774eea311a midx-write: use `revs->repo` inside `read_refs_snapshot`
 -:  ---------- >  4:  9ddfbdc0f8 write-midx: add repository field to `write_midx_context`
12:  5cdc724a0c !  5:  d25ea08541 midx-write: pass down repository to `write_midx_file[_only]`
    @@ Metadata
      ## Commit message ##
         midx-write: pass down repository to `write_midx_file[_only]`
     
    -    In the previous commit, we passed the repository field to all
    -    subcommands in the `builtin/` directory. We utilize this to pass the
    +    In a previous commit, we passed the repository field to all
    +    subcommands in the `builtin/` directory. Utilize this to pass the
         repository field down to the `write_midx_file[_only]` functions to
         remove the usage of `the_repository` global variables.
     
    -    With this, we remove all usage of global variables in `midx-write.c` and
    -    so we can remove the `USE_THE_REPOSITORY_VARIABLE` guard from the file.
    +    With this, all usage of global variables in `midx-write.c` is removed,
    +    hence, remove the `USE_THE_REPOSITORY_VARIABLE` guard from the file.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
13:  c6964becef !  6:  ca123e8c73 midx: cleanup internal usage of `the_repository` and `the_hash_algo`
    @@ Commit message
     
         In the `midx.c` file, there are multiple usages of `the_repository` and
         `the_hash_algo` within static functions of the file. Some of the usages
    -    can be simply swapped out with the available `repository` struct
    -    available. While some of them can be swapped out by passing the
    -    repository to the required functions.
    +    can be simply swapped out with the available `repository` struct. While
    +    some of them can be swapped out by passing the repository to the
    +    required functions.
     
         This leaves out only some other usages of `the_repository` and
         `the_hash_algo` in the file in non-static functions, which we'll tackle
14:  adcb6f0c6c !  7:  7e941aa0ba midx: pass `repository` to `load_multi_pack_index`
    @@ Commit message
         midx: pass `repository` to `load_multi_pack_index`
     
         The `load_multi_pack_index` function in midx uses `the_repository`
    -    variable to access the `repository` struct. Let's modify the function
    -    and its callee's to send the `repository` field.
    +    variable to access the `repository` struct. Modify the function and its
    +    callee's to send the `repository` field.
     
         This moves usage of `the_repository` to the `test-read-midx.c` file.
    -    While that is not optimal, it is okay, since we'll slowly move the usage
    -    of `the_repository` up the layers and eventually remove it entirely.
    +    While that is not optimal, it is okay, since the upcoming commits will
    +    slowly move the usage of `the_repository` up the layers and remove it
    +    eventually.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
15:  cea8e6c252 !  8:  3e6dd4e146 midx: pass down `hash_algo` to `get_midx_filename[_ext]`
    @@ Commit message
         midx: pass down `hash_algo` to `get_midx_filename[_ext]`
     
         The function `get_midx_filename_ext` uses `hash_to_hex` which internally
    -    uses the global variable `the_repository`. To remove this dependency, we
    +    uses the global variable `the_repository`. To remove this dependency,
         pass down the `hash_algo` to both `get_midx_filename` and
    -    `get_midx_filename_ext`. While we add `the_repository` usage to
    -    `midx-write.c` for this reason, we'll resolve this in a future commit.
    +    `get_midx_filename_ext`. This adds `the_repository` variable usage to
    +    `midx-write.c`, which will be resolved in a future commit.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
16:  66dd34c862 !  9:  704a5a4392 midx: pass down `hash_algo` to `get_split_midx_filename_ext`
    @@ Commit message
         midx: pass down `hash_algo` to `get_split_midx_filename_ext`
     
         Similar to the previous commit, pass down `hash_algo` to
    -    `get_split_midx_filename_ext` so we can use `hash_to_hex_algop` and not
    -    rely on the `the_repository` global variable.
    +    `get_split_midx_filename_ext` and use `hash_to_hex_algop`.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     
17:  1f0602aec6 ! 10:  0a4a68bcff midx: inline the `MIDX_MIN_SIZE` definition
    @@ Commit message
     
         The `MIDX_MIN_SIZE` definition is used to check the midx_size in
         `local_multi_pack_index_one`. This definitions relies on the
    -    `the_hash_algo` global variable. Let's inline this and remove the global
    +    `the_hash_algo` global variable. Inline this and remove the global
         variable usage.
     
