[PATCH 9/9] pack-revindex: write multi-pack reverse indexes

[Taylor Blau <me@ttaylorr.com>]

Implement the writing half of multi-pack reverse indexes. This is
nothing more than the format describe a few patches ago, with a new set
of helper functions that will be used to clear out stale .rev files
corresponding to old MIDXs.

Unfortunately, a very similar comparison function as the one implemented
recently in pack-revindex.c is reimplemented here, this time accepting a
MIDX-internal type. An effort to DRY these up would create more
indirection and overhead than is necessary, so it isn't pursued here.

Currently, there are no callers which pass the MIDX_WRITE_REV_INDEX
flag, meaning that this is all dead code. But, that won't be the case
for long, since subsequent patches will introduce the multi-pack bitmap,
which will begin passing this field.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 midx.h |   1 +
 2 files changed, 124 insertions(+)

diff --git a/midx.c b/midx.c
index 12bfce8bb1..3d79a1930a 100644
--- a/midx.c
+++ b/midx.c
@@ -11,6 +11,7 @@
 #include "trace2.h"
 #include "run-command.h"
 #include "repository.h"
+#include "pack.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -841,6 +842,78 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
+struct midx_pack_order_data {
+	struct pack_midx_entry *entries;
+	uint32_t *pack_perm;
+};
+
+static int midx_pack_order_cmp(const void *va, const void *vb, void *_data)
+{
+	struct midx_pack_order_data *data = _data;
+
+	struct pack_midx_entry *a = &data->entries[*(const uint32_t *)va];
+	struct pack_midx_entry *b = &data->entries[*(const uint32_t *)vb];
+
+	uint32_t perm_a = data->pack_perm[a->pack_int_id];
+	uint32_t perm_b = data->pack_perm[b->pack_int_id];
+
+	/* Sort objects in the preferred pack ahead of any others. */
+	if (a->preferred > b->preferred)
+		return -1;
+	if (a->preferred < b->preferred)
+		return 1;
+
+	/* Then, order objects by which packs they appear in. */
+	if (perm_a < perm_b)
+		return -1;
+	if (perm_a > perm_b)
+		return 1;
+
+	/* Then, disambiguate by their offset within each pack. */
+	if (a->offset < b->offset)
+		return -1;
+	if (a->offset > b->offset)
+		return 1;
+
+	return 0;
+}
+
+static uint32_t *midx_pack_order(struct pack_midx_entry *entries,
+				 uint32_t *pack_perm,
+				 uint32_t entries_nr)
+{
+	struct midx_pack_order_data data;
+	uint32_t *pack_order;
+	uint32_t i;
+
+	data.entries = entries;
+	data.pack_perm = pack_perm;
+
+	ALLOC_ARRAY(pack_order, entries_nr);
+	for (i = 0; i < entries_nr; i++)
+		pack_order[i] = i;
+	QSORT_S(pack_order, entries_nr, midx_pack_order_cmp, &data);
+
+	return pack_order;
+}
+
+static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
+				     uint32_t *pack_order,
+				     uint32_t entries_nr)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
+
+	write_rev_file_order(buf.buf, pack_order, entries_nr, midx_hash,
+			     WRITE_REV);
+
+	strbuf_release(&buf);
+}
+
+static void clear_midx_files_ext(struct repository *r, const char *ext,
+				 unsigned char *keep_hash);
+
 static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
 			       struct string_list *packs_to_drop,
 			       const char *preferred_pack_name,
@@ -854,6 +927,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	struct lock_file lk;
 	struct pack_list packs;
 	uint32_t *pack_perm = NULL;
+	uint32_t *pack_order = NULL;
 	uint64_t written = 0;
 	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
 	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
@@ -1111,6 +1185,14 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 		    chunk_offsets[num_chunks]);
 
 	finalize_hashfile(f, midx_hash, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	if (flags & MIDX_WRITE_REV_INDEX)
+		pack_order = midx_pack_order(entries, pack_perm, nr_entries);
+
+	if (flags & MIDX_WRITE_REV_INDEX)
+		write_midx_reverse_index(midx_name, midx_hash, pack_order,
+					 nr_entries);
+	clear_midx_files_ext(the_repository, ".rev", midx_hash);
+
 	commit_lock_file(&lk);
 
 cleanup:
@@ -1125,6 +1207,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	free(packs.info);
 	free(entries);
 	free(pack_perm);
+	free(pack_order);
 	free(midx_name);
 	return result;
 }
@@ -1137,6 +1220,44 @@ int write_midx_file(const char *object_dir,
 				   flags);
 }
 
+struct clear_midx_data {
+	char *keep;
+	const char *ext;
+};
+
+static void clear_midx_file_ext(const char *full_path, size_t full_path_len,
+				const char *file_name, void *_data)
+{
+	struct clear_midx_data *data = _data;
+
+	if (!(starts_with(file_name, "multi-pack-index-") &&
+	      ends_with(file_name, data->ext)))
+		return;
+	if (data->keep && !strcmp(data->keep, file_name))
+		return;
+
+	if (unlink(full_path))
+		die_errno(_("failed to remove %s"), full_path);
+}
+
+static void clear_midx_files_ext(struct repository *r, const char *ext,
+				 unsigned char *keep_hash)
+{
+	struct clear_midx_data data;
+	memset(&data, 0, sizeof(struct clear_midx_data));
+
+	if (keep_hash)
+		data.keep = xstrfmt("multi-pack-index-%s%s",
+				    hash_to_hex(keep_hash), ext);
+	data.ext = ext;
+
+	for_each_file_in_pack_dir(r->objects->odb->path,
+				  clear_midx_file_ext,
+				  &data);
+
+	free(data.keep);
+}
+
 void clear_midx_file(struct repository *r)
 {
 	char *midx = get_midx_filename(r->objects->odb->path);
@@ -1149,6 +1270,8 @@ void clear_midx_file(struct repository *r)
 	if (remove_path(midx))
 		die(_("failed to clear multi-pack-index at %s"), midx);
 
+	clear_midx_files_ext(r, ".rev", NULL);
+
 	free(midx);
 }
 
diff --git a/midx.h b/midx.h
index 0a8294d2ee..8684cf0fef 100644
--- a/midx.h
+++ b/midx.h
@@ -40,6 +40,7 @@ struct multi_pack_index {
 };
 
 #define MIDX_PROGRESS     (1 << 0)
+#define MIDX_WRITE_REV_INDEX (1 << 1)
 
 char *get_midx_rev_filename(struct multi_pack_index *m);
 
-- 
2.30.0.667.g81c0cbc6fd