read_packed_refs(): ensure that references are ordered when read

It doesn't actually matter now, because the references are only
iterated over to fill the associated `ref_cache`, which itself puts
them in the correct order. But we want to get rid of the `ref_cache`,
so we want to be able to iterate directly over the `packed-refs`
buffer, and then the iteration will need to be ordered correctly.

In fact, we already write the `packed-refs` file sorted, but it is
possible that other Git clients don't get it right. So let's not
assume that a `packed-refs` file is sorted unless it is explicitly
declared to be so via a `sorted` trait in its header line.

If it is *not* declared to be sorted, then scan quickly through the
file to check. If it is found to be out of order, then sort the
records into a new memory-only copy. This checking and sorting is done
quickly, without parsing the full file contents. However, it needs a
little bit of care to avoid reading past the end of the buffer even if
the `packed-refs` file is corrupt.

Since *we* always write the file correctly sorted, include that trait
when we write or rewrite a `packed-refs` file. This means that the
scan described in the previous paragraph should only have to be done
for `packed-refs` files that were written by older versions of the Git
command-line client, or by other clients that haven't yet learned to
write the `sorted` trait.

If `packed-refs` was already sorted, then (if the system allows it) we
can use the mmapped file contents directly. But if the system doesn't
allow a file that is currently mmapped to be replaced using
`rename()`, then it would be bad for us to keep the file mmapped for
any longer than necessary. So, on such systems, always make a copy of
the file contents, either as part of the sorting process, or
afterwards.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Michael Haggerty 2017-09-25 10:00:11 +02:00 committed by Junio C Hamano
parent 5b633610ec
commit 02b920f3f7

