Merge branch 'kg/packed-ref-cache-fix'

Avoid mmapping small files while using packed refs (especially ones
with zero size, which would cause later munmap() to fail).

* kg/packed-ref-cache-fix:
  packed_ref_cache: don't use mmap() for small files
  load_contents(): don't try to mmap an empty file
  packed_ref_iterator_begin(): make optimization more general
  find_reference_location(): make function safe for empty snapshots
  create_snapshot(): use `xmemdupz()` rather than a strbuf
  struct snapshot: store `start` rather than `header_len`

1 files changed, 55 insertions, 51 deletions

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 023243fd5f..65288c6472 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -68,17 +68,21 @@ struct snapshot {
 	int mmapped;
 
 	/*
-	 * 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.
+	 * The contents of the `packed-refs` file:
+	 *
+	 * - buf -- a pointer to the start of the memory
+	 * - start -- a pointer to the first byte of actual references
+	 *   (i.e., after the header line, if one is present)
+	 * - eof -- a pointer just past the end of the reference
+	 *   contents
+	 *
+	 * If the `packed-refs` file was already sorted, `buf` points
+	 * at the mmapped contents of the file. If not, it points at
+	 * heap-allocated memory containing the contents, sorted. If
+	 * there were no contents (e.g., because the file didn't
+	 * exist), `buf`, `start`, and `eof` are all NULL.
 	 */
-	char *buf, *eof;
-
-	/* The size of the header line, if any; otherwise, 0: */
-	size_t header_len;
+	char *buf, *start, *eof;
 
 	/*
 	 * What is the peeled state of the `packed-refs` file that
@@ -169,8 +173,7 @@ static void clear_snapshot_buffer(struct snapshot *snapshot)
 	} else {
 		free(snapshot->buf);
 	}
-	snapshot->buf = snapshot->eof = NULL;
-	snapshot->header_len = 0;
+	snapshot->buf = snapshot->start = snapshot->eof = NULL;
 }
 
 /*
@@ -319,13 +322,14 @@ static void sort_snapshot(struct snapshot *snapshot)
 	size_t len, i;
 	char *new_buffer, *dst;
 
-	pos = snapshot->buf + snapshot->header_len;
+	pos = snapshot->start;
 	eof = snapshot->eof;
-	len = eof - pos;
 
-	if (!len)
+	if (pos == eof)
 		return;
 
+	len = eof - pos;
+
 	/*
 	 * Initialize records based on a crude estimate of the number
 	 * of references in the file (we'll grow it below if needed):
@@ -391,9 +395,8 @@ static void sort_snapshot(struct snapshot *snapshot)
 	 * place:
 	 */
 	clear_snapshot_buffer(snapshot);
-	snapshot->buf = new_buffer;
+	snapshot->buf = snapshot->start = new_buffer;
 	snapshot->eof = new_buffer + len;
-	snapshot->header_len = 0;
 
 cleanup:
 	free(records);
@@ -442,23 +445,26 @@ static const char *find_end_of_record(const char *p, const char *end)
  */
 static void verify_buffer_safe(struct snapshot *snapshot)
 {
-	const char *buf = snapshot->buf + snapshot->header_len;
+	const char *start = snapshot->start;
 	const char *eof = snapshot->eof;
 	const char *last_line;
 
-	if (buf == eof)
+	if (start == eof)
 		return;
 
-	last_line = find_start_of_record(buf, eof - 1);
+	last_line = find_start_of_record(start, eof - 1);
 	if (*(eof - 1) != '\n' || eof - last_line < GIT_SHA1_HEXSZ + 2)
 		die_invalid_line(snapshot->refs->path,
 				 last_line, eof - last_line);
 }
 
+#define SMALL_FILE_SIZE (32*1024)
+
 /*
  * Depending on `mmap_strategy`, either mmap or read the contents of
  * 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.
+ * existed and was read, or 0 if the file was absent or empty. Die on
+ * errors.
  */
 static int load_contents(struct snapshot *snapshot)
 {
@@ -489,24 +495,23 @@ static int load_contents(struct snapshot *snapshot)
 		die_errno("couldn't stat %s", snapshot->refs->path);
 	size = xsize_t(st.st_size);
 
-	switch (mmap_strategy) {
-	case MMAP_NONE:
+	if (!size) {
+		return 0;
+	} else if (mmap_strategy == MMAP_NONE || size <= SMALL_FILE_SIZE) {
 		snapshot->buf = xmalloc(size);
 		bytes_read = read_in_full(fd, snapshot->buf, size);
 		if (bytes_read < 0 || bytes_read != size)
 			die_errno("couldn't read %s", snapshot->refs->path);
-		snapshot->eof = snapshot->buf + size;
 		snapshot->mmapped = 0;
-		break;
-	case MMAP_TEMPORARY:
-	case MMAP_OK:
+	} else {
 		snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
-		snapshot->eof = snapshot->buf + size;
 		snapshot->mmapped = 1;
-		break;
 	}
 	close(fd);
 
