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[PATCH 08/20] read_packed_refs(): read references with minimal copying

From
Michael Haggerty <mhagger@alum.mit.edu>
Date
Sep 13, 2017, 17:16 UTC
Message-ID
<597241da13f62d51fefd3396bfbab22a8f633a56.1505319366.git.mhagger@alum.mit.edu>
In-Reply-To
<cover.1505319366.git.mhagger@alum.mit.edu>

Instead of copying data from the `packed-refs` file one line at time and then processing it, process the data in place as much as possible.

Also, instead of processing one line per iteration of the main loop, process a reference line plus its corresponding peeled line (if present) together.

Note that this change slightly tightens up the parsing of the `parse-ref` file. Previously, the parser would have accepted multiple "peeled" lines for a single reference (ignoring all but the last one). Now it would reject that.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs/packed-backend.c | 101 ++++++++++++++++++++------------------------------
 1 file changed, 40 insertions(+), 61 deletions(-)
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index a45e3ff92f..2b80f244c8 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -134,33 +134,6 @@ static void clear_packed_ref_cache(struct packed_ref_store *refs)
 	}
 }
 
-/* The length of a peeled reference line in packed-refs, including EOL: */
-#define PEELED_LINE_LENGTH 42
-
-/*
- * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
- * Return a pointer to the refname within the line (null-terminated),
- * or NULL if there was a problem.
- */
-static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
-{
-	const char *ref;
-
-	if (parse_oid_hex(line->buf, oid, &ref) < 0)
-		return NULL;
-	if (!isspace(*ref++))
-		return NULL;
-
-	if (isspace(*ref))
-		return NULL;
-
-	if (line->buf[line->len - 1] != '\n')
-		return NULL;
-	line->buf[--line->len] = 0;
-
-	return ref;
-}
-
 static NORETURN void die_unterminated_line(const char *path,
 					   const char *p, size_t len)
 {
@@ -221,8 +194,7 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 	size_t size;
 	char *buf;
 	const char *pos, *eol, *eof;
-	struct ref_entry *last = NULL;
-	struct strbuf line = STRBUF_INIT;
+	struct strbuf tmp = STRBUF_INIT;
 	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
 	struct ref_dir *dir;
 
@@ -264,9 +236,9 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 		if (!eol)
 			die_unterminated_line(refs->path, pos, eof - pos);
 
-		strbuf_add(&line, pos, eol - pos);
+		strbuf_add(&tmp, pos, eol - pos);
 
-		if (!skip_prefix(line.buf, "# pack-refs with:", (const char **)&p))
+		if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
 			die_invalid_line(refs->path, pos, eof - pos);
 
 		string_list_split_in_place(&traits, p, ' ', -1);
@@ -281,61 +253,68 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 		pos = eol + 1;
 
 		string_list_clear(&traits, 0);
-		strbuf_reset(&line);
+		strbuf_reset(&tmp);
 	}
 
 	dir = get_ref_dir(packed_refs->cache->root);
 	while (pos < eof) {
+		const char *p = pos;
 		struct object_id oid;
 		const char *refname;
+		int flag = REF_ISPACKED;
+		struct ref_entry *entry = NULL;
 
-		eol = memchr(pos, '\n', eof - pos);
+		if (eof - pos < GIT_SHA1_HEXSZ + 2 ||
+		    parse_oid_hex(p, &oid, &p) ||
+		    !isspace(*p++))
+			die_invalid_line(refs->path, pos, eof - pos);
+
+		eol = memchr(p, '\n', eof - p);
 		if (!eol)
 			die_unterminated_line(refs->path, pos, eof - pos);
 
-		strbuf_add(&line, pos, eol + 1 - pos);
+		strbuf_add(&tmp, p, eol - p);
+		refname = tmp.buf;
+
+		if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+			if (!refname_is_safe(refname))
+				die("packed refname is dangerous: %s", refname);
+			oidclr(&oid);
+			flag |= REF_BAD_NAME | REF_ISBROKEN;
+		}
+		if (peeled == PEELED_FULLY ||
+		    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
+			flag |= REF_KNOWS_PEELED;
+		entry = create_ref_entry(refname, &oid, flag);
+		add_ref_entry(dir, entry);
 
-		refname = parse_ref_line(&line, &oid);
-		if (refname) {
-			int flag = REF_ISPACKED;
+		pos = eol + 1;
+
+		if (pos < eof && *pos == '^') {
+			p = pos + 1;
+			if (eof - p < GIT_SHA1_HEXSZ + 1 ||
+			    parse_oid_hex(p, &entry->u.value.peeled, &p) ||
+			    *p++ != '\n')
+				die_invalid_line(refs->path, pos, eof - pos);
 
