git/pack-revindex.c
Taylor Blau 7f514b7a5e midx: read RIDX chunk when present
When a MIDX contains the new `RIDX` chunk, ensure that the reverse index
is read from it instead of the on-disk .rev file. Since we need to
encode the object order in the MIDX itself for correctness reasons,
there is no point in storing the same data again outside of the MIDX.

So, this patch stops writing separate .rev files, and reads it out of
the MIDX itself. This is possible to do with relatively little new code,
since the format of the RIDX chunk is identical to the data in the .rev
file. In other words, we can implement this by pointing the
`revindex_data` field at the reverse index chunk of the MIDX instead of
the .rev file without any other changes.

Note that we have two knobs that are adjusted for the new tests:
GIT_TEST_MIDX_WRITE_REV and GIT_TEST_MIDX_READ_RIDX. The former controls
whether the MIDX .rev is written at all, and the latter controls whether
we read the MIDX's RIDX chunk.

Both are necessary to ensure that the test added at the beginning of
this series continues to work. This is because we always need to write
the RIDX chunk in the MIDX in order to change its checksum, but we want
to make sure reading the existing .rev file still works (since the RIDX
chunk takes precedence by default).

Arguably this isn't a very interesting mode to test, because the
precedence rules mean that we'll always read the RIDX chunk over the
.rev file. But it makes it impossible for a user to induce corruption in
their repository by adjusting the test knobs (since if we had an
either/or knob they could stop writing the RIDX chunk, allowing them to
tweak the MIDX's object order without changing its checksum).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-01-27 12:07:53 -08:00

