midx-write: handle noop writes when converting incremental chains

When updating a MIDX, we optimize out writes that will result in an
identical MIDX as the one we already have on disk. See b3bab9d
(midx-write: extract function to test whether MIDX needs updating,
2025-12-10) for more details on exactly which writes are optimized out.

If `midx_needs_update()` can't rule out any of the obvious cases (e.g.,
the checksum is invalid, we're requesting a different version, or
performing compaction which always requires an update), then we compare
the packs we're writing to the packs we already know about. If there are
an equal number of packs being written as there are in any existing
MIDX layer(s), then we compare the packs by their name.

This comparison fails when we have an incremental MIDX chain with
at least two layers, since we do not recursively peel through earlier
layers, instead treating the `->pack_names` array of the tip MIDX layer
as containing all `m->num_packs + m->num_packs_in_base` packs.

Adjust this to instead look through the MIDX layers one by one when
comparing pack names. While we're at it, fix a typo above in the same
function.

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

Author: ttaylorr

midx-write.c

@@ -1152,7 +1152,7 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 
 	/*
 	 * Ensure that we have a valid checksum before consulting the
-	 * exisiting MIDX in order to determine if we can avoid an
+	 * existing MIDX in order to determine if we can avoid an
 	 * update.
 	 *
 	 * This is necessary because the given MIDX is loaded directly
@@ -1208,14 +1208,16 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 			BUG("same pack added twice?");
 	}
 
-	for (uint32_t i = 0; i < ctx->nr; i++) {
-		strbuf_reset(&buf);
-		strbuf_addstr(&buf, midx->pack_names[i]);
-		strbuf_strip_suffix(&buf, ".idx");
+	for (struct multi_pack_index *m = midx; m; m = m->base_midx) {
+		for (uint32_t i = 0; i < m->num_packs; i++) {
+			strbuf_reset(&buf);
+			strbuf_addstr(&buf, m->pack_names[i]);
+			strbuf_strip_suffix(&buf, ".idx");
 
-		if (!strset_contains(&packs, buf.buf))
-			goto out;
-		strset_remove(&packs, buf.buf);
+			if (!strset_contains(&packs, buf.buf))
+				goto out;
+			strset_remove(&packs, buf.buf);
+		}
 	}
 
 	needed = false;

t/t5334-incremental-multi-pack-index.sh

@@ -132,4 +132,20 @@ test_expect_success 'relink existing MIDX layer' '
 
 '
 
+test_expect_success 'non-incremental write with existing incremental chain' '
+	git init non-incremental-write-with-existing &&
+	test_when_finished "rm -fr non-incremental-write-with-existing" &&
+
+	(
+		cd non-incremental-write-with-existing &&
+
+		git config set maintenance.auto false &&
+
+		write_midx_layer &&
+		write_midx_layer &&
+
+		git multi-pack-index write
+	)
+'
+
 test_done