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![Jeff King](/assets/img/avatar_default.png)
When an object lookup fails, we re-read the objects/pack directory to pick up any new packfiles that may have been created since our last read. We also discard any pack revindex structs we've allocated. The discarding is a problem for the pack-bitmap code, which keeps a pointer to the revindex for the bitmapped pack. After the discard, the pointer is invalid, and we may read free()d memory. Other revindex users do not keep a bare pointer to the revindex; instead, they always access it through revindex_for_pack(), which lazily builds the revindex. So one solution is to teach the pack-bitmap code a similar trick. It would be slightly less efficient, but probably not all that noticeable. However, it turns out this discarding is not actually necessary. When we call reprepare_packed_git, we do not throw away our old pack list. We keep the existing entries, and only add in new ones. So there is no safety problem; we will still have the pack struct that matches each revindex. The packfile itself may go away, of course, but we are already prepared to handle that, and it may happen outside of reprepare_packed_git anyway. Throwing away the revindex may save some RAM if the pack never gets reused (about 12 bytes per object). But it also wastes some CPU time (to regenerate the index) if the pack does get reused. It's hard to say which is more valuable, but in either case, it happens very rarely (only when we race with a simultaneous repack). Just leaving the revindex in place is simple and safe both for current and future code. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
248 lines
7 KiB
C
248 lines
7 KiB
C
#include "cache.h"
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#include "pack-revindex.h"
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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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* We build a hashtable of existing packs (pack_revindex), and keep reverse
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* index here -- pack index file is sorted by object name mapping to offset;
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* this pack_revindex[].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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static struct pack_revindex *pack_revindex;
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static int pack_revindex_hashsz;
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static int pack_revindex_ix(struct packed_git *p)
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{
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unsigned long ui = (unsigned long)p;
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int i;
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ui = ui ^ (ui >> 16); /* defeat structure alignment */
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i = (int)(ui % pack_revindex_hashsz);
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while (pack_revindex[i].p) {
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if (pack_revindex[i].p == p)
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return i;
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if (++i == pack_revindex_hashsz)
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i = 0;
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}
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return -1 - i;
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}
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static void init_pack_revindex(void)
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{
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int num;
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struct packed_git *p;
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for (num = 0, p = packed_git; p; p = p->next)
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num++;
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if (!num)
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return;
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pack_revindex_hashsz = num * 11;
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pack_revindex = xcalloc(sizeof(*pack_revindex), pack_revindex_hashsz);
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for (p = packed_git; p; p = p->next) {
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num = pack_revindex_ix(p);
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num = - 1 - num;
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pack_revindex[num].p = p;
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}
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/* revindex elements are lazily initialized */
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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 = xmalloc(n * sizeof(*tmp));
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struct revindex_entry *from = entries, *to = tmp;
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int bits;
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unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
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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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struct revindex_entry *swap;
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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;
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from = to;
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to = swap;
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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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memcpy(entries, tmp, n * sizeof(*entries));
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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 pack_revindex *rix)
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{
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struct packed_git *p = rix->p;
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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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rix->revindex = xmalloc(sizeof(*rix->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 + p->num_objects * (20 + 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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uint32_t off = ntohl(*off_32++);
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if (!(off & 0x80000000)) {
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rix->revindex[i].offset = off;
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} else {
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rix->revindex[i].offset =
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((uint64_t)ntohl(*off_64++)) << 32;
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rix->revindex[i].offset |=
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ntohl(*off_64++);
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}
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rix->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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uint32_t hl = *((uint32_t *)(index + 24 * i));
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rix->revindex[i].offset = ntohl(hl);
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rix->revindex[i].nr = i;
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}
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}
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/* This knows the pack format -- the 20-byte trailer
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* follows immediately after the last object data.
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*/
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rix->revindex[num_ent].offset = p->pack_size - 20;
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rix->revindex[num_ent].nr = -1;
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sort_revindex(rix->revindex, num_ent, p->pack_size);
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}
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struct pack_revindex *revindex_for_pack(struct packed_git *p)
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{
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int num;
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struct pack_revindex *rix;
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if (!pack_revindex_hashsz)
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init_pack_revindex();
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num = pack_revindex_ix(p);
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if (num < 0)
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die("internal error: pack revindex fubar");
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rix = &pack_revindex[num];
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if (!rix->revindex)
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create_pack_revindex(rix);
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return rix;
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}
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int find_revindex_position(struct pack_revindex *pridx, off_t ofs)
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{
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int lo = 0;
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int hi = pridx->p->num_objects + 1;
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struct revindex_entry *revindex = pridx->revindex;
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do {
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unsigned mi = lo + (hi - lo) / 2;
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if (revindex[mi].offset == ofs) {
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return mi;
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} else if (ofs < revindex[mi].offset)
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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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struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
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{
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struct pack_revindex *pridx = revindex_for_pack(p);
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int pos = find_revindex_position(pridx, ofs);
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if (pos < 0)
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return NULL;
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return pridx->revindex + pos;
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}
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