packed-backend.c: rename a bunch of things and update comments

We've made huge changes to this file, and some of the old names and
comments are no longer very fitting. So rename a bunch of things:

* `struct packed_ref_cache` → `struct snapshot`
* `acquire_packed_ref_cache()` → `acquire_snapshot()`
* `release_packed_ref_buffer()` → `clear_snapshot_buffer()`
* `release_packed_ref_cache()` → `release_snapshot()`
* `clear_packed_ref_cache()` → `clear_snapshot()`
* `struct packed_ref_entry` → `struct snapshot_record`
* `cmp_packed_ref_entries()` → `cmp_packed_ref_records()`
* `cmp_entry_to_refname()` → `cmp_record_to_refname()`
* `sort_packed_refs()` → `sort_snapshot()`
* `read_packed_refs()` → `create_snapshot()`
* `validate_packed_ref_cache()` → `validate_snapshot()`
* `get_packed_ref_cache()` → `get_snapshot()`
* Renamed local variables and struct members accordingly.

Also update a bunch of comments to reflect the renaming and the
accumulated changes that the code has undergone.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 1ed52d7eca6939e0bb023e06a5db0ea9841d481d..d500ebfaa5311fc37ae0b35144c8a6bd130b54be 100644 (file)
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -37,10 +37,30 @@ static enum mmap_strategy mmap_strategy = MMAP_OK;
 
 struct packed_ref_store;
 
 
 struct packed_ref_store;
 

-struct packed_ref_cache {
+/*
+ * A `snapshot` represents one snapshot of a `packed-refs` file.
+ *
+ * Normally, this will be a mmapped view of the contents of the
+ * `packed-refs` file at the time the snapshot was created. However,
+ * if the `packed-refs` file was not sorted, this might point at heap
+ * memory holding the contents of the `packed-refs` file with its
+ * records sorted by refname.
+ *
+ * `snapshot` instances are reference counted (via
+ * `acquire_snapshot()` and `release_snapshot()`). This is to prevent
+ * an instance from disappearing while an iterator is still iterating
+ * over it. Instances are garbage collected when their `referrers`
+ * count goes to zero.
+ *
+ * The most recent `snapshot`, if available, is referenced by the
+ * `packed_ref_store`. Its freshness is checked whenever
+ * `get_snapshot()` is called; if the existing snapshot is obsolete, a
+ * new snapshot is taken.
+ */
+struct snapshot {

        /*
         * A back-pointer to the packed_ref_store with which this
        /*
         * A back-pointer to the packed_ref_store with which this

-        * cache is associated:
+        * snapshot is associated:

         */
        struct packed_ref_store *refs;
 
         */
        struct packed_ref_store *refs;
 


@@ -61,26 +81,42 @@ struct packed_ref_cache {
        size_t header_len;
 
        /*
        size_t header_len;
 
        /*

-        * What is the peeled state of this cache? (This is usually
-        * determined from the header of the "packed-refs" file.)
+        * What is the peeled state of the `packed-refs` file that
+        * this snapshot represents? (This is usually determined from
+        * the file's header.)

         */
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled;
 
        /*
         */
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled;
 
        /*

-        * Count of references to the data structure in this instance,
-        * including the pointer from files_ref_store::packed if any.
-        * The data will not be freed as long as the reference count
-        * is nonzero.
+        * Count of references to this instance, including the pointer
+        * from `packed_ref_store::snapshot`, if any. The instance
+        * will not be freed as long as the reference count is
+        * nonzero.

         */
        unsigned int referrers;
 
         */
        unsigned int referrers;
 

-       /* The metadata from when this packed-refs cache was read */
+       /*
+        * The metadata of the `packed-refs` file from which this
+        * snapshot was created, used to tell if the file has been
+        * replaced since we read it.
+        */

        struct stat_validity validity;
 };
 
 /*
        struct stat_validity validity;
 };
 
 /*

- * A container for `packed-refs`-related data. It is not (yet) a
- * `ref_store`.
+ * A `ref_store` representing references stored in a `packed-refs`
+ * file. It implements the `ref_store` interface, though it has some
+ * limitations:
+ *
+ * - It cannot store symbolic references.
+ *
+ * - It cannot store reflogs.
+ *
+ * - It does not support reference renaming (though it could).
+ *
+ * On the other hand, it can be locked outside of a reference
+ * transaction. In that case, it remains locked even after the
+ * transaction is done and the new `packed-refs` file is activated.

  */
 struct packed_ref_store {
        struct ref_store base;
  */
 struct packed_ref_store {
        struct ref_store base;


@@ -91,10 +127,10 @@ struct packed_ref_store {
        char *path;
 
        /*
        char *path;
 
        /*

-        * A cache of the values read from the `packed-refs` file, if
-        * it might still be current; otherwise, NULL.
+        * A snapshot of the values read from the `packed-refs` file,
+        * if it might still be current; otherwise, NULL.

         */
         */

-       struct packed_ref_cache *cache;
+       struct snapshot *snapshot;

 
        /*
         * Lock used for the "packed-refs" file. Note that this (and
 
        /*
         * Lock used for the "packed-refs" file. Note that this (and


@@ -111,43 +147,42 @@ struct packed_ref_store {
 };
 
 /*
 };
 
 /*

- * Increment the reference count of *packed_refs.
+ * Increment the reference count of `*snapshot`.

