Add a pack index 'virtual function' to search by file offset

In addition to searching the index by oid, we need to search by
'.pack' file offset, particularly when processing OBJ_OFS_DELTA
objects. Since the v1 and v2 file formats differ in the layout
of the object records, we provide two implementations of the
search function and initialise the (virtual) function pointer
appropriately.

Note that, as part of the creation of the 'offset index', we also
add a check that the offset data in the index is within the bounds
of the '.pack' file. Having sorted the file offsets, while creating
the index, we only need to check the smallest and largest values.

The offset index consists of the im_off_idx array, which contains
the index entry numbers sorted into file offset order, and the
im_off_next mapping array. The im_off_next array maps an index
entry number to the 'next' index entry in file offset order.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

src/odb.c

@@ -40,6 +40,10 @@ struct git_pack {
 		uint32_t *,
 		struct git_pack *,
 		const git_oid *);
+	int (*idx_search_offset)(
+		uint32_t *,
+		struct git_pack *,
+		off_t);
 
 	/** The .idx file, mapped into memory. */
 	git_file idx_fd;
@@ -49,10 +53,18 @@ struct git_pack {
 	uint32_t *im_crc;
 	uint32_t *im_offset32;
 	uint32_t *im_offset64;
+	uint32_t *im_off_idx;
+	uint32_t *im_off_next;
 
 	/** Number of objects in this pack. */
 	uint32_t obj_cnt;
 
+	/** The size of the .pack file. */
+	off_t pack_size;
+
+	/** The mtime of the .pack file. */
+	time_t pack_mtime;
+
 	/** Number of git_packlist we appear in. */
 	unsigned int refcnt;
 
@@ -663,6 +675,51 @@ static int pack_openidx_map(git_pack *p)
 	return GIT_SUCCESS;
 }
 
+typedef struct {
+	off_t offset;
+	uint32_t n;
+} offset_idx_info;
+
+static int cmp_offset_idx_info(const void *lhs, const void *rhs)
+{
+	const offset_idx_info *a = lhs;
+	const offset_idx_info *b = rhs;
+	return (a->offset < b->offset) ? -1 : (a->offset > b->offset) ? 1 : 0;
+}
+
+static int make_offset_index(git_pack *p, offset_idx_info *data)
+{
+	off_t min_off = 3 * 4, max_off = p->pack_size - GIT_OID_RAWSZ;
+	uint32_t *idx, *next;
+	uint32_t j;
+
+	qsort(data, p->obj_cnt, sizeof(*data), cmp_offset_idx_info);
+
+	if (data[0].offset < min_off || data[p->obj_cnt].offset > max_off)
+		return GIT_ERROR;
+
+	if ((idx = git__malloc(sizeof(*idx) * (p->obj_cnt+1))) == NULL)
+		return GIT_ERROR;
+	if ((next = git__malloc(sizeof(*next) * p->obj_cnt)) == NULL) {
+		free(idx);
+		return GIT_ERROR;
+	}
+
+	for (j = 0; j < p->obj_cnt+1; j++)
+		idx[j] = data[j].n;
+
+	for (j = 0; j < p->obj_cnt; j++) {
+		assert(idx[j]   < p->obj_cnt);
+		assert(idx[j+1] < p->obj_cnt+1);
+
+		next[idx[j]] = idx[j+1];
+	}
+
+	p->im_off_idx = idx;
+	p->im_off_next = next;
+	return GIT_SUCCESS;
+}
+
 static int idxv1_search(uint32_t *out, git_pack *p, const git_oid *id)
 {
 	unsigned char *data = p->im_oid;
@@ -684,12 +741,37 @@ static int idxv1_search(uint32_t *out, git_pack *p, const git_oid *id)
 	return GIT_ENOTFOUND;
 }
 
