repack: allow `--write-midx=incremental` without `--geometric`

Previously, `--write-midx=incremental` required `--geometric` and would
die() without it. Relax this restriction so that incremental MIDX
repacking can be used independently.

Without `--geometric`, the behavior is append-only: a single new MIDX
layer is created containing whatever packs were written by the repack
and appended to the existing chain (or a new chain is started). Existing
layers are preserved as-is with no compaction or merging.

Implement this via a new repack_make_midx_append_plan() that builds a
plan consisting of a WRITE step for the freshly written packs followed
by COPY steps for every existing MIDX layer. The existing compaction
plan (repack_make_midx_compaction_plan) is used only when `--geometric`
is active.

Update the documentation to describe the behavior with and without
`--geometric`, and replace the test that enforced the old restriction
with one exercising append-only incremental MIDX repacking.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repack.adoc

@@ -263,14 +263,19 @@ linkgit:git-multi-pack-index[1]).
 
 	`incremental`;;
 		Write an incremental MIDX chain instead of a single
-		flat MIDX. This mode requires `--geometric`.
+		flat MIDX.
 +
-The incremental mode maintains a chain of MIDX layers that is compacted
-over time using a geometric merging strategy. Each repack creates a new
-tip layer containing the newly written pack(s). Adjacent layers are then
-merged whenever the newer layer's object count exceeds
-`1/repack.midxSplitFactor` of the next deeper layer's count. Layers
-that do not meet this condition are retained as-is.
+Without `--geometric`, a new MIDX layer is appended to the existing
+chain (or a new chain is started) containing whatever packs were written
+by the repack. Existing layers are preserved as-is.
++
+When combined with `--geometric`, the incremental mode maintains a chain
+of MIDX layers that is compacted over time using a geometric merging
+strategy. Each repack creates a new tip layer containing the newly
+written pack(s). Adjacent layers are then merged whenever the newer
+layer's object count exceeds `1/repack.midxSplitFactor` of the next
+deeper layer's count. Layers that do not meet this condition are
+retained as-is.
 +
 The result is that newer (tip) layers tend to contain many small packs
 with relatively few objects, while older (deeper) layers contain fewer,

builtin/repack.c

@@ -263,9 +263,6 @@ int cmd_repack(int argc,
 	if (pack_everything & PACK_CRUFT)
 		pack_everything |= ALL_INTO_ONE;
 
-	if (write_midx == REPACK_WRITE_MIDX_INCREMENTAL && !geometry.split_factor)
-		die(_("--write-midx=incremental requires --geometric"));
-
 	if (write_bitmaps < 0) {
 		if (write_midx == REPACK_WRITE_MIDX_NONE &&
 		    (!(pack_everything & ALL_INTO_ONE) || !is_bare_repository()))

repack-midx.c

@@ -548,6 +548,60 @@ static void midx_compaction_step_release(struct midx_compaction_step *step)
 	free(step->csum);
 }
 
+/*
+ * Build an append-only MIDX plan: a single WRITE step for the freshly
+ * written packs, plus COPY steps for every existing layer.  No
+ * compaction or merging is performed.
+ */
+static void repack_make_midx_append_plan(struct repack_write_midx_opts *opts,
+					 struct midx_compaction_step **steps_p,
+					 size_t *steps_nr_p)
+{
+	struct multi_pack_index *m;
+	struct midx_compaction_step *steps = NULL;
+	struct midx_compaction_step *step;
+	size_t steps_nr = 0, steps_alloc = 0;
+
+	odb_reprepare(opts->existing->repo->objects);
+	m = get_multi_pack_index(opts->existing->source);
+
+	if (opts->names->nr) {
+		struct strbuf buf = STRBUF_INIT;
+		uint32_t i;
+
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_WRITE;
+		string_list_init_dup(&step->u.write);
+
+		for (i = 0; i < opts->names->nr; i++) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "pack-%s.idx",
+				    opts->names->items[i].string);
+			string_list_append(&step->u.write, buf.buf);
+		}
+
+		strbuf_release(&buf);
+	}
+
+	for (; m; m = m->base_midx) {
+		ALLOC_GROW(steps, st_add(steps_nr, 1), steps_alloc);
+
+		step = &steps[steps_nr++];
+		memset(step, 0, sizeof(*step));
+
+		step->type = MIDX_COMPACTION_STEP_COPY;
+		step->u.copy = m;
+		step->objects_nr = m->num_objects;
+	}
+
+	*steps_p = steps;
+	*steps_nr_p = steps_nr;
+}
+
 static int repack_make_midx_compaction_plan(struct repack_write_midx_opts *opts,
 					    struct midx_compaction_step **steps_p,
 					    size_t *steps_nr_p)
@@ -904,9 +958,13 @@ static int write_midx_incremental(struct repack_write_midx_opts *opts)
 		goto done;
 	}
 