495 lines
13 KiB
C

#include "cache.h"
#include "pack-revindex.h"
#include "object-store.h"
#include "packfile.h"
#include "config.h"
#include "midx.h"
struct revindex_entry {
off_t offset;
unsigned int nr;
};
/*
* Pack index for existing packs give us easy access to the offsets into
* corresponding pack file where each object's data starts, but the entries
* do not store the size of the compressed representation (uncompressed
* size is easily available by examining the pack entry header). It is
* also rather expensive to find the sha1 for an object given its offset.
*
* The pack index file is sorted by object name mapping to offset;
* this revindex array is a list of offset/index_nr pairs
* ordered by offset, so if you know the offset of an object, next offset
* is where its packed representation ends and the index_nr can be used to
* get the object sha1 from the main index.
*/
/*
* This is a least-significant-digit radix sort.
*
* It sorts each of the "n" items in "entries" by its offset field. The "max"
* parameter must be at least as large as the largest offset in the array,
* and lets us quit the sort early.
*/
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
{
/*
* We use a "digit" size of 16 bits. That keeps our memory
* usage reasonable, and we can generally (for a 4G or smaller
* packfile) quit after two rounds of radix-sorting.
*/
#define DIGIT_SIZE (16)
#define BUCKETS (1 << DIGIT_SIZE)
/*
* We want to know the bucket that a[i] will go into when we are using
* the digit that is N bits from the (least significant) end.
*/
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
/*
* We need O(n) temporary storage. Rather than do an extra copy of the
* partial results into "entries", we sort back and forth between the
* real array and temporary storage. In each iteration of the loop, we
* keep track of them with alias pointers, always sorting from "from"
* to "to".
*/
struct revindex_entry *tmp, *from, *to;
int bits;
unsigned *pos;
ALLOC_ARRAY(pos, BUCKETS);
ALLOC_ARRAY(tmp, n);
from = entries;
to = tmp;
/*
* If (max >> bits) is zero, then we know that the radix digit we are
* on (and any higher) will be zero for all entries, and our loop will
* be a no-op, as everybody lands in the same zero-th bucket.
*/
for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
unsigned i;
memset(pos, 0, BUCKETS * sizeof(*pos));
/*
* We want pos[i] to store the index of the last element that
* will go in bucket "i" (actually one past the last element).
* To do this, we first count the items that will go in each
* bucket, which gives us a relative offset from the last
* bucket. We can then cumulatively add the index from the
* previous bucket to get the true index.
*/
for (i = 0; i < n; i++)
pos[BUCKET_FOR(from, i, bits)]++;
for (i = 1; i < BUCKETS; i++)
pos[i] += pos[i-1];
/*
* Now we can drop the elements into their correct buckets (in
* our temporary array). We iterate the pos counter backwards
* to avoid using an extra index to count up. And since we are
* going backwards there, we must also go backwards through the
* array itself, to keep the sort stable.
*
* Note that we use an unsigned iterator to make sure we can
* handle 2^32-1 objects, even on a 32-bit system. But this
* means we cannot use the more obvious "i >= 0" loop condition
* for counting backwards, and must instead check for
* wrap-around with UINT_MAX.
*/
for (i = n - 1; i != UINT_MAX; i--)
to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
/*
* Now "to" contains the most sorted list, so we swap "from" and
* "to" for the next iteration.
*/
SWAP(from, to);
}
/*
* If we ended with our data in the original array, great. If not,
* we have to move it back from the temporary storage.
*/
if (from != entries)
COPY_ARRAY(entries, tmp, n);
free(tmp);
free(pos);
#undef BUCKET_FOR
#undef BUCKETS
#undef DIGIT_SIZE
}
/*
* Ordered list of offsets of objects in the pack.
*/
static void create_pack_revindex(struct packed_git *p)
{
const unsigned num_ent = p->num_objects;
unsigned i;
const char *index = p->index_data;
const unsigned hashsz = the_hash_algo->rawsz;
ALLOC_ARRAY(p->revindex, num_ent + 1);
index += 4 * 256;
if (p->index_version > 1) {
const uint32_t *off_32 =
(uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
const uint32_t *off_64 = off_32 + p->num_objects;
for (i = 0; i < num_ent; i++) {
const uint32_t off = ntohl(*off_32++);
if (!(off & 0x80000000)) {
p->revindex[i].offset = off;
} else {
p->revindex[i].offset = get_be64(off_64);
off_64 += 2;
}
p->revindex[i].nr = i;
}
} else {
for (i = 0; i < num_ent; i++) {
const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
p->revindex[i].offset = ntohl(hl);
p->revindex[i].nr = i;
}
}
/*
* This knows the pack format -- the hash trailer
* follows immediately after the last object data.
*/
p->revindex[num_ent].offset = p->pack_size - hashsz;
p->revindex[num_ent].nr = -1;
sort_revindex(p->revindex, num_ent, p->pack_size);
}
static int create_pack_revindex_in_memory(struct packed_git *p)
{
if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
die("dying as requested by '%s'",
GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
if (open_pack_index(p))
return -1;
create_pack_revindex(p);
return 0;
}
static char *pack_revindex_filename(struct packed_git *p)
{
size_t len;
if (!strip_suffix(p->pack_name, ".pack", &len))
BUG("pack_name does not end in .pack");
return xstrfmt("%.*s.rev", (int)len, p->pack_name);
}
#define RIDX_HEADER_SIZE (12)
#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
struct revindex_header {
uint32_t signature;
uint32_t version;
uint32_t hash_id;
};
static int load_revindex_from_disk(char *revindex_name,
uint32_t num_objects,
const uint32_t **data_p, size_t *len_p)
{
int fd, ret = 0;
struct stat st;
void *data = NULL;
size_t revindex_size;
struct revindex_header *hdr;
fd = git_open(revindex_name);
if (fd < 0) {
ret = -1;
goto cleanup;
}
if (fstat(fd, &st)) {
ret = error_errno(_("failed to read %s"), revindex_name);
goto cleanup;
}
revindex_size = xsize_t(st.st_size);
if (revindex_size < RIDX_MIN_SIZE) {