+static int idxv1_search_offset(uint32_t *out, git_pack *p, off_t offset)
+{
+	if (offset > 0 && offset < (p->pack_size - GIT_OID_RAWSZ)) {
+		uint32_t lo = 0, hi = p->obj_cnt+1;
+		unsigned char *data = p->im_oid;
+		uint32_t *idx = p->im_off_idx;
+		do {
+			uint32_t mid = (lo + hi) >> 1;
+			uint32_t n = idx[mid];
+			uint32_t pos = n * (GIT_OID_RAWSZ + 4);
+			off_t here = decode32(data + pos);
+			if (offset < here)
+				hi = mid;
+			else if (offset == here) {
+				*out = n;
+				return GIT_SUCCESS;
+			} else
+				lo = mid + 1;
+		} while (lo < hi);
+	}
+	return GIT_ENOTFOUND;
+}
+
 static int pack_openidx_v1(git_pack *p)
 {
 	uint32_t *src_fanout = p->idx_map.data;
 	uint32_t *im_fanout;
+	offset_idx_info *info;
 	size_t expsz;
-	int j;
+	uint32_t j;
+
 
 	if ((im_fanout = git__malloc(sizeof(*im_fanout) * 256)) == NULL)
 		return GIT_ERROR;
@@ -711,8 +793,30 @@ static int pack_openidx_v1(git_pack *p)
 	}
 
 	p->idx_search = idxv1_search;
+	p->idx_search_offset = idxv1_search_offset;
 	p->im_fanout = im_fanout;
 	p->im_oid = (unsigned char *)(src_fanout + 256);
+
+	if ((info = git__malloc(sizeof(*info) * (p->obj_cnt+1))) == NULL) {
+		free(im_fanout);
+		return GIT_ERROR;
+	}
+
+	for (j = 0; j < p->obj_cnt; j++) {
+		uint32_t pos = j * (GIT_OID_RAWSZ + 4);
+		info[j].offset = decode32(p->im_oid + pos);
+		info[j].n = j;
+	}
+	info[p->obj_cnt].offset = p->pack_size - GIT_OID_RAWSZ;
+	info[p->obj_cnt].n = p->obj_cnt;
+
+	if (make_offset_index(p, info)) {
+		free(im_fanout);
+		free(info);
+		return GIT_ERROR;
+	}
+	free(info);
+
 	return GIT_SUCCESS;
 }
 
@@ -737,11 +841,40 @@ static int idxv2_search(uint32_t *out, git_pack *p, const git_oid *id)
 	return GIT_ENOTFOUND;
 }
 
+static int idxv2_search_offset(uint32_t *out, git_pack *p, off_t offset)
+{
+	if (offset > 0 && offset < (p->pack_size - GIT_OID_RAWSZ)) {
+		uint32_t lo = 0, hi = p->obj_cnt+1;
+		uint32_t *idx = p->im_off_idx;
+		do {
+			uint32_t mid = (lo + hi) >> 1;
+			uint32_t n = idx[mid];
+			uint32_t o32 = decode32(p->im_offset32 + n);
+			off_t here = o32;
+
+			if (o32 & 0x80000000) {
+				uint32_t o64_idx = (o32 & ~0x80000000);
+				here = decode64(p->im_offset64 + 2*o64_idx);
+			}
+
+			if (offset < here)
+				hi = mid;
+			else if (offset == here) {
+				*out = n;
+				return GIT_SUCCESS;
+			} else
+				lo = mid + 1;
+		} while (lo < hi);
+	}
+	return GIT_ENOTFOUND;
+}
+
 static int pack_openidx_v2(git_pack *p)
 {
 	unsigned char *data = p->idx_map.data;
 	uint32_t *src_fanout = (uint32_t *)(data + 8);
 	uint32_t *im_fanout;
+	offset_idx_info *info;
 	size_t sz, o64_sz, o64_len;
 	uint32_t j;
 
@@ -766,25 +899,67 @@ static int pack_openidx_v2(git_pack *p)
 	}
 
 	p->idx_search = idxv2_search;
+	p->idx_search_offset = idxv2_search_offset;
 	p->im_fanout = im_fanout;
 	p->im_oid = (unsigned char *)(src_fanout + 256);
 	p->im_crc = (uint32_t *)(p->im_oid + 20 * p->obj_cnt);
 	p->im_offset32 = p->im_crc + p->obj_cnt;
 	p->im_offset64 = p->im_offset32 + p->obj_cnt;
 