View file

@ -51,11 +51,12 @@ struct packed_ref_cache {
int mmapped;
/*
* The contents of the `packed-refs` file. If the file is
* mmapped, this points at the mmapped contents of the file.
* If not, this points at heap-allocated memory containing the
* contents. If there were no contents (e.g., because the file
* didn't exist), `buf` and `eof` are both NULL.
* The contents of the `packed-refs` file. If the file was
* already sorted, this points at the mmapped contents of the
* file. If not, this points at heap-allocated memory
* containing the contents, sorted. If there were no contents
* (e.g., because the file didn't exist), `buf` and `eof` are
* both NULL.
*/
char *buf, *eof;
@ -358,7 +359,7 @@ struct ref_iterator *mmapped_ref_iterator_begin(
if (!packed_refs->buf)
return empty_ref_iterator_begin();
base_ref_iterator_init(ref_iterator, &mmapped_ref_iterator_vtable, 0);
base_ref_iterator_init(ref_iterator, &mmapped_ref_iterator_vtable, 1);
iter->packed_refs = packed_refs;
acquire_packed_ref_cache(iter->packed_refs);
@ -371,6 +372,170 @@ struct ref_iterator *mmapped_ref_iterator_begin(
return ref_iterator;
}
struct packed_ref_entry {
const char *start;
size_t len;
};
static int cmp_packed_ref_entries(const void *v1, const void *v2)
{
const struct packed_ref_entry *e1 = v1, *e2 = v2;
const char *r1 = e1->start + GIT_SHA1_HEXSZ + 1;
const char *r2 = e2->start + GIT_SHA1_HEXSZ + 1;
while (1) {
if (*r1 == '\n')
return *r2 == '\n' ? 0 : -1;
if (*r1 != *r2) {
if (*r2 == '\n')
return 1;
else
return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
}
r1++;
r2++;
}
}
/*
* `packed_refs->buf` is not known to be sorted. Check whether it is,
* and if not, sort it into new memory and munmap/free the old
* storage.
*/
static void sort_packed_refs(struct packed_ref_cache *packed_refs)
{
struct packed_ref_entry *entries = NULL;
size_t alloc = 0, nr = 0;
int sorted = 1;
const char *pos, *eof, *eol;
size_t len, i;
char *new_buffer, *dst;
pos = packed_refs->buf + packed_refs->header_len;
eof = packed_refs->eof;
len = eof - pos;
if (!len)
return;
/*
* Initialize entries based on a crude estimate of the number
* of references in the file (we'll grow it below if needed):
*/
ALLOC_GROW(entries, len / 80 + 20, alloc);
while (pos < eof) {
eol = memchr(pos, '\n', eof - pos);
if (!eol)
/* The safety check should prevent this. */
BUG("unterminated line found in packed-refs");
if (eol - pos < GIT_SHA1_HEXSZ + 2)
die_invalid_line(packed_refs->refs->path,
pos, eof - pos);
eol++;
if (eol < eof && *eol == '^') {
/*
* Keep any peeled line together with its
* reference:
*/
const char *peeled_start = eol;
eol = memchr(peeled_start, '\n', eof - peeled_start);
if (!eol)
/* The safety check should prevent this. */
BUG("unterminated peeled line found in packed-refs");
eol++;
}
ALLOC_GROW(entries, nr + 1, alloc);
entries[nr].start = pos;
entries[nr].len = eol - pos;
nr++;
if (sorted &&
nr > 1 &&
cmp_packed_ref_entries(&entries[nr - 2],
&entries[nr - 1]) >= 0)
sorted = 0;
pos = eol;
}
if (sorted)
goto cleanup;
/* We need to sort the memory. First we sort the entries array: */
QSORT(entries, nr, cmp_packed_ref_entries);
/*
* Allocate a new chunk of memory, and copy the old memory to
* the new in the order indicated by `entries` (not bothering
* with the header line):
*/
new_buffer = xmalloc(len);
for (dst = new_buffer, i = 0; i < nr; i++) {
memcpy(dst, entries[i].start, entries[i].len);
dst += entries[i].len;
}
/*
* Now munmap the old buffer and use the sorted buffer in its
* place:
*/
release_packed_ref_buffer(packed_refs);
packed_refs->buf = new_buffer;
packed_refs->eof = new_buffer + len;
packed_refs->header_len = 0;
cleanup:
free(entries);
}
/*
* Return a pointer to the start of the record that contains the
* character `*p` (which must be within the buffer). If no other
* record start is found, return `buf`.
*/
static const char *find_start_of_record(const char *buf, const char *p)
{
while (p > buf && (p[-1] != '\n' || p[0] == '^'))
p--;
return p;
}
/*
* We want to be able to compare mmapped reference records quickly,
* without totally parsing them. We can do so because the records are
* LF-terminated, and the refname should start exactly (GIT_SHA1_HEXSZ
* + 1) bytes past the beginning of the record.
*
* But what if the `packed-refs` file contains garbage? We're willing
* to tolerate not detecting the problem, as long as we don't produce
* totally garbled output (we can't afford to check the integrity of
* the whole file during every Git invocation). But we do want to be
* sure that we never read past the end of the buffer in memory and
* perform an illegal memory access.
*
* Guarantee that minimum level of safety by verifying that the last
* record in the file is LF-terminated, and that it has at least
* (GIT_SHA1_HEXSZ + 1) characters before the LF. Die if either of
* these checks fails.
*/
static void verify_buffer_safe(struct packed_ref_cache *packed_refs)
{
const char *buf = packed_refs->buf + packed_refs->header_len;
const char *eof = packed_refs->eof;
const char *last_line;
if (buf == eof)
return;
last_line = find_start_of_record(buf, eof - 1);
if (*(eof - 1) != '\n' || eof - last_line < GIT_SHA1_HEXSZ + 2)
die_invalid_line(packed_refs->refs->path,
last_line, eof - last_line);
}
/*
* Depending on `mmap_strategy`, either mmap or read the contents of
* the `packed-refs` file into the `packed_refs` instance. Return 1 if
@ -408,7 +573,6 @@ static int load_contents(struct packed_ref_cache *packed_refs)
switch (mmap_strategy) {
case MMAP_NONE:
case MMAP_TEMPORARY:
packed_refs->buf = xmalloc(size);
bytes_read = read_in_full(fd, packed_refs->buf, size);
if (bytes_read < 0 || bytes_read != size)
@ -416,6 +580,7 @@ static int load_contents(struct packed_ref_cache *packed_refs)
packed_refs->eof = packed_refs->buf + size;
packed_refs->mmapped = 0;
break;
case MMAP_TEMPORARY:
case MMAP_OK:
packed_refs->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
packed_refs->eof = packed_refs->buf + size;
@ -435,19 +600,19 @@ static int load_contents(struct packed_ref_cache *packed_refs)
* A comment line of the form "# pack-refs with: " may contain zero or
* more traits. We interpret the traits as follows:
*
* No traits:
* Neither `peeled` nor `fully-peeled`:
*
* Probably no references are peeled. But if the file contains a
* peeled value for a reference, we will use it.
*
* peeled:
* `peeled`:
*
* References under "refs/tags/", if they *can* be peeled, *are*
* peeled in this file. References outside of "refs/tags/" are
* probably not peeled even if they could have been, but if we find
* a peeled value for such a reference we will use it.
*
* fully-peeled:
* `fully-peeled`:
*
* All references in the file that can be peeled are peeled.
* Inversely (and this is more important), any references in the
@ -455,12 +620,17 @@ static int load_contents(struct packed_ref_cache *packed_refs)
* trait should typically be written alongside "peeled" for
* compatibility with older clients, but we do not require it
* (i.e., "peeled" is a no-op if "fully-peeled" is set).
*
* `sorted`:
*
* The references in this file are known to be sorted by refname.
*/
static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
{
struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
struct ref_dir *dir;
struct ref_iterator *iter;
int sorted = 0;
int ok;
packed_refs->refs = refs;
@ -499,6 +669,9 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
packed_refs->peeled = PEELED_FULLY;
else if (unsorted_string_list_has_string(&traits, "peeled"))
packed_refs->peeled = PEELED_TAGS;
sorted = unsorted_string_list_has_string(&traits, "sorted");
/* perhaps other traits later as well */
/* The "+ 1" is for the LF character. */
@ -508,6 +681,34 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
strbuf_release(&tmp);
}
verify_buffer_safe(packed_refs);
if (!sorted) {
sort_packed_refs(packed_refs);
/*
* Reordering the records might have moved a short one
* to the end of the buffer, so verify the buffer's
* safety again:
*/
verify_buffer_safe(packed_refs);
}
if (mmap_strategy != MMAP_OK && packed_refs->mmapped) {
/*
* We don't want to leave the file mmapped, so we are
* forced to make a copy now:
*/
size_t size = packed_refs->eof -
(packed_refs->buf + packed_refs->header_len);
char *buf_copy = xmalloc(size);
memcpy(buf_copy, packed_refs->buf + packed_refs->header_len, size);
release_packed_ref_buffer(packed_refs);
packed_refs->buf = buf_copy;
packed_refs->eof = buf_copy + size;
}
dir = get_ref_dir(packed_refs->cache->root);
iter = mmapped_ref_iterator_begin(
packed_refs,
@ -811,7 +1012,7 @@ int packed_refs_is_locked(struct ref_store *ref_store)
* the colon and the trailing space are required.
*/
static const char PACKED_REFS_HEADER[] =
"# pack-refs with: peeled fully-peeled \n";
"# pack-refs with: peeled fully-peeled sorted \n";
static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
{