  */
  */

-static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
+static void acquire_snapshot(struct snapshot *snapshot)

 {
 {

-       packed_refs->referrers++;
+       snapshot->referrers++;

 }
 
 /*
 }
 
 /*

- * If the buffer in `packed_refs` is active, then either munmap the
+ * If the buffer in `snapshot` is active, then either munmap the

  * memory and close the file, or free the memory. Then set the buffer
  * pointers to NULL.
  */
  * memory and close the file, or free the memory. Then set the buffer
  * pointers to NULL.
  */

-static void release_packed_ref_buffer(struct packed_ref_cache *packed_refs)
+static void clear_snapshot_buffer(struct snapshot *snapshot)

 {
 {

-       if (packed_refs->mmapped) {
-               if (munmap(packed_refs->buf,
-                          packed_refs->eof - packed_refs->buf))
+       if (snapshot->mmapped) {
+               if (munmap(snapshot->buf, snapshot->eof - snapshot->buf))

                        die_errno("error ummapping packed-refs file %s",
                        die_errno("error ummapping packed-refs file %s",

-                                 packed_refs->refs->path);
-               packed_refs->mmapped = 0;
+                                 snapshot->refs->path);
+               snapshot->mmapped = 0;

        } else {
        } else {

-               free(packed_refs->buf);
+               free(snapshot->buf);

        }
        }

-       packed_refs->buf = packed_refs->eof = NULL;
-       packed_refs->header_len = 0;
+       snapshot->buf = snapshot->eof = NULL;
+       snapshot->header_len = 0;

 }
 
 /*
 }
 
 /*

- * Decrease the reference count of *packed_refs.  If it goes to zero,
- * free *packed_refs and return true; otherwise return false.
+ * Decrease the reference count of `*snapshot`. If it goes to zero,
+ * free `*snapshot` and return true; otherwise return false.

  */
  */

-static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
+static int release_snapshot(struct snapshot *snapshot)

 {
 {

-       if (!--packed_refs->referrers) {
-               stat_validity_clear(&packed_refs->validity);
-               release_packed_ref_buffer(packed_refs);
-               free(packed_refs);
+       if (!--snapshot->referrers) {
+               stat_validity_clear(&snapshot->validity);
+               clear_snapshot_buffer(snapshot);
+               free(snapshot);

                return 1;
        } else {
                return 0;
                return 1;
        } else {
                return 0;


@@ -192,13 +227,13 @@ static struct packed_ref_store *packed_downcast(struct ref_store *ref_store,
        return refs;
 }
 
        return refs;
 }
 

-static void clear_packed_ref_cache(struct packed_ref_store *refs)
+static void clear_snapshot(struct packed_ref_store *refs)

 {
 {

-       if (refs->cache) {
-               struct packed_ref_cache *cache = refs->cache;
+       if (refs->snapshot) {
+               struct snapshot *snapshot = refs->snapshot;

 
 

-               refs->cache = NULL;
-               release_packed_ref_cache(cache);
+               refs->snapshot = NULL;
+               release_snapshot(snapshot);

        }
 }
 
        }
 }
 


@@ -225,14 +260,14 @@ static NORETURN void die_invalid_line(const char *path,
 
 }
 
 
 }
 

-struct packed_ref_entry {
+struct snapshot_record {

        const char *start;
        size_t len;
 };
 
        const char *start;
        size_t len;
 };
 

-static int cmp_packed_ref_entries(const void *v1, const void *v2)
+static int cmp_packed_ref_records(const void *v1, const void *v2)

 {
 {

-       const struct packed_ref_entry *e1 = v1, *e2 = v2;
+       const struct snapshot_record *e1 = v1, *e2 = v2;

        const char *r1 = e1->start + GIT_SHA1_HEXSZ + 1;
        const char *r2 = e2->start + GIT_SHA1_HEXSZ + 1;
 
        const char *r1 = e1->start + GIT_SHA1_HEXSZ + 1;
        const char *r2 = e2->start + GIT_SHA1_HEXSZ + 1;
 


@@ -251,10 +286,10 @@ static int cmp_packed_ref_entries(const void *v1, const void *v2)
 }
 
 /*
 }
 
 /*

- * Compare a packed-refs record pointed to by `rec` to the specified
- * NUL-terminated refname.
+ * Compare a snapshot record at `rec` to the specified NUL-terminated
+ * refname.

