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[PATCH v4] pkt-line: allow writing of LARGE_PACKET_MAX buffers

From
Jeff King <peff@peff.net>
Date
Dec 10, 2014, 09:47 UTC
Message-ID
<20141210094702.GA8917@peff.net>
In-Reply-To
<CAPig+cQQThA7wiz8iwkKX=ipg1n5w+gyeS8NqtbjGui986Hn+g@mail.gmail.com>
On Wed, Dec 10, 2014 at 03:36:31AM -0500, Eric Sunshine wrote:
Show 15 quoted lines
> On Wed, Dec 10, 2014 at 2:34 AM, Jeff King <peff@peff.net> wrote:
> > Below is a another iteration on the patch. The actual code changes are
> > the same as the strbuf one, but the tests take care to avoid assuming
> > the filesystem can handle such a long path. Testing on Windows and OS X
> > is appreciated.
> 
> All three new tests fail on OS X. Thus far brief examination of the
> first failing tests shows that 'expect' and 'actual' differ:
> 
> expect:
>     long
>     master
> 
> actual:
>     master

Ugh. It looks like the packed-refs reader uses a PATH_MAX-sized buffer to read each line, and simply omits the ref. That may be something we want to work on, but it's a separate topic. I think there are platforms whose PATH_MAX is much smaller than what they can actually represent. So ideally we would lift the arbitrary PATH_MAX limitations inside git, and just let the OS complain when we exceed its limits.

Even if we fix that, though, I think the push test would still fail on systems that have a true limit on the filesystem. Writing to a ref requires taking a lock in the filesystem, which will involve creating the too-long path. I think there are simply some systems that will not support long refs well, and the tests need to be skipped there.

So here's a re-roll that uses prerequisites.
-Peff
-- >8 --
Subject: pkt-line: allow writing of LARGE_PACKET_MAX buffers

When we send out pkt-lines with refnames, we use a static 1000-byte buffer. This means that the maximum size of a ref over the git protocol is around 950 bytes (the exact size depends on the protocol line being written, but figure on a sha1 plus some boilerplate).

This is enough for any sane workflow, but occasionally odd things happen (e.g., a bug may create a ref "foo/foo/foo/..." accidentally). With the current code, you cannot even use "push" to delete such a ref from a remote.

Let's switch to using a strbuf, with a hard-limit of LARGE_PACKET_MAX (which is specified by the protocol). This matches the size of the readers, as of 74543a0 (pkt-line: provide a LARGE_PACKET_MAX static buffer, 2013-02-20). Versions of git older than that will complain about our large packets, but it's really no worse than the current behavior. Right now the sender barfs with "impossibly long line" trying to send the packet, and afterwards the reader will barf with "protocol error: bad line length %d", which is arguably better anyway.

Note that we're not really _solving_ the problem here, but just bumping the limits. In theory, the length of a ref is unbounded, and pkt-line can only represent sizes up to 65531 bytes. So we are just bumping the limit, not removing it. But hopefully 64K should be enough for anyone.

As a bonus, by using a strbuf for the formatting we can eliminate an unnecessary copy in format_buf_write.

Signed-off-by: Jeff King <peff@peff.net>
---
 pkt-line.c                | 37 +++++++++++++++++++------------------
 t/t5527-fetch-odd-refs.sh | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 18 deletions(-)
diff --git a/pkt-line.c b/pkt-line.c
index 8bc89b1..187a229 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -64,44 +64,45 @@ void packet_buf_flush(struct strbuf *buf)
 }
 
 #define hex(a) (hexchar[(a) & 15])
-static char buffer[1000];
-static unsigned format_packet(const char *fmt, va_list args)
+static void format_packet(struct strbuf *out, const char *fmt, va_list args)
 {
 	static char hexchar[] = "0123456789abcdef";
-	unsigned n;
+	size_t orig_len, n;
 
