pack-bitmap.c: read pseudo-merge extension

Now that the scaffolding for reading the pseudo-merge extension has been
laid, teach the pack-bitmap machinery to read the pseudo-merge extension
when present.

Note that pseudo-merges themselves are not yet used during traversal,
this step will be taken by a future commit.

In the meantime, read the table and initialize the pseudo_merge_map
structure introduced by a previous commit. When the pseudo-merge
extension is present, `load_bitmap_header()` performs basic sanity
checks to make sure that the table is well-formed.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

pack-bitmap.c

@@ -20,6 +20,7 @@
 #include "list-objects-filter-options.h"
 #include "midx.h"
 #include "config.h"
+#include "pseudo-merge.h"
 
 /*
  * An entry on the bitmap index, representing the bitmap for a given
@@ -86,6 +87,9 @@ struct bitmap_index {
 	 */
 	unsigned char *table_lookup;
 
+	/* This contains the pseudo-merge cache within 'map' (if found). */
+	struct pseudo_merge_map pseudo_merges;
+
 	/*
 	 * Extended index.
 	 *
@@ -205,6 +209,41 @@ static int load_bitmap_header(struct bitmap_index *index)
 				index->table_lookup = (void *)(index_end - table_size);
 			index_end -= table_size;
 		}
+
+		if (flags & BITMAP_OPT_PSEUDO_MERGES) {
+			unsigned char *pseudo_merge_ofs;
+			size_t table_size;
+			uint32_t i;
+
+			if (sizeof(table_size) > index_end - index->map - header_size)
+				return error(_("corrupted bitmap index file (too short to fit pseudo-merge table header)"));
+
+			table_size = get_be64(index_end - 8);
+			if (table_size > index_end - index->map - header_size)
+				return error(_("corrupted bitmap index file (too short to fit pseudo-merge table)"));
+
+			if (git_env_bool("GIT_TEST_USE_PSEUDO_MERGES", 1)) {
+				const unsigned char *ext = (index_end - table_size);
+
+				index->pseudo_merges.map = index->map;
+				index->pseudo_merges.map_size = index->map_size;
+				index->pseudo_merges.commits = ext + get_be64(index_end - 16);
+				index->pseudo_merges.commits_nr = get_be32(index_end - 20);
+				index->pseudo_merges.nr = get_be32(index_end - 24);
+
+				CALLOC_ARRAY(index->pseudo_merges.v,
+					     index->pseudo_merges.nr);
+
+				pseudo_merge_ofs = index_end - 24 -
+					(index->pseudo_merges.nr * sizeof(uint64_t));
+				for (i = 0; i < index->pseudo_merges.nr; i++) {
+					index->pseudo_merges.v[i].at = get_be64(pseudo_merge_ofs);
+					pseudo_merge_ofs += sizeof(uint64_t);
+				}
+			}
+
+			index_end -= table_size;
+		}
 	}
 
 	index->entry_count = ntohl(header->entry_count);