  */
  */

-static int cmp_entry_to_refname(const char *rec, const char *refname)
+static int cmp_record_to_refname(const char *rec, const char *refname)

 {
        const char *r1 = rec + GIT_SHA1_HEXSZ + 1;
        const char *r2 = refname;
 {
        const char *r1 = rec + GIT_SHA1_HEXSZ + 1;
        const char *r2 = refname;


@@ -272,31 +307,30 @@ static int cmp_entry_to_refname(const char *rec, const char *refname)
 }
 
 /*
 }
 
 /*

- * `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.
+ * `snapshot->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)
+static void sort_snapshot(struct snapshot *snapshot)

 {
 {

-       struct packed_ref_entry *entries = NULL;
+       struct snapshot_record *records = NULL;

        size_t alloc = 0, nr = 0;
        int sorted = 1;
        const char *pos, *eof, *eol;
        size_t len, i;
        char *new_buffer, *dst;
 
        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;
+       pos = snapshot->buf + snapshot->header_len;
+       eof = snapshot->eof;

        len = eof - pos;
 
        if (!len)
                return;
 
        /*
        len = eof - pos;
 
        if (!len)
                return;
 
        /*

-        * Initialize entries based on a crude estimate of the number
+        * Initialize records based on a crude estimate of the number

         * of references in the file (we'll grow it below if needed):
         */
         * of references in the file (we'll grow it below if needed):
         */

-       ALLOC_GROW(entries, len / 80 + 20, alloc);
+       ALLOC_GROW(records, len / 80 + 20, alloc);

 
        while (pos < eof) {
                eol = memchr(pos, '\n', eof - pos);
 
        while (pos < eof) {
                eol = memchr(pos, '\n', eof - pos);


@@ -304,7 +338,7 @@ static void sort_packed_refs(struct packed_ref_cache *packed_refs)
                        /* The safety check should prevent this. */
                        BUG("unterminated line found in packed-refs");
                if (eol - pos < GIT_SHA1_HEXSZ + 2)
                        /* 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,
+                       die_invalid_line(snapshot->refs->path,

                                         pos, eof - pos);
                eol++;
                if (eol < eof && *eol == '^') {
                                         pos, eof - pos);
                eol++;
                if (eol < eof && *eol == '^') {


@@ -321,15 +355,15 @@ static void sort_packed_refs(struct packed_ref_cache *packed_refs)
                        eol++;
                }
 
                        eol++;
                }
 

-               ALLOC_GROW(entries, nr + 1, alloc);
-               entries[nr].start = pos;
-               entries[nr].len = eol - pos;
+               ALLOC_GROW(records, nr + 1, alloc);
+               records[nr].start = pos;
+               records[nr].len = eol - pos;

                nr++;
 
                if (sorted &&
                    nr > 1 &&
                nr++;
 
                if (sorted &&
                    nr > 1 &&

-                   cmp_packed_ref_entries(&entries[nr - 2],
-                                          &entries[nr - 1]) >= 0)
+                   cmp_packed_ref_records(&records[nr - 2],
+                                          &records[nr - 1]) >= 0)

                        sorted = 0;
 
                pos = eol;
                        sorted = 0;
 
                pos = eol;


@@ -338,31 +372,31 @@ static void sort_packed_refs(struct packed_ref_cache *packed_refs)
        if (sorted)
                goto cleanup;
 
        if (sorted)
                goto cleanup;
 

-       /* We need to sort the memory. First we sort the entries array: */
-       QSORT(entries, nr, cmp_packed_ref_entries);
+       /* We need to sort the memory. First we sort the records array: */
+       QSORT(records, nr, cmp_packed_ref_records);

 
        /*
         * Allocate a new chunk of memory, and copy the old memory to
 
        /*
         * Allocate a new chunk of memory, and copy the old memory to

-        * the new in the order indicated by `entries` (not bothering
+        * the new in the order indicated by `records` (not bothering

         * with the header line):
         */
        new_buffer = xmalloc(len);
        for (dst = new_buffer, i = 0; i < nr; i++) {
         * 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;
+               memcpy(dst, records[i].start, records[i].len);
+               dst += records[i].len;

        }
 
        /*
         * Now munmap the old buffer and use the sorted buffer in its
         * place:
         */
        }
 
        /*
         * 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;
+       clear_snapshot_buffer(snapshot);
+       snapshot->buf = new_buffer;
+       snapshot->eof = new_buffer + len;
+       snapshot->header_len = 0;

 
 cleanup:
 
 cleanup:

-       free(entries);
+       free(records);

 }
 
 /*
 }
 
 /*


@@ -406,10 +440,10 @@ static const char *find_end_of_record(const char *p, const char *end)
  * (GIT_SHA1_HEXSZ + 1) characters before the LF. Die if either of
  * these checks fails.
  */
  * (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)
+static void verify_buffer_safe(struct snapshot *snapshot)

 {
 {

-       const char *buf = packed_refs->buf + packed_refs->header_len;
-       const char *eof = packed_refs->eof;
+       const char *buf = snapshot->buf + snapshot->header_len;
+       const char *eof = snapshot->eof;

        const char *last_line;
 
        if (buf == eof)
        const char *last_line;
 
        if (buf == eof)


@@ -417,24 +451,23 @@ static void verify_buffer_safe(struct packed_ref_cache *packed_refs)
 
        last_line = find_start_of_record(buf, eof - 1);
        if (*(eof - 1) != '\n' || eof - last_line < GIT_SHA1_HEXSZ + 2)
 
        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,
+               die_invalid_line(snapshot->refs->path,

                                 last_line, eof - last_line);
 }
 
 /*
  * Depending on `mmap_strategy`, either mmap or read the contents of
                                 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
- * the file existed and was read, or 0 if the file was absent. Die on
- * errors.
+ * the `packed-refs` file into the snapshot. Return 1 if the file
+ * existed and was read, or 0 if the file was absent. Die on errors.