-	n = vsnprintf(buffer + 4, sizeof(buffer) - 4, fmt, args);
-	if (n >= sizeof(buffer)-4)
+	orig_len = out->len;
+	strbuf_addstr(out, "0000");
+	strbuf_vaddf(out, fmt, args);
+	n = out->len - orig_len;
+
+	if (n > LARGE_PACKET_MAX)
 		die("protocol error: impossibly long line");
-	n += 4;
-	buffer[0] = hex(n >> 12);
-	buffer[1] = hex(n >> 8);
-	buffer[2] = hex(n >> 4);
-	buffer[3] = hex(n);
-	packet_trace(buffer+4, n-4, 1);
-	return n;
+
+	out->buf[orig_len + 0] = hex(n >> 12);
+	out->buf[orig_len + 1] = hex(n >> 8);
+	out->buf[orig_len + 2] = hex(n >> 4);
+	out->buf[orig_len + 3] = hex(n);
+	packet_trace(out->buf + orig_len + 4, n - 4, 1);
 }
 
 void packet_write(int fd, const char *fmt, ...)
 {
+	static struct strbuf buf = STRBUF_INIT;
 	va_list args;
-	unsigned n;
 
+	strbuf_reset(&buf);
 	va_start(args, fmt);
-	n = format_packet(fmt, args);
+	format_packet(&buf, fmt, args);
 	va_end(args);
-	write_or_die(fd, buffer, n);
+	write_or_die(fd, buf.buf, buf.len);
 }
 
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...)
 {
 	va_list args;
-	unsigned n;
 
 	va_start(args, fmt);
-	n = format_packet(fmt, args);
+	format_packet(buf, fmt, args);
 	va_end(args);
-	strbuf_add(buf, buffer, n);
 }
 
 static int get_packet_data(int fd, char **src_buf, size_t *src_size,
diff --git a/t/t5527-fetch-odd-refs.sh b/t/t5527-fetch-odd-refs.sh
index edea9f9..85bcb2e 100755
--- a/t/t5527-fetch-odd-refs.sh
+++ b/t/t5527-fetch-odd-refs.sh
@@ -26,4 +26,37 @@ test_expect_success 'suffix ref is ignored during fetch' '
 	test_cmp expect actual
 '
 
+test_expect_success 'try to create repo with absurdly long refname' '
+	ref240=$_z40/$_z40/$_z40/$_z40/$_z40/$_z40
+	ref1440=$ref240/$ref240/$ref240/$ref240/$ref240/$ref240 &&
+	git init long &&
+	(
+		cd long &&
+		test_commit long &&
+		test_commit master
+	) &&
+	if git -C long update-ref refs/heads/$ref1440 long; then
+		test_set_prereq LONG_REF
+	else
+		echo >&2 "long refs not supported"
+	fi
+'
+
+test_expect_success LONG_REF 'fetch handles extremely long refname' '
+	git fetch long refs/heads/*:refs/remotes/long/* &&
+	cat >expect <<-\EOF &&
+	long
+	master
+	EOF
+	git for-each-ref --format="%(subject)" refs/remotes/long >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success LONG_REF 'push handles extremely long refname' '
+	git push long :refs/heads/$ref1440 &&
+	git -C long for-each-ref --format="%(subject)" refs/heads >actual &&
+	echo master >expect &&
+	test_cmp expect actual
+'
+
 test_done

-- 
2.2.0.454.g7eca6b7
Previous: Junio C HamanoNext: Eric Sunshine
Message 15 of 20 in “pkt-line: allow writing of LARGE_PACKET_MAX buffers”
  1. pkt-line: allow writing of LARGE_PACKET_MAX buffersJeff King, Dec 9, 2014
  2. Jeff KingDec 9, 2014
  3. Junio C HamanoDec 9, 2014
  4. Michael BlumeDec 10, 2014
  5. pkt-line: allow writing of LARGE_PACKET_MAX buffersJeff King, Dec 10, 2014
  6. Eric SunshineDec 10, 2014
  7. Eric SunshineDec 10, 2014
  8. Eric SunshineDec 10, 2014
  9. Jeff KingDec 10, 2014
  10. 0/3 convert read_packed_refs to use strbufJeff King, Dec 10, 2014
  11. 1/3 read_packed_refs: use a strbuf for reading linesJeff King, Dec 10, 2014
  12. 2/3 read_packed_refs: pass strbuf to parse_ref_lineJeff King, Dec 10, 2014
  13. 3/3 read_packed_refs: use skip_prefix instead of static arrayJeff King, Dec 10, 2014
  14. Junio C HamanoDec 10, 2014
  15. pkt-line: allow writing of LARGE_PACKET_MAX buffersJeff King, Dec 10, 2014
  16. Eric SunshineDec 10, 2014
  17. Eric SunshineDec 10, 2014
  18. Jeff KingDec 10, 2014
  19. Johannes SixtDec 9, 2014
  20. Jeff KingDec 9, 2014

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