path: root/pkt-line.c
diff options
authorJeff King <>2020-10-28 09:33:24 (GMT)
committerJunio C Hamano <>2020-10-29 16:23:29 (GMT)
commitaf22a63c3995e4113963aa756c580bb111d99176 (patch)
tree5388c0adde0e323a211c1e95217739006c28b236 /pkt-line.c
parent712b0377db60e0d3ac8bf3332022dfcd01e29afa (diff)
sideband: diagnose more sideband anomalies
In demultiplex_sideband(), there are two oddities when we check an incoming packet: - if it has zero length, then we assume it's a flush packet. This means we fail to notice the difference between a real flush and a true zero-length packet that's missing its sideband designator. It's not a huge problem in practice because we'd never send a zero-length data packet (even our keepalives are otherwise-empty sideband-1 packets). But it would be nice to detect and report the error, since it's likely to cause other confusion (we think the other side flushed, but they do not). - we try to detect packets missing their designator by checking for "if (len < 1)". But this will never trigger for "len == 0"; we've already detected that and left the function before then. It _could_ detect a negative "len" parameter. But in that case, the error message is wrong. The issue is not "no sideband" but rather "eof while reading the packet". However, this can't actually be triggered in practice, because neither of the two callers uses pkt_read's GENTLE_ON_EOF flag. Which means they'd die with "the remote end hung up unexpectedly" before we even get here. So this truly is dead code. We can improve these cases by passing in a pkt-line status to the demultiplexer, and by having recv_sideband() use GENTLE_ON_EOF. This gives us two improvements: - we can now reliably detect flush packets, and will report a normal packet missing its sideband designator as an error - we'll report an eof with a more detailed "protocol error: eof while reading sideband packet", rather than the generic "the remote end hung up unexpectedly" - when we see an eof, we'll flush the sideband scratch buffer, which may provide some hints from the remote about why they hung up (though note we already flush on newlines, so it's likely that most such messages already made it through) In some sense this patch goes against fbd76cd450 (sideband: reverse its dependency on pkt-line, 2019-01-16), which caused the sideband code not to depend on the pkt-line code. But that commit was really just trying to deal with the circular header dependency. The two modules are conceptually interlinked, and it was just trying to keep things compiling. And indeed, there's a sticking point in this patch: because pkt-line.h includes sideband.h, we can't add the reverse include we need for the sideband code to have an "enum packet_read_status" parameter. Nor can we forward declare it, because you can't forward declare an enum in C. However, C does guarantee that enums fit in an int, so we can just use that type. One alternative would be for the callers to check themselves that they got something sane from the pkt-line code. But besides duplicating logic, this gets quite tricky. Any error condition requires flushing the sideband #2 scratch buffer, which only demultiplex_sideband() knows how to do. Signed-off-by: Jeff King <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'pkt-line.c')
1 files changed, 8 insertions, 6 deletions
diff --git a/pkt-line.c b/pkt-line.c
index 657a702..d633005 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -461,9 +461,11 @@ int recv_sideband(const char *me, int in_stream, int out)
enum sideband_type sideband_type;
while (1) {
- len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
- 0);
- if (!demultiplex_sideband(me, buf, len, 0, &scratch,
+ int status = packet_read_with_status(in_stream, NULL, NULL,
+ &len,
+ if (!demultiplex_sideband(me, status, buf, len, 0, &scratch,
switch (sideband_type) {
@@ -520,9 +522,9 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
if (!reader->use_sideband)
- if (demultiplex_sideband(reader->me, reader->buffer,
- reader->pktlen, 1, &scratch,
- &sideband_type))
+ if (demultiplex_sideband(reader->me, reader->status,
+ reader->buffer, reader->pktlen, 1,
+ &scratch, &sideband_type))