  */
  */

-static int load_contents(struct packed_ref_cache *packed_refs)
+static int load_contents(struct snapshot *snapshot)

 {
        int fd;
        struct stat st;
        size_t size;
        ssize_t bytes_read;
 
 {
        int fd;
        struct stat st;
        size_t size;
        ssize_t bytes_read;
 

-       fd = open(packed_refs->refs->path, O_RDONLY);
+       fd = open(snapshot->refs->path, O_RDONLY);

        if (fd < 0) {
                if (errno == ENOENT) {
                        /*
        if (fd < 0) {
                if (errno == ENOENT) {
                        /*


@@ -446,30 +479,30 @@ static int load_contents(struct packed_ref_cache *packed_refs)
                         */
                        return 0;
                } else {
                         */
                        return 0;
                } else {

-                       die_errno("couldn't read %s", packed_refs->refs->path);
+                       die_errno("couldn't read %s", snapshot->refs->path);

                }
        }
 
                }
        }
 

-       stat_validity_update(&packed_refs->validity, fd);
+       stat_validity_update(&snapshot->validity, fd);

 
        if (fstat(fd, &st) < 0)
 
        if (fstat(fd, &st) < 0)

-               die_errno("couldn't stat %s", packed_refs->refs->path);
+               die_errno("couldn't stat %s", snapshot->refs->path);

        size = xsize_t(st.st_size);
 
        switch (mmap_strategy) {
        case MMAP_NONE:
        size = xsize_t(st.st_size);
 
        switch (mmap_strategy) {
        case MMAP_NONE:

-               packed_refs->buf = xmalloc(size);
-               bytes_read = read_in_full(fd, packed_refs->buf, size);
+               snapshot->buf = xmalloc(size);
+               bytes_read = read_in_full(fd, snapshot->buf, size);

                if (bytes_read < 0 || bytes_read != size)
                if (bytes_read < 0 || bytes_read != size)

-                       die_errno("couldn't read %s", packed_refs->refs->path);
-               packed_refs->eof = packed_refs->buf + size;
-               packed_refs->mmapped = 0;
+                       die_errno("couldn't read %s", snapshot->refs->path);
+               snapshot->eof = snapshot->buf + size;
+               snapshot->mmapped = 0;

                break;
        case MMAP_TEMPORARY:
        case MMAP_OK:
                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;
-               packed_refs->mmapped = 1;
+               snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+               snapshot->eof = snapshot->buf + size;
+               snapshot->mmapped = 1;

                break;
        }
        close(fd);
                break;
        }
        close(fd);


@@ -478,7 +511,7 @@ static int load_contents(struct packed_ref_cache *packed_refs)
 }
 
 /*
 }
 
 /*

- * Find the place in `cache->buf` where the start of the record for
+ * Find the place in `snapshot->buf` where the start of the record for

  * `refname` starts. If `mustexist` is true and the reference doesn't
  * exist, then return NULL. If `mustexist` is false and the reference
  * doesn't exist, then return the point where that reference would be
  * `refname` starts. If `mustexist` is true and the reference doesn't
  * exist, then return NULL. If `mustexist` is false and the reference
  * doesn't exist, then return the point where that reference would be


@@ -486,10 +519,10 @@ static int load_contents(struct packed_ref_cache *packed_refs)
  * reference name; for example, one could search for "refs/replace/"
  * to find the start of any replace references.
  *
  * reference name; for example, one could search for "refs/replace/"
  * to find the start of any replace references.
  *

- * The record is sought using a binary search, so `cache->buf` must be
- * sorted.
+ * The record is sought using a binary search, so `snapshot->buf` must
+ * be sorted.

  */
  */

-static const char *find_reference_location(struct packed_ref_cache *cache,
+static const char *find_reference_location(struct snapshot *snapshot,

                                           const char *refname, int mustexist)
 {
        /*
                                           const char *refname, int mustexist)
 {
        /*


@@ -506,13 +539,13 @@ static const char *find_reference_location(struct packed_ref_cache *cache,
         * preceding records all have reference names that come
         * *before* `refname`.
         */
         * preceding records all have reference names that come
         * *before* `refname`.
         */

-       const char *lo = cache->buf + cache->header_len;
+       const char *lo = snapshot->buf + snapshot->header_len;

 
        /*
         * A pointer to a the first character of a record whose
         * reference name comes *after* `refname`.
         */
 
        /*
         * A pointer to a the first character of a record whose
         * reference name comes *after* `refname`.
         */

-       const char *hi = cache->eof;
+       const char *hi = snapshot->eof;

 
        while (lo < hi) {
                const char *mid, *rec;
 
        while (lo < hi) {
                const char *mid, *rec;


@@ -520,7 +553,7 @@ static const char *find_reference_location(struct packed_ref_cache *cache,
 
                mid = lo + (hi - lo) / 2;
                rec = find_start_of_record(lo, mid);
 
                mid = lo + (hi - lo) / 2;
                rec = find_start_of_record(lo, mid);

-               cmp = cmp_entry_to_refname(rec, refname);
+               cmp = cmp_record_to_refname(rec, refname);

                if (cmp < 0) {
                        lo = find_end_of_record(mid, hi);
                } else if (cmp > 0) {
                if (cmp < 0) {
                        lo = find_end_of_record(mid, hi);
                } else if (cmp > 0) {


@@ -537,9 +570,9 @@ static const char *find_reference_location(struct packed_ref_cache *cache,
 }
 
 /*
 }
 
 /*

- * Read from the `packed-refs` file into a newly-allocated
- * `packed_ref_cache` and return it. The return value will already
- * have its reference count incremented.
+ * Create a newly-allocated `snapshot` of the `packed-refs` file in
+ * its current state and return it. The return value will already have
+ * its reference count incremented.