+	snapshot->start = snapshot->buf;
+	snapshot->eof = snapshot->buf + size;
+
 	return 1;
 }
 
@@ -515,9 +520,11 @@ static int load_contents(struct snapshot *snapshot)
  * `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
- * inserted. In the latter mode, `refname` doesn't have to be a proper
- * reference name; for example, one could search for "refs/replace/"
- * to find the start of any replace references.
+ * inserted, or `snapshot->eof` (which might be NULL) if it would be
+ * inserted at the end of the file. In the latter mode, `refname`
+ * doesn't have to be a proper 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 `snapshot->buf` must
  * be sorted.
@@ -539,7 +546,7 @@ static const char *find_reference_location(struct snapshot *snapshot,
 	 * preceding records all have reference names that come
 	 * *before* `refname`.
 	 */
-	const char *lo = snapshot->buf + snapshot->header_len;
+	const char *lo = snapshot->start;
 
 	/*
 	 * A pointer to a the first character of a record whose
@@ -547,7 +554,7 @@ static const char *find_reference_location(struct snapshot *snapshot,
 	 */
 	const char *hi = snapshot->eof;
 
-	while (lo < hi) {
+	while (lo != hi) {
 		const char *mid, *rec;
 		int cmp;
 
@@ -616,9 +623,7 @@ static struct snapshot *create_snapshot(struct packed_ref_store *refs)
 
 	/* If the file has a header line, process it: */
 	if (snapshot->buf < snapshot->eof && *snapshot->buf == '#') {
-		struct strbuf tmp = STRBUF_INIT;
-		char *p;
-		const char *eol;
+		char *tmp, *p, *eol;
 		struct string_list traits = STRING_LIST_INIT_NODUP;
 
 		eol = memchr(snapshot->buf, '\n',
@@ -628,9 +633,9 @@ static struct snapshot *create_snapshot(struct packed_ref_store *refs)
 					      snapshot->buf,
 					      snapshot->eof - snapshot->buf);
 
-		strbuf_add(&tmp, snapshot->buf, eol - snapshot->buf);
+		tmp = xmemdupz(snapshot->buf, eol - snapshot->buf);
 
-		if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
+		if (!skip_prefix(tmp, "# pack-refs with:", (const char **)&p))
 			die_invalid_line(refs->path,
 					 snapshot->buf,
 					 snapshot->eof - snapshot->buf);
@@ -647,10 +652,10 @@ static struct snapshot *create_snapshot(struct packed_ref_store *refs)
 		/* perhaps other traits later as well */
 
 		/* The "+ 1" is for the LF character. */
-		snapshot->header_len = eol + 1 - snapshot->buf;
+		snapshot->start = eol + 1;
 
 		string_list_clear(&traits, 0);
-		strbuf_release(&tmp);
+		free(tmp);
 	}
 
 	verify_buffer_safe(snapshot);
@@ -671,13 +676,12 @@ static struct snapshot *create_snapshot(struct packed_ref_store *refs)
 		 * We don't want to leave the file mmapped, so we are
 		 * forced to make a copy now:
 		 */
-		size_t size = snapshot->eof -
-			(snapshot->buf + snapshot->header_len);
+		size_t size = snapshot->eof - snapshot->start;
 		char *buf_copy = xmalloc(size);
 
-		memcpy(buf_copy, snapshot->buf + snapshot->header_len, size);
+		memcpy(buf_copy, snapshot->start, size);
 		clear_snapshot_buffer(snapshot);
-		snapshot->buf = buf_copy;
+		snapshot->buf = snapshot->start = buf_copy;
 		snapshot->eof = buf_copy + size;
 	}
 
@@ -924,7 +928,12 @@ static struct ref_iterator *packed_ref_iterator_begin(
 	 */
 	snapshot = get_snapshot(refs);
 
-	if (!snapshot->buf)
+	if (prefix && *prefix)
+		start = find_reference_location(snapshot, prefix, 0);
+	else
+		start = snapshot->start;
+
+	if (start == snapshot->eof)
 		return empty_ref_iterator_begin();
 
 	iter = xcalloc(1, sizeof(*iter));
@@ -934,11 +943,6 @@ static struct ref_iterator *packed_ref_iterator_begin(
 	iter->snapshot = snapshot;
 	acquire_snapshot(snapshot);
 
-	if (prefix && *prefix)
-		start = find_reference_location(snapshot, prefix, 0);
-	else
-		start = snapshot->buf + snapshot->header_len;
-
 	iter->pos = start;
 	iter->eof = snapshot->eof;
 	strbuf_init(&iter->refname_buf, 0);