pack: move open_pack_index(), parse_pack_index()

alloc_packed_git() in packfile.c is duplicated from sha1_file.c. In a
subsequent commit, alloc_packed_git() will be removed from sha1_file.c.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "mru.h"
+#include "pack.h"
 
 char *odb_pack_name(struct strbuf *buf,
 		    const unsigned char *sha1,
@@ -59,3 +60,151 @@ void pack_report(void)
 		pack_open_windows, peak_pack_open_windows,
 		sz_fmt(pack_mapped), sz_fmt(peak_pack_mapped));
 }
+
+/*
+ * Open and mmap the index file at path, perform a couple of
+ * consistency checks, then record its information to p.  Return 0 on
+ * success.
+ */
+static int check_packed_git_idx(const char *path, struct packed_git *p)
+{
+	void *idx_map;
+	struct pack_idx_header *hdr;
+	size_t idx_size;
+	uint32_t version, nr, i, *index;
+	int fd = git_open(path);
+	struct stat st;
+
+	if (fd < 0)
+		return -1;
+	if (fstat(fd, &st)) {
+		close(fd);
+		return -1;
+	}
+	idx_size = xsize_t(st.st_size);
+	if (idx_size < 4 * 256 + 20 + 20) {
+		close(fd);
+		return error("index file %s is too small", path);
+	}
+	idx_map = xmmap(NULL, idx_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	close(fd);
+
+	hdr = idx_map;
+	if (hdr->idx_signature == htonl(PACK_IDX_SIGNATURE)) {
+		version = ntohl(hdr->idx_version);
+		if (version < 2 || version > 2) {
+			munmap(idx_map, idx_size);
+			return error("index file %s is version %"PRIu32
+				     " and is not supported by this binary"
+				     " (try upgrading GIT to a newer version)",
+				     path, version);
+		}
+	} else
+		version = 1;
+
+	nr = 0;
+	index = idx_map;
+	if (version > 1)
+		index += 2;  /* skip index header */
+	for (i = 0; i < 256; i++) {
+		uint32_t n = ntohl(index[i]);
+		if (n < nr) {
+			munmap(idx_map, idx_size);
+			return error("non-monotonic index %s", path);
+		}
+		nr = n;
+	}
+
+	if (version == 1) {
+		/*
+		 * Total size:
+		 *  - 256 index entries 4 bytes each
+		 *  - 24-byte entries * nr (20-byte sha1 + 4-byte offset)
+		 *  - 20-byte SHA1 of the packfile
+		 *  - 20-byte SHA1 file checksum
+		 */
+		if (idx_size != 4*256 + nr * 24 + 20 + 20) {
+			munmap(idx_map, idx_size);
+			return error("wrong index v1 file size in %s", path);
+		}
+	} else if (version == 2) {
+		/*
+		 * Minimum size:
+		 *  - 8 bytes of header
+		 *  - 256 index entries 4 bytes each
+		 *  - 20-byte sha1 entry * nr
+		 *  - 4-byte crc entry * nr
+		 *  - 4-byte offset entry * nr
+		 *  - 20-byte SHA1 of the packfile
+		 *  - 20-byte SHA1 file checksum
+		 * And after the 4-byte offset table might be a
+		 * variable sized table containing 8-byte entries
+		 * for offsets larger than 2^31.
+		 */
+		unsigned long min_size = 8 + 4*256 + nr*(20 + 4 + 4) + 20 + 20;
+		unsigned long max_size = min_size;
+		if (nr)
+			max_size += (nr - 1)*8;
+		if (idx_size < min_size || idx_size > max_size) {
+			munmap(idx_map, idx_size);
+			return error("wrong index v2 file size in %s", path);
+		}
+		if (idx_size != min_size &&
+		    /*
+		     * make sure we can deal with large pack offsets.
+		     * 31-bit signed offset won't be enough, neither
+		     * 32-bit unsigned one will be.
+		     */
+		    (sizeof(off_t) <= 4)) {
+			munmap(idx_map, idx_size);
+			return error("pack too large for current definition of off_t in %s", path);
+		}
+	}
+
+	p->index_version = version;
+	p->index_data = idx_map;
+	p->index_size = idx_size;
+	p->num_objects = nr;
+	return 0;
+}
+
+int open_pack_index(struct packed_git *p)
+{
+	char *idx_name;
+	size_t len;
+	int ret;
+
+	if (p->index_data)
+		return 0;
+
+	if (!strip_suffix(p->pack_name, ".pack", &len))
+		die("BUG: pack_name does not end in .pack");
+	idx_name = xstrfmt("%.*s.idx", (int)len, p->pack_name);
+	ret = check_packed_git_idx(idx_name, p);
+	free(idx_name);
+	return ret;
+}
+
+static struct packed_git *alloc_packed_git(int extra)
+{
+	struct packed_git *p = xmalloc(st_add(sizeof(*p), extra));
+	memset(p, 0, sizeof(*p));
+	p->pack_fd = -1;
+	return p;
+}
+
+struct packed_git *parse_pack_index(unsigned char *sha1, const char *idx_path)
+{
+	const char *path = sha1_pack_name(sha1);
+	size_t alloc = st_add(strlen(path), 1);
+	struct packed_git *p = alloc_packed_git(alloc);
+
+	memcpy(p->pack_name, path, alloc); /* includes NUL */
+	hashcpy(p->sha1, sha1);
+	if (check_packed_git_idx(idx_path, p)) {
+		free(p);
+		return NULL;
+	}
+
+	return p;
+}