  *
  * A comment line of the form "# pack-refs with: " may contain zero or
  * more traits. We interpret the traits as follows:
  *
  * A comment line of the form "# pack-refs with: " may contain zero or
  * more traits. We interpret the traits as follows:


@@ -569,116 +602,117 @@ static const char *find_reference_location(struct packed_ref_cache *cache,
  *
  *      The references in this file are known to be sorted by refname.
  */
  *
  *      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)
+static struct snapshot *create_snapshot(struct packed_ref_store *refs)

 {
 {

-       struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
+       struct snapshot *snapshot = xcalloc(1, sizeof(*snapshot));

        int sorted = 0;
 
        int sorted = 0;
 

-       packed_refs->refs = refs;
-       acquire_packed_ref_cache(packed_refs);
-       packed_refs->peeled = PEELED_NONE;
+       snapshot->refs = refs;
+       acquire_snapshot(snapshot);
+       snapshot->peeled = PEELED_NONE;

 
 

-       if (!load_contents(packed_refs))
-               return packed_refs;
+       if (!load_contents(snapshot))
+               return snapshot;

 
        /* If the file has a header line, process it: */
 
        /* If the file has a header line, process it: */

-       if (packed_refs->buf < packed_refs->eof && *packed_refs->buf == '#') {
+       if (snapshot->buf < snapshot->eof && *snapshot->buf == '#') {

                struct strbuf tmp = STRBUF_INIT;
                char *p;
                const char *eol;
                struct string_list traits = STRING_LIST_INIT_NODUP;
 
                struct strbuf tmp = STRBUF_INIT;
                char *p;
                const char *eol;
                struct string_list traits = STRING_LIST_INIT_NODUP;
 

-               eol = memchr(packed_refs->buf, '\n',
-                            packed_refs->eof - packed_refs->buf);
+               eol = memchr(snapshot->buf, '\n',
+                            snapshot->eof - snapshot->buf);

                if (!eol)
                        die_unterminated_line(refs->path,
                if (!eol)
                        die_unterminated_line(refs->path,

-                                             packed_refs->buf,
-                                             packed_refs->eof - packed_refs->buf);
+                                             snapshot->buf,
+                                             snapshot->eof - snapshot->buf);

 
 

-               strbuf_add(&tmp, packed_refs->buf, eol - packed_refs->buf);
+               strbuf_add(&tmp, snapshot->buf, eol - snapshot->buf);

 
                if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
                        die_invalid_line(refs->path,
 
                if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
                        die_invalid_line(refs->path,

-                                        packed_refs->buf,
-                                        packed_refs->eof - packed_refs->buf);
+                                        snapshot->buf,
+                                        snapshot->eof - snapshot->buf);

 
                string_list_split_in_place(&traits, p, ' ', -1);
 
                if (unsorted_string_list_has_string(&traits, "fully-peeled"))
 
                string_list_split_in_place(&traits, p, ' ', -1);
 
                if (unsorted_string_list_has_string(&traits, "fully-peeled"))

-                       packed_refs->peeled = PEELED_FULLY;
+                       snapshot->peeled = PEELED_FULLY;

                else if (unsorted_string_list_has_string(&traits, "peeled"))
                else if (unsorted_string_list_has_string(&traits, "peeled"))

-                       packed_refs->peeled = PEELED_TAGS;
+                       snapshot->peeled = PEELED_TAGS;

 
                sorted = unsorted_string_list_has_string(&traits, "sorted");
 
                /* perhaps other traits later as well */
 
                /* The "+ 1" is for the LF character. */
 
                sorted = unsorted_string_list_has_string(&traits, "sorted");
 
                /* perhaps other traits later as well */
 
                /* The "+ 1" is for the LF character. */

-               packed_refs->header_len = eol + 1 - packed_refs->buf;
+               snapshot->header_len = eol + 1 - snapshot->buf;

 
                string_list_clear(&traits, 0);
                strbuf_release(&tmp);
        }
 
 
                string_list_clear(&traits, 0);
                strbuf_release(&tmp);
        }
 

-       verify_buffer_safe(packed_refs);
+       verify_buffer_safe(snapshot);

 
        if (!sorted) {
 
        if (!sorted) {

-               sort_packed_refs(packed_refs);
+               sort_snapshot(snapshot);

 
                /*
                 * Reordering the records might have moved a short one
                 * to the end of the buffer, so verify the buffer's
                 * safety again:
                 */
 
                /*
                 * 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);
+               verify_buffer_safe(snapshot);

        }
 
        }
 

-       if (mmap_strategy != MMAP_OK && packed_refs->mmapped) {
+       if (mmap_strategy != MMAP_OK && snapshot->mmapped) {