-	if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
-		ret = error(_("unable to generate compaction plan"));
-		goto done;
+	if (opts->geometry->split_factor) {
+		if (repack_make_midx_compaction_plan(opts, &steps, &steps_nr) < 0) {
+			ret = error(_("unable to generate compaction plan"));
+			goto done;
+		}
+	} else {
+		repack_make_midx_append_plan(opts, &steps, &steps_nr);
 	}
 
 	for (i = 0; i < steps_nr; i++) {

t/t7705-repack-incremental-midx.sh

@@ -63,10 +63,36 @@ create_layers () {
 	done
 }
 
-test_expect_success '--write-midx=incremental requires --geometric' '
-	test_must_fail git repack --write-midx=incremental 2>err &&
+test_expect_success '--write-midx=incremental without --geometric' '
+	git init incremental-without-geometric &&
+	(
+		cd incremental-without-geometric &&
 
-	test_grep -- "--write-midx=incremental requires --geometric" err
+		git config maintenance.auto false &&
+
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+		cp $midx_chain $midx_chain.before &&
+
+		# A second repack appends a new layer without
+		# disturbing the existing one.
+		test_commit third &&
+		git repack --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 2 $midx_chain &&
+		head -n 1 $midx_chain.before >expect &&
+		head -n 1 $midx_chain >actual &&
+		test_cmp expect actual &&
+
+		git fsck
+	)
 '
 
 test_expect_success 'below layer threshold, tip packs excluded' '
@@ -334,8 +360,7 @@ test_expect_success 'kept packs are excluded from repack' '
 		# entirely, so no rollup occurs as there is only one
 		# non-kept pack. A new MIDX layer is written containing
 		# that pack.
-		git repack --geometric=2 -d --write-midx=incremental \
-			--write-bitmap-index &&
+		git repack --geometric=2 -d --write-midx=incremental &&
 
 		test-tool read-midx $objdir >actual &&
 		grep "^pack-.*\.idx$" actual >actual.packs &&
@@ -433,6 +458,36 @@ test_expect_success 'repack -ad removes stale incremental chain' '
 	)
 '
 
+test_expect_success 'repack -ad --write-midx=incremental is safe' '
+	git init ad-incremental-midx &&
+	(
+		cd ad-incremental-midx &&
+
+		git config maintenance.auto false &&
+
+		# Build a MIDX chain with multiple layers referencing
+		# distinct packs.
+		test_commit first &&
+		git repack -d &&
+
+		test_commit second &&
+		git repack -d --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		test_line_count = 1 $midx_chain &&
+
+		# Now do a full -ad repack. The new pack contains all
+		# objects, but any retained MIDX layers still reference
+		# the now-deleted packs.
+		test_commit third &&
+		git repack -ad --write-midx=incremental &&
+
+		git multi-pack-index verify &&
+		git fsck &&
+		git rev-list --all --objects >/dev/null
+	)
+'
+
 test_expect_success 'repack rejects invalid midxSplitFactor' '
 	test_when_finished "rm -fr bad-split-factor" &&
 	git init bad-split-factor &&