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 <mhag...@alum.mit.edu>
---
 refs/packed-backend.c | 392 ++++++++++++++++++++++++++++----------------------
 1 file changed, 217 insertions(+), 175 deletions(-)

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index a3f8a19b9b..8235ac8506 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -8,10 +8,30 @@
 
 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+ * `get_snapshot()` is called; if the existing snapshot is obsolete, a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
-        * cache is associated:
+        * snapshot is associated:
         */
        struct packed_ref_store *refs;
 
@@ -35,26 +55,42 @@ struct packed_ref_cache {
        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;
 
        /*
-        * 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;
 
-       /* 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;
 };
 
 /*
- * 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;
@@ -65,10 +101,10 @@ struct packed_ref_store {
        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
@@ -85,44 +121,43 @@ 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.
  */
-static void release_packed_ref_buffer(struct packed_ref_cache *packed_refs)
+static void clear_snapshot_buffer(struct snapshot *snapshot)
 {
-       if (packed_refs->fd >= 0) {
-               if (munmap(packed_refs->buf,
-                          packed_refs->eof - packed_refs->buf))
+       if (snapshot->fd >= 0) {
+               if (munmap(snapshot->buf, snapshot->eof - snapshot->buf))
                        die_errno("error ummapping packed-refs file %s",
-                                 packed_refs->refs->path);
-               close(packed_refs->fd);
-               packed_refs->fd = -1;
+                                 snapshot->refs->path);
+               close(snapshot->fd);
+               snapshot->fd = -1;
        } 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;
@@ -167,13 +202,13 @@ static struct packed_ref_store *packed_downcast(struct 
ref_store *ref_store,
        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);
        }
 }
 
@@ -200,14 +235,14 @@ static NORETURN void die_invalid_line(const char *path,
 
 }
 
-struct packed_ref_entry {
+struct snapshot_record {
        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;
 
@@ -226,10 +261,10 @@ 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;
@@ -247,31 +282,30 @@ 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;
 
-       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;
 
        /*
-        * 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):
         */
-       ALLOC_GROW(entries, len / 80 + 20, alloc);
+       ALLOC_GROW(records, len / 80 + 20, alloc);
 
        while (pos < eof) {
                eol = memchr(pos, '\n', eof - pos);
@@ -279,7 +313,7 @@ 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)
-                       die_invalid_line(packed_refs->refs->path,
+                       die_invalid_line(snapshot->refs->path,
                                         pos, eof - pos);
                eol++;
                if (eol < eof && *eol == '^') {
@@ -296,15 +330,15 @@ static void sort_packed_refs(struct packed_ref_cache 
*packed_refs)
                        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 &&
-                   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;
@@ -313,31 +347,31 @@ static void sort_packed_refs(struct packed_ref_cache 
*packed_refs)
        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
-        * 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++) {
-               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:
         */
-       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:
-       free(entries);
+       free(records);
 }
 
 /*
@@ -381,10 +415,10 @@ 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.
  */
-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)
@@ -392,12 +426,12 @@ 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)
-               die_invalid_line(packed_refs->refs->path,
+               die_invalid_line(snapshot->refs->path,
                                 last_line, eof - last_line);
 }
 
 /*
- * 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
@@ -405,10 +439,10 @@ static void verify_buffer_safe(struct packed_ref_cache 
*packed_refs)
  * 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)
 {
        /*
@@ -425,13 +459,13 @@ static const char *find_reference_location(struct 
packed_ref_cache *cache,
         * 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`.
         */
-       const char *hi = cache->eof;
+       const char *hi = snapshot->eof;
 
        while (lo < hi) {
                const char *mid, *rec;
@@ -439,7 +473,7 @@ static const char *find_reference_location(struct 
packed_ref_cache *cache,
 
                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) {
@@ -456,9 +490,9 @@ 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:
@@ -488,19 +522,19 @@ static const char *find_reference_location(struct 
packed_ref_cache *cache,
  *
  *      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));
        struct stat st;
        size_t size;
        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;
 
-       packed_refs->fd = open(refs->path, O_RDONLY);
-       if (packed_refs->fd < 0) {
+       snapshot->fd = open(refs->path, O_RDONLY);
+       if (snapshot->fd < 0) {
                if (errno == ENOENT) {
                        /*
                         * This is OK; it just means that no
@@ -509,106 +543,106 @@ static struct packed_ref_cache *read_packed_refs(struct 
packed_ref_store *refs)
                         * which is its state when initialized with
                         * zeros.
                         */
-                       return packed_refs;
+                       return snapshot;
                } else {
                        die_errno("couldn't read %s", refs->path);
                }
        }
 