                /*
                 * We don't want to leave the file mmapped, so we are
                 * forced to make a copy now:
                 */
                /*
                 * 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);
+               size_t size = snapshot->eof -
+                       (snapshot->buf + snapshot->header_len);

                char *buf_copy = xmalloc(size);
 
                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;
+               memcpy(buf_copy, snapshot->buf + snapshot->header_len, size);
+               clear_snapshot_buffer(snapshot);
+               snapshot->buf = buf_copy;
+               snapshot->eof = buf_copy + size;

        }
 
        }
 

-       return packed_refs;
+       return snapshot;

 }
 
 /*
 }
 
 /*

- * Check that the packed refs cache (if any) still reflects the
- * contents of the file. If not, clear the cache.
+ * Check that `refs->snapshot` (if present) still reflects the
+ * contents of the `packed-refs` file. If not, clear the snapshot.

  */
  */

-static void validate_packed_ref_cache(struct packed_ref_store *refs)
+static void validate_snapshot(struct packed_ref_store *refs)

 {
 {

-       if (refs->cache &&
-           !stat_validity_check(&refs->cache->validity, refs->path))
-               clear_packed_ref_cache(refs);
+       if (refs->snapshot &&
+           !stat_validity_check(&refs->snapshot->validity, refs->path))
+               clear_snapshot(refs);

 }
 
 /*
 }
 
 /*

- * Get the packed_ref_cache for the specified packed_ref_store,
- * creating and populating it if it hasn't been read before or if the
- * file has been changed (according to its `validity` field) since it
- * was last read. On the other hand, if we hold the lock, then assume
- * that the file hasn't been changed out from under us, so skip the
- * extra `stat()` call in `stat_validity_check()`.
+ * Get the `snapshot` for the specified packed_ref_store, creating and
+ * populating it if it hasn't been read before or if the file has been
+ * changed (according to its `validity` field) since it was last read.
+ * On the other hand, if we hold the lock, then assume that the file
+ * hasn't been changed out from under us, so skip the extra `stat()`
+ * call in `stat_validity_check()`. This function does *not* increase
+ * the snapshot's reference count on behalf of the caller.

  */
  */

-static struct packed_ref_cache *get_packed_ref_cache(struct packed_ref_store *refs)
+static struct snapshot *get_snapshot(struct packed_ref_store *refs)

 {
        if (!is_lock_file_locked(&refs->lock))
 {
        if (!is_lock_file_locked(&refs->lock))

-               validate_packed_ref_cache(refs);
+               validate_snapshot(refs);

 
 

-       if (!refs->cache)
-               refs->cache = read_packed_refs(refs);
+       if (!refs->snapshot)
+               refs->snapshot = create_snapshot(refs);

 
 

-       return refs->cache;
+       return refs->snapshot;

 }
 
 static int packed_read_raw_ref(struct ref_store *ref_store,
 }
 
 static int packed_read_raw_ref(struct ref_store *ref_store,


@@ -687,12 +721,12 @@ static int packed_read_raw_ref(struct ref_store *ref_store,
 {
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
 {
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");

-       struct packed_ref_cache *packed_refs = get_packed_ref_cache(refs);
+       struct snapshot *snapshot = get_snapshot(refs);

        const char *rec;
 
        *type = 0;
 
        const char *rec;
 
        *type = 0;
 

-       rec = find_reference_location(packed_refs, refname, 1);
+       rec = find_reference_location(snapshot, refname, 1);

 
        if (!rec) {
                /* refname is not a packed reference. */
 
        if (!rec) {
                /* refname is not a packed reference. */


@@ -701,7 +735,7 @@ static int packed_read_raw_ref(struct ref_store *ref_store,
        }
 
        if (get_sha1_hex(rec, sha1))
        }
 
        if (get_sha1_hex(rec, sha1))

-               die_invalid_line(refs->path, rec, packed_refs->eof - rec);
+               die_invalid_line(refs->path, rec, snapshot->eof - rec);

 
        *type = REF_ISPACKED;
        return 0;
 
        *type = REF_ISPACKED;
        return 0;


@@ -716,26 +750,33 @@ static int packed_read_raw_ref(struct ref_store *ref_store,
 #define REF_KNOWS_PEELED 0x40
 
 /*
 #define REF_KNOWS_PEELED 0x40
 
 /*

- * An iterator over a packed-refs file that is currently mmapped.
+ * An iterator over a snapshot of a `packed-refs` file.

  */
 struct packed_ref_iterator {
        struct ref_iterator base;
 
  */
 struct packed_ref_iterator {
        struct ref_iterator base;
 

-       struct packed_ref_cache *packed_refs;
+       struct snapshot *snapshot;

 
 

-       /* The current position in the mmapped file: */
+       /* The current position in the snapshot's buffer: */

        const char *pos;
 
        const char *pos;
 

-       /* The end of the mmapped file: */
+       /* The end of the part of the buffer that will be iterated over: */

        const char *eof;
 
        const char *eof;
 

+       /* Scratch space for current values: */

        struct object_id oid, peeled;
        struct object_id oid, peeled;

-

        struct strbuf refname_buf;
 
        unsigned int flags;
 };
 
        struct strbuf refname_buf;
 
        unsigned int flags;
 };
 

