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@xxxxxxxxxxxx>
---
refs/packed-backend.c | 223 +++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 212 insertions(+), 11 deletions(-)
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4981516f1e..f336690f16 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -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
@@ -407,7 +572,6 @@ 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)
@@ -415,6 +579,7 @@ 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;
@@ -434,19 +599,19 @@ 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
@@ -454,12 +619,17 @@ 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;
@@ -498,6 +668,9 @@ 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. */
@@ -507,6 +680,34 @@ 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,
@@ -810,7 +1011,7 @@ 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)
{
--
2.14.1
