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![Taylor Blau](/assets/img/avatar_default.png)
When an on-disk reverse index exists, there is no need to generate one in memory. In fact, doing so can be slow, and require large amounts of the heap. Let's make sure that we treat the on-disk reverse index with precedence (i.e., that when it exists, we don't bother trying to generate an equivalent one in memory) by teaching Git how to conditionally die() when generating a reverse index in memory. Then, add a test to ensure that when (a) an on-disk reverse index exists, and (b) when setting GIT_TEST_REV_INDEX_DIE_IN_MEMORY, that we do not die, implying that we read from the on-disk one. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
349 lines
9.2 KiB
C
349 lines
9.2 KiB
C
#include "cache.h"
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#include "pack-revindex.h"
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#include "object-store.h"
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#include "packfile.h"
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#include "config.h"
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struct revindex_entry {
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off_t offset;
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unsigned int nr;
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};
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/*
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* Pack index for existing packs give us easy access to the offsets into
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* corresponding pack file where each object's data starts, but the entries
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* do not store the size of the compressed representation (uncompressed
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* size is easily available by examining the pack entry header). It is
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* also rather expensive to find the sha1 for an object given its offset.
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*
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* The pack index file is sorted by object name mapping to offset;
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* this revindex array is a list of offset/index_nr pairs
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* ordered by offset, so if you know the offset of an object, next offset
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* is where its packed representation ends and the index_nr can be used to
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* get the object sha1 from the main index.
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*/
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/*
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* This is a least-significant-digit radix sort.
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*
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* It sorts each of the "n" items in "entries" by its offset field. The "max"
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* parameter must be at least as large as the largest offset in the array,
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* and lets us quit the sort early.
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*/
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static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
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{
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/*
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* We use a "digit" size of 16 bits. That keeps our memory
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* usage reasonable, and we can generally (for a 4G or smaller
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* packfile) quit after two rounds of radix-sorting.
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*/
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#define DIGIT_SIZE (16)
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#define BUCKETS (1 << DIGIT_SIZE)
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/*
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* We want to know the bucket that a[i] will go into when we are using
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* the digit that is N bits from the (least significant) end.
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*/
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#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
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/*
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* We need O(n) temporary storage. Rather than do an extra copy of the
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* partial results into "entries", we sort back and forth between the
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* real array and temporary storage. In each iteration of the loop, we
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* keep track of them with alias pointers, always sorting from "from"
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* to "to".
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*/
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struct revindex_entry *tmp, *from, *to;
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int bits;
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unsigned *pos;
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ALLOC_ARRAY(pos, BUCKETS);
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ALLOC_ARRAY(tmp, n);
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from = entries;
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to = tmp;
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/*
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* If (max >> bits) is zero, then we know that the radix digit we are
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* on (and any higher) will be zero for all entries, and our loop will
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* be a no-op, as everybody lands in the same zero-th bucket.
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*/
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for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
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unsigned i;
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memset(pos, 0, BUCKETS * sizeof(*pos));
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/*
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* We want pos[i] to store the index of the last element that
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* will go in bucket "i" (actually one past the last element).
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* To do this, we first count the items that will go in each
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* bucket, which gives us a relative offset from the last
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* bucket. We can then cumulatively add the index from the
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* previous bucket to get the true index.
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*/
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for (i = 0; i < n; i++)
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pos[BUCKET_FOR(from, i, bits)]++;
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for (i = 1; i < BUCKETS; i++)
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pos[i] += pos[i-1];
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/*
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* Now we can drop the elements into their correct buckets (in
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* our temporary array). We iterate the pos counter backwards
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* to avoid using an extra index to count up. And since we are
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* going backwards there, we must also go backwards through the
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* array itself, to keep the sort stable.
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*
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* Note that we use an unsigned iterator to make sure we can
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* handle 2^32-1 objects, even on a 32-bit system. But this
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* means we cannot use the more obvious "i >= 0" loop condition
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* for counting backwards, and must instead check for
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* wrap-around with UINT_MAX.
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*/
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for (i = n - 1; i != UINT_MAX; i--)
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to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
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/*
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* Now "to" contains the most sorted list, so we swap "from" and
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* "to" for the next iteration.
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*/
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SWAP(from, to);
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}
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/*
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* If we ended with our data in the original array, great. If not,
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* we have to move it back from the temporary storage.
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*/
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if (from != entries)
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COPY_ARRAY(entries, tmp, n);
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free(tmp);
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free(pos);
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#undef BUCKET_FOR
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#undef BUCKETS
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#undef DIGIT_SIZE
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}
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/*
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* Ordered list of offsets of objects in the pack.