+/*
+ * Move the iterator to the next record in the snapshot, without
+ * respect for whether the record is actually required by the current
+ * iteration. Adjust the fields in `iter` and return `ITER_OK` or
+ * `ITER_DONE`. This function does not free the iterator in the case
+ * of `ITER_DONE`.
+ */

 static int next_record(struct packed_ref_iterator *iter)
 {
        const char *p = iter->pos, *eol;
 static int next_record(struct packed_ref_iterator *iter)
 {
        const char *p = iter->pos, *eol;


@@ -750,12 +791,12 @@ static int next_record(struct packed_ref_iterator *iter)
        if (iter->eof - p < GIT_SHA1_HEXSZ + 2 ||
            parse_oid_hex(p, &iter->oid, &p) ||
            !isspace(*p++))
        if (iter->eof - p < GIT_SHA1_HEXSZ + 2 ||
            parse_oid_hex(p, &iter->oid, &p) ||
            !isspace(*p++))

-               die_invalid_line(iter->packed_refs->refs->path,
+               die_invalid_line(iter->snapshot->refs->path,

                                 iter->pos, iter->eof - iter->pos);
 
        eol = memchr(p, '\n', iter->eof - p);
        if (!eol)
                                 iter->pos, iter->eof - iter->pos);
 
        eol = memchr(p, '\n', iter->eof - p);
        if (!eol)

-               die_unterminated_line(iter->packed_refs->refs->path,
+               die_unterminated_line(iter->snapshot->refs->path,

                                      iter->pos, iter->eof - iter->pos);
 
        strbuf_add(&iter->refname_buf, p, eol - p);
                                      iter->pos, iter->eof - iter->pos);
 
        strbuf_add(&iter->refname_buf, p, eol - p);


@@ -768,8 +809,8 @@ static int next_record(struct packed_ref_iterator *iter)
                oidclr(&iter->oid);
                iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
        }
                oidclr(&iter->oid);
                iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
        }

-       if (iter->packed_refs->peeled == PEELED_FULLY ||
-           (iter->packed_refs->peeled == PEELED_TAGS &&
+       if (iter->snapshot->peeled == PEELED_FULLY ||
+           (iter->snapshot->peeled == PEELED_TAGS &&

             starts_with(iter->base.refname, "refs/tags/")))
                iter->base.flags |= REF_KNOWS_PEELED;
 
             starts_with(iter->base.refname, "refs/tags/")))
                iter->base.flags |= REF_KNOWS_PEELED;
 


@@ -780,7 +821,7 @@ static int next_record(struct packed_ref_iterator *iter)
                if (iter->eof - p < GIT_SHA1_HEXSZ + 1 ||
                    parse_oid_hex(p, &iter->peeled, &p) ||
                    *p++ != '\n')
                if (iter->eof - p < GIT_SHA1_HEXSZ + 1 ||
                    parse_oid_hex(p, &iter->peeled, &p) ||
                    *p++ != '\n')

-                       die_invalid_line(iter->packed_refs->refs->path,
+                       die_invalid_line(iter->snapshot->refs->path,

                                         iter->pos, iter->eof - iter->pos);
                iter->pos = p;
 
                                         iter->pos, iter->eof - iter->pos);
                iter->pos = p;
 


@@ -850,7 +891,7 @@ static int packed_ref_iterator_abort(struct ref_iterator *ref_iterator)
        int ok = ITER_DONE;
 
        strbuf_release(&iter->refname_buf);
        int ok = ITER_DONE;
 
        strbuf_release(&iter->refname_buf);

-       release_packed_ref_cache(iter->packed_refs);
+       release_snapshot(iter->snapshot);

        base_ref_iterator_free(ref_iterator);
        return ok;
 }
        base_ref_iterator_free(ref_iterator);
        return ok;
 }


@@ -866,7 +907,7 @@ static struct ref_iterator *packed_ref_iterator_begin(
                const char *prefix, unsigned int flags)
 {
        struct packed_ref_store *refs;
                const char *prefix, unsigned int flags)
 {
        struct packed_ref_store *refs;

-       struct packed_ref_cache *packed_refs;
+       struct snapshot *snapshot;

        const char *start;
        struct packed_ref_iterator *iter;
        struct ref_iterator *ref_iterator;
        const char *start;
        struct packed_ref_iterator *iter;
        struct ref_iterator *ref_iterator;


@@ -876,30 +917,30 @@ static struct ref_iterator *packed_ref_iterator_begin(
                required_flags |= REF_STORE_ODB;
        refs = packed_downcast(ref_store, required_flags, "ref_iterator_begin");
 
                required_flags |= REF_STORE_ODB;
        refs = packed_downcast(ref_store, required_flags, "ref_iterator_begin");
 

-       packed_refs = get_packed_ref_cache(refs);
+       /*
+        * Note that `get_snapshot()` internally checks whether the
+        * snapshot is up to date with what is on disk, and re-reads
+        * it if not.
+        */
+       snapshot = get_snapshot(refs);

 
 