    -    With this, we can remove `USE_THE_REPOSITORY_VARIABLE` usage from
    -    `midx.c`.
    +    With this, remove `USE_THE_REPOSITORY_VARIABLE` usage from `midx.c`.
     
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
     


--- 

base-commit: b31fb630c0fc6869a33ed717163e8a1210460d94
change-id: 20241111-374-refactor-midx-c-and-midx-write-c-to-not-depend-on-global-state-a88498c2590f

Thanks
- Karthik

Comments

Christian Couder Nov. 20, 2024, 6:20 p.m. UTC | #1
On Tue, Nov 19, 2024 at 4:38 PM Karthik Nayak <karthik.188@gmail.com> wrote:
>
> Similar to the earlier patch series on cleaning up packfile.c and
> removing usage of global variables [1], we change the midx.c and
> midx-write.c files to no longer use global variables.
>
> This is done by the following:
>   - Usage of repository variable already available in existing structs.
>   - Passing down repository variable from other subsystems.
>   - Modifying all subcommands to obtain repository variable from the
>   command in `builtins/` and passing down the variable from there.
>
> The biggest change is in the first commit, wherein we modify all
> subcommands to add the repository variable. Since the subcommand
> definition are not often changed, it shouldn't cause too many conflicts
> with in flight topics.

Overall I like the way it's done. I found a few improvements that
could be made to some commit messages though.

Thanks.
Taylor Blau Nov. 20, 2024, 10:24 p.m. UTC | #2
On Wed, Nov 20, 2024 at 07:20:33PM +0100, Christian Couder wrote:
> On Tue, Nov 19, 2024 at 4:38 PM Karthik Nayak <karthik.188@gmail.com> wrote:
> >
> > Similar to the earlier patch series on cleaning up packfile.c and
> > removing usage of global variables [1], we change the midx.c and
> > midx-write.c files to no longer use global variables.
> >
> > This is done by the following:
> >   - Usage of repository variable already available in existing structs.
> >   - Passing down repository variable from other subsystems.
> >   - Modifying all subcommands to obtain repository variable from the
> >   command in `builtins/` and passing down the variable from there.
> >
> > The biggest change is in the first commit, wherein we modify all
> > subcommands to add the repository variable. Since the subcommand
> > definition are not often changed, it shouldn't cause too many conflicts
> > with in flight topics.
>
> Overall I like the way it's done. I found a few improvements that
> could be made to some commit messages though.

Same here!

Thanks,
Taylor
Junio C Hamano Nov. 21, 2024, 12:09 a.m. UTC | #3
Taylor Blau <me@ttaylorr.com> writes:

> On Wed, Nov 20, 2024 at 07:20:33PM +0100, Christian Couder wrote:
>
>> Overall I like the way it's done. I found a few improvements that
>> could be made to some commit messages though.
>
> Same here!

Thanks, all.  It seems to me that we are getting fairly close to the
goal post on this topic?
Junio C Hamano Nov. 21, 2024, 2:19 a.m. UTC | #4
Karthik Nayak <karthik.188@gmail.com> writes:

> Since the `packfile.c` cleanup is still in flight, this series is based
> on top of master: b31fb630c0 (Merge https://github.com/j6t/git-gui,
> 2024-11-11) with those patches merged in. This applies cleanly on top 
> of next, but conflicts with `bf/set-head-symref` in seen, the conflict
> is mostly straight forward. I'll merge the topic in if it is merged into
> next soon.

I think set_head() part is fairly trivial.  But there seems to be a
much bigger conflicts with the incremental midx topic.

I actually do not understand offhand why doing anything to the midx
layer needs to change the calling convention of set_head() or many
other things that are unrelated to what midx layer does, and that do
*not* use the new parameter *anyway*.  Shouldn't the "subsubcommand
can inherit the repository from the builtin subcommand
implementation" be split out as a separate topic, which the midx
thing later builds on?

Thanks.