-       stat_validity_update(&packed_refs->validity, packed_refs->fd);
+       stat_validity_update(&snapshot->validity, snapshot->fd);
 
-       if (fstat(packed_refs->fd, &st) < 0)
+       if (fstat(snapshot->fd, &st) < 0)
                die_errno("couldn't stat %s", refs->path);
 
        size = xsize_t(st.st_size);
-       packed_refs->buf = xmmap(NULL, size,
-                                PROT_READ, MAP_PRIVATE,
-                                packed_refs->fd, 0);
-       packed_refs->eof = packed_refs->buf + size;
+       snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE,
+                             snapshot->fd, 0);
+       snapshot->eof = snapshot->buf + size;
 
        /* 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;
 
-               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,
-                                             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,
-                                        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"))
-                       packed_refs->peeled = PEELED_FULLY;
+                       snapshot->peeled = PEELED_FULLY;
                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. */
-               packed_refs->header_len = eol + 1 - packed_refs->buf;
+               snapshot->header_len = eol + 1 - snapshot->buf;
 
                string_list_clear(&traits, 0);
                strbuf_release(&tmp);
        }
 
-       verify_buffer_safe(packed_refs);
+       verify_buffer_safe(snapshot);
 
        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:
                 */
-               verify_buffer_safe(packed_refs);
+               verify_buffer_safe(snapshot);
        }
 
-       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))
-               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,
@@ -617,12 +651,12 @@ 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_cache *packed_refs = get_packed_ref_cache(refs);
+       struct snapshot *snapshot = get_snapshot(refs);
        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. */
@@ -631,7 +665,7 @@ static int packed_read_raw_ref(struct ref_store *ref_store,
        }
 
        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;
@@ -646,26 +680,33 @@ static int packed_read_raw_ref(struct ref_store 
*ref_store,
 #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_cache *packed_refs;
+       struct snapshot *snapshot;
 
-       /* The current position in the mmapped file: */
+       /* The current position in the snapshot's buffer: */
        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;
 
+       /* Scratch space for current values: */
        struct object_id oid, peeled;
-
        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;
@@ -680,12 +721,12 @@ 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++))
-               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)
-               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);
@@ -698,8 +739,8 @@ static int next_record(struct packed_ref_iterator *iter)
                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;
 
@@ -710,7 +751,7 @@ 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')
-                       die_invalid_line(iter->packed_refs->refs->path,
+                       die_invalid_line(iter->snapshot->refs->path,
                                         iter->pos, iter->eof - iter->pos);
                iter->pos = p;
 
@@ -780,7 +821,7 @@ static int packed_ref_iterator_abort(struct ref_iterator 
*ref_iterator)
        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;
 }
@@ -796,7 +837,7 @@ static struct ref_iterator *packed_ref_iterator_begin(
                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;
@@ -806,30 +847,30 @@ static struct ref_iterator *packed_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);
 
-       /*
-        * 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)
-               start = find_reference_location(packed_refs, prefix, 0);
+               start = find_reference_location(snapshot, prefix, 0);
        else
-               start = packed_refs->buf + packed_refs->header_len;
+               start = snapshot->buf + snapshot->header_len;
 
        iter->pos = start;
-       iter->eof = packed_refs->eof;
+       iter->eof = snapshot->eof;
        strbuf_init(&iter->refname_buf, 0);
 
        iter->base.oid = &iter->oid;
@@ -893,19 +934,19 @@ 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
-        * 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
-        * locked state is loaded into the cache:
+        * locked state is loaded into the snapshot:
         */
-       get_packed_ref_cache(refs);
+       get_snapshot(refs);
        return 0;
 }
 
@@ -932,8 +973,8 @@ 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
@@ -949,9 +990,9 @@ 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.
  *
@@ -1192,9 +1233,10 @@ 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
-        * 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));
@@ -1267,7 +1309,7 @@ static int packed_transaction_finish(struct ref_store 
*ref_store,
                goto cleanup;
        }
 
-       clear_packed_ref_cache(refs);
+       clear_snapshot(refs);
        ret = 0;
 
 cleanup:
-- 
2.14.1

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