+	if ((info = git__malloc(sizeof(*info) * (p->obj_cnt+1))) == NULL) {
+		free(im_fanout);
+		return GIT_ERROR;
+	}
+
 	/* check 64-bit offset table index values are within bounds */
 	o64_sz = p->idx_map.len - sz;
 	o64_len = o64_sz / 8;
 	for (j = 0; j < p->obj_cnt; j++) {
 		uint32_t o32 = decode32(p->im_offset32 + j);
+		off_t offset = o32;
 		if (o32 & 0x80000000) {
 			uint32_t o64_idx = (o32 & ~0x80000000);
 			if (o64_idx >= o64_len) {
 				free(im_fanout);
+				free(info);
 				return GIT_ERROR;
 			}
+			offset = decode64(p->im_offset64 + 2*o64_idx);
 		}
+		info[j].offset = offset;
+		info[j].n = j;
+	}
+	info[p->obj_cnt].offset = p->pack_size - GIT_OID_RAWSZ;
+	info[p->obj_cnt].n = p->obj_cnt;
+
+	if (make_offset_index(p, info)) {
+		free(im_fanout);
+		free(info);
+		return GIT_ERROR;
 	}
+	free(info);
+
+	return GIT_SUCCESS;
+}
+
+static int pack_stat(git_pack *p)
+{
+	char pb[GIT_PATH_MAX];
+	struct stat sb;
+
+	if (git__fmt(pb, sizeof(pb), "%s/pack/%s.pack",
+			p->db->objects_dir,
+			p->pack_name) < 0)
+		return GIT_ERROR;
+
+	if (stat(pb, &sb) || !S_ISREG(sb.st_mode))
+		return GIT_ERROR;
+
+	if (sb.st_size < (3 * 4 + GIT_OID_RAWSZ))
+		return GIT_ERROR;
+
+	p->pack_size = sb.st_size;
+	p->pack_mtime = sb.st_mtime;
 
 	return GIT_SUCCESS;
 }
@@ -792,40 +967,51 @@ static int pack_openidx_v2(git_pack *p)
 static int pack_openidx(git_pack *p)
 {
 	gitlck_lock(&p->lock);
-	if (p->invalid)
-		goto unlock_fail;
+
+	if (p->invalid) {
+		gitlck_unlock(&p->lock);
+		return GIT_ERROR;
+	}
+
 	if (++p->idxcnt == 1 && !p->idx_search) {
+		int status, version;
 		uint32_t *data;
 
-		if (pack_openidx_map(p))
-			goto invalid_fail;
+		if (pack_stat(p) || pack_openidx_map(p)) {
+			p->invalid = 1;
+			p->idxcnt--;
+			gitlck_unlock(&p->lock);
+			return GIT_ERROR;
+		}
 		data = p->idx_map.data;
+		status = GIT_SUCCESS;
+		version = 1;
 
-		if (decode32(&data[0]) == PACK_TOC) {
-			switch (decode32(&data[1])) {
-			case 2:
-				if (pack_openidx_v2(p))
-					goto unmap_fail;
-				break;
-			default:
-				goto unmap_fail;
-			}
-		} else if (pack_openidx_v1(p))
-			goto unmap_fail;
-	}
-	gitlck_unlock(&p->lock);
-	return GIT_SUCCESS;
+		if (decode32(&data[0]) == PACK_TOC)
+			version = decode32(&data[1]);
 
-unmap_fail:
-	gitfo_free_map(&p->idx_map);
+		switch (version) {
+		case 1:
+			status = pack_openidx_v1(p);
+			break;
+		case 2:
+			status = pack_openidx_v2(p);
+			break;
+		default:
+			status = GIT_ERROR;
+		}
 
-invalid_fail:
-	p->invalid = 1;
-	p->idxcnt--;
+		if (status != GIT_SUCCESS) {
+			gitfo_free_map(&p->idx_map);
+			p->invalid = 1;
+			p->idxcnt--;
+			gitlck_unlock(&p->lock);
+			return status;
+		}
+	}
 
-unlock_fail:
 	gitlck_unlock(&p->lock);
-	return GIT_ERROR;
+	return GIT_SUCCESS;
 }
 
 static void pack_decidx(git_pack *p)