tag 697682 = patch
thanks
On Thu, Aug 07, 2014 at 04:32:08PM +0300, Niko Tyni wrote:
> On Sat, Mar 30, 2013 at 03:26:55PM +0000, Dominic Hargreaves wrote:
> > This doesn't seem to be limited to s390; I've seen it in on i386 during
> > perl 5.16 rebuilds.
>
> As a data point, t/apache/read3.t fails for me consistently on the
> kfreebsd-{i386,amd64} porter boxes (fischer and falla) but interestingly
> not on the kfreebsd buildds.
>
> The test is getting an internal server error, with this in the
> error log:
>
> [Thu Aug 07 13:30:28.768239 2014] [perl:error] [pid 81995:tid 34593212160]
> [client 127.0.0.1:12832] Apache2::RequestIO::read: (70007) The timeout
> specified has expired at
> /home/ntyni/libapache2-mod-perl2-2.0.8+httpd24-r1449661/t/response/TestApache/read3.pm
> line 30
As this was fully reproducible on the Debian/kFreeBSD boxes, I spent
some time debugging it.
The test is about a client sending a 12000 byte POST request to the
server, which reads it in a loop 100 bytes at a time and checks that it
all came through.
Starting the server with
t/TEST -httpd_conf $(pwd)/debian/apache2.conf -one-process -start-httpd
attaching ktrace to it
ktrace -p $(pgrep apache2) -f ktrace.apache2.2000
and then running the client part under ktrace as well:
ktrace -f ktrace.client.2000 -- perl -Iblib/lib -Iblib/arch t/apache/read3.t
gives nice traces to inspect with kdump.
The failure only happens when the POSTed string is large enough that
it doesn't fit in the same write() call with the HTTP headers. So
"foobar"x1300 doesn't exhibit the problem but "foobar"x2000 does.
There seems to be a deadlock here:
client writes the HTTP headers of the POST request but not the data yet
server reads HTTP headers
server writes HTTP OK + part of the body
client calls select(), gets 2 so there are two descriptors ready (read+write)
client reads HTTP OK + part of the body
server tries to read the POST data but gets EAGAIN (no data yet)
server calls poll() and blocks until there's something to read
client calls select() again and blocks
The client is using LWP::Protocol::http under the hood, and I see it
indeed stops writing to the server when it gets a full HTTP header (up
to two newlines.) This is around line 386 of lib/LWP/Protocol/http.pm
in libwww-perl_6.08-1. Also, I note that it sends the whole POST request
in one write() unless it's over eight kilobytes long (see line 276.)
I'm unsure if there's a fault with the client (it gets an FD that's
ready for writing from the first select() call but ignores it, calls
select() again and blocks because the server has already called poll()
on the same descriptor), but the deadlock can be fixed/worked around by
making the server not send a response before the full POST request has
been read. See the attached patch, which fixes this completely for me.
I can also reproduce the issue on amd64 by running the test in a loop and
putting some load on the host. The patch makes it go away there too.
Cc'ing the modperl dev list. Please consider applying the patch.
--
Niko Tyni nt...@debian.org
>From 2be468161de0b09df1eb10940abe531439d311e5 Mon Sep 17 00:00:00 2001
From: Niko Tyni <nt...@debian.org>
Date: Fri, 8 Aug 2014 17:22:33 +0000
Subject: [PATCH] Don't answer anything to the client before it's done sending
the POST data
LWP::Protocol::http sends long POST requests with several write() calls.
If it gets a response with a full HTTP header (up to the two newlines)
before it's done with the later write() calls sending the actual POST data,
it will stop writing. This leads to a deadlock and a test failure after
timeout.
Bug-Debian: https://bugs.debian.org/697682
---
t/response/TestApache/read3.pm | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/t/response/TestApache/read3.pm b/t/response/TestApache/read3.pm
index 8ad9f26..2994093 100644
--- a/t/response/TestApache/read3.pm
+++ b/t/response/TestApache/read3.pm
@@ -20,8 +20,6 @@ my $expected = "foobar"x2000;
sub handler {
my $r = shift;
- plan $r, tests => 1;
-
# test to read data up to end of file is signaled
my $data = '';
my $where = 0;
@@ -31,6 +29,8 @@ sub handler {
$where += $len;
} while ($len > 0);
+ plan $r, tests => 1;
+
ok t_cmp($data, $expected, "reading up to end of file");
Apache2::Const::OK;
--
2.1.0.rc1
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