-       if (!packed_refs->buf)
+       if (!snapshot->buf)

                return empty_ref_iterator_begin();
 
        iter = xcalloc(1, sizeof(*iter));
        ref_iterator = &iter->base;
        base_ref_iterator_init(ref_iterator, &packed_ref_iterator_vtable, 1);
 
                return empty_ref_iterator_begin();
 
        iter = xcalloc(1, sizeof(*iter));
        ref_iterator = &iter->base;
        base_ref_iterator_init(ref_iterator, &packed_ref_iterator_vtable, 1);
 

-       /*
-        * Note that get_packed_ref_cache() internally checks whether
-        * the packed-ref cache is up to date with what is on disk,
-        * and re-reads it if not.
-        */
-       iter->packed_refs = packed_refs;
-       acquire_packed_ref_cache(packed_refs);
+       iter->snapshot = snapshot;
+       acquire_snapshot(snapshot);

 
        if (prefix && *prefix)
 
        if (prefix && *prefix)

-               start = find_reference_location(packed_refs, prefix, 0);
+               start = find_reference_location(snapshot, prefix, 0);

        else
        else

-               start = packed_refs->buf + packed_refs->header_len;
+               start = snapshot->buf + snapshot->header_len;

 
        iter->pos = start;
 
        iter->pos = start;

-       iter->eof = packed_refs->eof;
+       iter->eof = snapshot->eof;

        strbuf_init(&iter->refname_buf, 0);
 
        iter->base.oid = &iter->oid;
        strbuf_init(&iter->refname_buf, 0);
 
        iter->base.oid = &iter->oid;


@@ -963,19 +1004,19 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 
        /*
         * Now that we hold the `packed-refs` lock, make sure that our
 
        /*
         * Now that we hold the `packed-refs` lock, make sure that our

-        * cache matches the current version of the file. Normally
-        * `get_packed_ref_cache()` does that for us, but that
-        * function assumes that when the file is locked, any existing
-        * cache is still valid. We've just locked the file, but it
-        * might have changed the moment *before* we locked it.
+        * snapshot matches the current version of the file. Normally
+        * `get_snapshot()` does that for us, but that function
+        * assumes that when the file is locked, any existing snapshot
+        * is still valid. We've just locked the file, but it might
+        * have changed the moment *before* we locked it.

         */
         */

-       validate_packed_ref_cache(refs);
+       validate_snapshot(refs);

 
        /*
         * Now make sure that the packed-refs file as it exists in the
 
        /*
         * Now make sure that the packed-refs file as it exists in the

-        * locked state is loaded into the cache:
+        * locked state is loaded into the snapshot:

         */
         */

-       get_packed_ref_cache(refs);
+       get_snapshot(refs);

        return 0;
 }
 
        return 0;
 }
 


@@ -1002,8 +1043,8 @@ int packed_refs_is_locked(struct ref_store *ref_store)
 }
 
 /*
 }
 
 /*

- * The packed-refs header line that we write out.  Perhaps other
- * traits will be added later.
+ * The packed-refs header line that we write out. Perhaps other traits
+ * will be added later.

  *
  * Note that earlier versions of Git used to parse these traits by
  * looking for " trait " in the line. For this reason, the space after
  *
  * Note that earlier versions of Git used to parse these traits by
  * looking for " trait " in the line. For this reason, the space after


@@ -1019,9 +1060,9 @@ static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
 }
 
 /*
 }
 
 /*

- * Write the packed-refs from the cache to the packed-refs tempfile,
- * incorporating any changes from `updates`. `updates` must be a
- * sorted string list whose keys are the refnames and whose util
+ * Write the packed refs from the current snapshot to the packed-refs
+ * tempfile, incorporating any changes from `updates`. `updates` must
+ * be a sorted string list whose keys are the refnames and whose util

  * values are `struct ref_update *`. On error, rollback the tempfile,
  * write an error message to `err`, and return a nonzero value.
  *
  * values are `struct ref_update *`. On error, rollback the tempfile,
  * write an error message to `err`, and return a nonzero value.
  *


@@ -1262,9 +1303,10 @@ static int packed_transaction_prepare(struct ref_store *ref_store,
        /*
         * Note that we *don't* skip transactions with zero updates,
         * because such a transaction might be executed for the side
        /*
         * Note that we *don't* skip transactions with zero updates,
         * because such a transaction might be executed for the side

-        * effect of ensuring that all of the references are peeled.
-        * If the caller wants to optimize away empty transactions, it
-        * should do so itself.
+        * effect of ensuring that all of the references are peeled or
+        * ensuring that the `packed-refs` file is sorted. If the
+        * caller wants to optimize away empty transactions, it should
+        * do so itself.

         */
 
        data = xcalloc(1, sizeof(*data));
         */
 
        data = xcalloc(1, sizeof(*data));


@@ -1330,7 +1372,7 @@ static int packed_transaction_finish(struct ref_store *ref_store,
        int ret = TRANSACTION_GENERIC_ERROR;
        char *packed_refs_path;
 
        int ret = TRANSACTION_GENERIC_ERROR;
        char *packed_refs_path;
 

-       clear_packed_ref_cache(refs);
+       clear_snapshot(refs);

 
        packed_refs_path = get_locked_file_path(&refs->lock);
        if (rename_tempfile(&refs->tempfile, packed_refs_path)) {
 
        packed_refs_path = get_locked_file_path(&refs->lock);
        if (rename_tempfile(&refs->tempfile, packed_refs_path)) {