[PATCH 1/3] pkt-line: teach packet_get_line a no-op mode

You can use packet_get_line to parse a single packet out of
a stream and into a buffer. However, if you just want to
throw away a set of packets from the stream, there's no need
to even bother copying the bytes. This patch treats a NULL
output buffer as a hint that the caller does not even want
to see the output.

We have to tweak the packet_trace call, too, since it showed
the trace from the copied buffer, which now might not exist.
The new code is actually more correct, though, as it shows
just what we parsed, not any cruft that may have been in the
output buffer before (it never mattered, though, because all
callers gave us a fresh buffer).

Signed-off-by: Jeff King <redacted>
---
 pkt-line.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/pkt-line.c b/pkt-line.c
index eaba15f..7f28701 100644
--- a/pkt-line.c
+++ b/pkt-line.c

@@ -234,9 +234,10 @@ int packet_get_line(struct strbuf *out,
 	*src_len -= 4;
 	len -= 4;
 
-	strbuf_add(out, *src_buf, len);
+	if (out)
+		strbuf_add(out, *src_buf, len);
+	packet_trace(*src_buf, len, 0);
 	*src_buf += len;
 	*src_len -= len;
-	packet_trace(out->buf, out->len, 0);
 	return len;
 }

-- 
1.8.1.2.11.g1a2f572