-			if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
-				if (!refname_is_safe(refname))
-					die("packed refname is dangerous: %s", refname);
-				oidclr(&oid);
-				flag |= REF_BAD_NAME | REF_ISBROKEN;
-			}
-			last = create_ref_entry(refname, &oid, flag);
-			if (peeled == PEELED_FULLY ||
-			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
-				last->flag |= REF_KNOWS_PEELED;
-			add_ref_entry(dir, last);
-		} else if (last &&
-		    line.buf[0] == '^' &&
-		    line.len == PEELED_LINE_LENGTH &&
-		    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
-		    !get_oid_hex(line.buf + 1, &oid)) {
-			oidcpy(&last->u.value.peeled, &oid);
 			/*
 			 * Regardless of what the file header said,
 			 * we definitely know the value of *this*
 			 * reference:
 			 */
-			last->flag |= REF_KNOWS_PEELED;
-		} else {
-			die_invalid_line(refs->path, line.buf, line.len);
+			entry->flag |= REF_KNOWS_PEELED;
+
+			pos = p;
 		}
 
-		/* The "+ 1" is for the LF character. */
-		pos = eol + 1;
-		strbuf_reset(&line);
+		strbuf_reset(&tmp);
 	}
 
 	if (munmap(buf, size))
 		die_errno("error ummapping packed-refs file");
 	close(fd);
 
-	strbuf_release(&line);
+	strbuf_release(&tmp);
 	return packed_refs;
 }
 

-- 
2.14.1
Previous: Michael HaggertyNext: Junio C Hamano
Message 22 of 27 in “Read `packed-refs` using mmap()”
  1. 00/20 Read `packed-refs` using mmap()Michael Haggerty, Sep 13, 2017
  2. 01/20 ref_iterator: keep track of whether the iterator output is orderedMichael Haggerty, Sep 13, 2017
  3. 02/20 prefix_ref_iterator: break when we leave the prefixMichael Haggerty, Sep 13, 2017
  4. 03/20 packed_ref_cache: add a backlink to the associated `packed_ref_store`Michael Haggerty, Sep 13, 2017
  5. 05/20 read_packed_refs(): use mmap to read the `packed-refs` fileMichael Haggerty, Sep 13, 2017
  6. 06/20 read_packed_refs(): only check for a header at the top of the fileMichael Haggerty, Sep 13, 2017
  7. 07/20 read_packed_refs(): make parsing of the header line more robustMichael Haggerty, Sep 13, 2017
  8. 09/20 packed_ref_cache: remember the file-wide peeling stateMichael Haggerty, Sep 13, 2017
  9. 11/20 mmapped_ref_iterator_advance(): no peeled value for broken refsMichael Haggerty, Sep 13, 2017
  10. 12/20 packed_ref_cache: keep the `packed-refs` file open and mmappedMichael Haggerty, Sep 13, 2017
  11. 17/20 packed_ref_store: get rid of the `ref_cache` entirelyMichael Haggerty, Sep 13, 2017
  12. 19/20 mmapped_ref_iterator: inline into `packed_ref_iterator`Michael Haggerty, Sep 13, 2017
  13. 20/20 packed-backend.c: rename a bunch of things and update commentsMichael Haggerty, Sep 13, 2017
  14. Stefan BellerSep 13, 2017
  15. 14/20 packed_ref_iterator_begin(): iterate using `mmapped_ref_iterator`Michael Haggerty, Sep 13, 2017
  16. 18/20 ref_cache: remove support for storing peeled valuesMichael Haggerty, Sep 13, 2017
  17. 10/20 mmapped_ref_iterator: add iterator over a packed-refs fileMichael Haggerty, Sep 13, 2017
  18. 15/20 packed_read_raw_ref(): read the reference from the mmapped bufferMichael Haggerty, Sep 13, 2017
  19. 16/20 ref_store: implement `refs_peel_ref()` genericallyMichael Haggerty, Sep 13, 2017
  20. 04/20 die_unterminated_line(), die_invalid_line(): new functionsMichael Haggerty, Sep 13, 2017
  21. 13/20 read_packed_refs(): ensure that references are ordered when readMichael Haggerty, Sep 13, 2017
  22. 08/20 read_packed_refs(): read references with minimal copyingMichael Haggerty, Sep 13, 2017
  23. Junio C HamanoSep 13, 2017
  24. Michael HaggertySep 14, 2017
  25. Johannes SchindelinSep 14, 2017
  26. Johannes SchindelinSep 14, 2017
  27. Michael HaggertySep 15, 2017

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