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*/
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static void create_pack_revindex(struct packed_git *p)
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{
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const unsigned num_ent = p->num_objects;
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unsigned i;
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const char *index = p->index_data;
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const unsigned hashsz = the_hash_algo->rawsz;
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ALLOC_ARRAY(p->revindex, num_ent + 1);
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index += 4 * 256;
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if (p->index_version > 1) {
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const uint32_t *off_32 =
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(uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
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const uint32_t *off_64 = off_32 + p->num_objects;
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for (i = 0; i < num_ent; i++) {
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const uint32_t off = ntohl(*off_32++);
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if (!(off & 0x80000000)) {
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p->revindex[i].offset = off;
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} else {
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p->revindex[i].offset = get_be64(off_64);
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off_64 += 2;
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}
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p->revindex[i].nr = i;
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}
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} else {
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for (i = 0; i < num_ent; i++) {
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const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
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p->revindex[i].offset = ntohl(hl);
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p->revindex[i].nr = i;
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}
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}
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/*
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* This knows the pack format -- the hash trailer
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* follows immediately after the last object data.
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*/
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p->revindex[num_ent].offset = p->pack_size - hashsz;
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p->revindex[num_ent].nr = -1;
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sort_revindex(p->revindex, num_ent, p->pack_size);
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}
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static int create_pack_revindex_in_memory(struct packed_git *p)
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{
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if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
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die("dying as requested by '%s'",
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GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
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if (open_pack_index(p))
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return -1;
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create_pack_revindex(p);
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return 0;
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}
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static char *pack_revindex_filename(struct packed_git *p)
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{
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size_t len;
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if (!strip_suffix(p->pack_name, ".pack", &len))
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BUG("pack_name does not end in .pack");
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return xstrfmt("%.*s.rev", (int)len, p->pack_name);
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}
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#define RIDX_HEADER_SIZE (12)
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#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
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struct revindex_header {
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uint32_t signature;
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uint32_t version;
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uint32_t hash_id;
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};
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static int load_revindex_from_disk(char *revindex_name,
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uint32_t num_objects,
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const uint32_t **data_p, size_t *len_p)
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{
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int fd, ret = 0;
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struct stat st;
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void *data = NULL;
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size_t revindex_size;
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struct revindex_header *hdr;
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fd = git_open(revindex_name);
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if (fd < 0) {
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ret = -1;
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goto cleanup;
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}
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if (fstat(fd, &st)) {
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ret = error_errno(_("failed to read %s"), revindex_name);
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goto cleanup;
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}
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revindex_size = xsize_t(st.st_size);
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if (revindex_size < RIDX_MIN_SIZE) {
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ret = error(_("reverse-index file %s is too small"), revindex_name);
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goto cleanup;
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}
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if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
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ret = error(_("reverse-index file %s is corrupt"), revindex_name);
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goto cleanup;
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}
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data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
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hdr = data;
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if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
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ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
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goto cleanup;
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}
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if (ntohl(hdr->version) != 1) {
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ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
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revindex_name, ntohl(hdr->version));
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goto cleanup;
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}
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if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
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ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
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revindex_name, ntohl(hdr->hash_id));
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goto cleanup;
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}
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cleanup:
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if (ret) {
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if (data)
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munmap(data, revindex_size);
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} else {
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*len_p = revindex_size;
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*data_p = (const uint32_t *)data;
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}
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close(fd);
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return ret;
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}
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static int load_pack_revindex_from_disk(struct packed_git *p)
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{
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char *revindex_name;
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int ret;
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if (open_pack_index(p))
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return -1;
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revindex_name = pack_revindex_filename(p);
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ret = load_revindex_from_disk(revindex_name,
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p->num_objects,
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&p->revindex_map,
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&p->revindex_size);
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if (ret)
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goto cleanup;
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p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
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cleanup:
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free(revindex_name);
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return ret;
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}
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int load_pack_revindex(struct packed_git *p)
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{
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if (p->revindex || p->revindex_data)
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return 0;
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if (!load_pack_revindex_from_disk(p))
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return 0;
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else if (!create_pack_revindex_in_memory(p))
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return 0;
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return -1;
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}
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int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
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{
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unsigned lo, hi;
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if (load_pack_revindex(p) < 0)
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return -1;
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lo = 0;
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hi = p->num_objects + 1;
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do {
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const unsigned mi = lo + (hi - lo) / 2;
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off_t got = pack_pos_to_offset(p, mi);
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if (got == ofs) {
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*pos = mi;
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return 0;
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} else if (ofs < got)
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hi = mi;
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else
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lo = mi + 1;
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} while (lo < hi);
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error("bad offset for revindex");
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return -1;
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}
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uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
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{
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if (!(p->revindex || p->revindex_data))
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BUG("pack_pos_to_index: reverse index not yet loaded");
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if (p->num_objects <= pos)
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BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
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if (p->revindex)
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return p->revindex[pos].nr;
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else
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return get_be32(p->revindex_data + pos);
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}
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off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
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{
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if (!(p->revindex || p->revindex_data))
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BUG("pack_pos_to_index: reverse index not yet loaded");
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if (p->num_objects < pos)
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BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
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if (p->revindex)
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return p->revindex[pos].offset;
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else if (pos == p->num_objects)
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return p->pack_size - the_hash_algo->rawsz;
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else
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return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
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}
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