ret = error(_("reverse-index file %s is too small"), revindex_name);
goto cleanup;
}
if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
ret = error(_("reverse-index file %s is corrupt"), revindex_name);
goto cleanup;
}
data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
hdr = data;
if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
goto cleanup;
}
if (ntohl(hdr->version) != 1) {
ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
revindex_name, ntohl(hdr->version));
goto cleanup;
}
if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
revindex_name, ntohl(hdr->hash_id));
goto cleanup;
}
cleanup:
if (ret) {
if (data)
munmap(data, revindex_size);
} else {
*len_p = revindex_size;
*data_p = (const uint32_t *)data;
}
if (fd >= 0)
close(fd);
return ret;
}
static int load_pack_revindex_from_disk(struct packed_git *p)
{
char *revindex_name;
int ret;
if (open_pack_index(p))
return -1;
revindex_name = pack_revindex_filename(p);
ret = load_revindex_from_disk(revindex_name,
p->num_objects,
&p->revindex_map,
&p->revindex_size);
if (ret)
goto cleanup;
p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
cleanup:
free(revindex_name);
return ret;
}
int load_pack_revindex(struct packed_git *p)
{
if (p->revindex || p->revindex_data)
return 0;
if (!load_pack_revindex_from_disk(p))
return 0;
else if (!create_pack_revindex_in_memory(p))
return 0;
return -1;
}
int load_midx_revindex(struct multi_pack_index *m)
{
struct strbuf revindex_name = STRBUF_INIT;
int ret;
if (m->revindex_data)
return 0;
if (m->chunk_revindex) {
/*
* If the MIDX `m` has a `RIDX` chunk, then use its contents for
* the reverse index instead of trying to load a separate `.rev`
* file.
*
* Note that we do *not* set `m->revindex_map` here, since we do
* not want to accidentally call munmap() in the middle of the
* MIDX.
*/
trace2_data_string("load_midx_revindex", the_repository,
"source", "midx");
m->revindex_data = (const uint32_t *)m->chunk_revindex;
return 0;
}
trace2_data_string("load_midx_revindex", the_repository,
"source", "rev");
get_midx_rev_filename(&revindex_name, m);
ret = load_revindex_from_disk(revindex_name.buf,
m->num_objects,
&m->revindex_map,
&m->revindex_len);
if (ret)
goto cleanup;
m->revindex_data = (const uint32_t *)((const char *)m->revindex_map + RIDX_HEADER_SIZE);
cleanup:
strbuf_release(&revindex_name);
return ret;
}
int close_midx_revindex(struct multi_pack_index *m)
{
if (!m || !m->revindex_map)
return 0;
munmap((void*)m->revindex_map, m->revindex_len);
m->revindex_map = NULL;
m->revindex_data = NULL;
m->revindex_len = 0;
return 0;
}
int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
{
unsigned lo, hi;
if (load_pack_revindex(p) < 0)
return -1;
lo = 0;
hi = p->num_objects + 1;
do {
const unsigned mi = lo + (hi - lo) / 2;
off_t got = pack_pos_to_offset(p, mi);
if (got == ofs) {
*pos = mi;
return 0;
} else if (ofs < got)
hi = mi;
else
lo = mi + 1;
} while (lo < hi);
error("bad offset for revindex");
return -1;
}
uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
{
if (!(p->revindex || p->revindex_data))
BUG("pack_pos_to_index: reverse index not yet loaded");
if (p->num_objects <= pos)
BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
if (p->revindex)
return p->revindex[pos].nr;
else
return get_be32(p->revindex_data + pos);
}
off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
{
if (!(p->revindex || p->revindex_data))
BUG("pack_pos_to_index: reverse index not yet loaded");
if (p->num_objects < pos)
BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
if (p->revindex)
return p->revindex[pos].offset;
else if (pos == p->num_objects)
return p->pack_size - the_hash_algo->rawsz;
else
return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
}
uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos)
{
if (!m->revindex_data)
BUG("pack_pos_to_midx: reverse index not yet loaded");
if (m->num_objects <= pos)
BUG("pack_pos_to_midx: out-of-bounds object at %"PRIu32, pos);
return get_be32(m->revindex_data + pos);
}
struct midx_pack_key {
uint32_t pack;
off_t offset;
uint32_t preferred_pack;
struct multi_pack_index *midx;
};
static int midx_pack_order_cmp(const void *va, const void *vb)
{
const struct midx_pack_key *key = va;
struct multi_pack_index *midx = key->midx;
uint32_t versus = pack_pos_to_midx(midx, (uint32_t*)vb - (const uint32_t *)midx->revindex_data);
uint32_t versus_pack = nth_midxed_pack_int_id(midx, versus);
off_t versus_offset;
uint32_t key_preferred = key->pack == key->preferred_pack;
uint32_t versus_preferred = versus_pack == key->preferred_pack;
/*
* First, compare the preferred-ness, noting that the preferred pack
* comes first.
*/
if (key_preferred && !versus_preferred)
return -1;
else if (!key_preferred && versus_preferred)
return 1;
/* Then, break ties first by comparing the pack IDs. */
if (key->pack < versus_pack)
return -1;
else if (key->pack > versus_pack)
return 1;
/* Finally, break ties by comparing offsets within a pack. */
versus_offset = nth_midxed_offset(midx, versus);
if (key->offset < versus_offset)
return -1;
else if (key->offset > versus_offset)
return 1;
return 0;
}
int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
{
struct midx_pack_key key;
uint32_t *found;
if (!m->revindex_data)
BUG("midx_to_pack_pos: reverse index not yet loaded");
if (m->num_objects <= at)
BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
key.pack = nth_midxed_pack_int_id(m, at);
key.offset = nth_midxed_offset(m, at);
key.midx = m;
/*
* The preferred pack sorts first, so determine its identifier by
* looking at the first object in pseudo-pack order.
*
* Note that if no --preferred-pack is explicitly given when writing a
* multi-pack index, then whichever pack has the lowest identifier
* implicitly is preferred (and includes all its objects, since ties are
* broken first by pack identifier).
*/
key.preferred_pack = nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
found = bsearch(&key, m->revindex_data, m->num_objects,
sizeof(*m->revindex_data), midx_pack_order_cmp);
if (!found)
return error("bad offset for revindex");
*pos = found - m->revindex_data;
return 0;
}