[Libwebsockets] HTTP keep-alive

Carl Stehle droid at appception.com
Fri Aug 15 20:05:48 CEST 2014


When an HTTP request header contains 'Connection: keep-alive',
a persistent HTTP connection is established. The initial HTTP
request is processed fine, but in subsequent requests (on the
same connection), the URI header (as read with lws_hdr_copy())
is corrupted. This occurs on a Windows platform (not yet tested
elsewhere).

It results from an artifact of sharing in the union, described
below:

union u {
		struct _lws_http_mode_related http;
		struct _lws_header_related hdr;
		struct _lws_websocket_related ws;
	} u;

While processing the headers, u.hdr is valid. Once the headers
are processed, a 'transition' from u.hdr to u.http is made (in
lws_handshake_server()) so that u.hdr becomes invalid (except for
the retained ah structure). At this point, u.hdr should not be
used for the remainder of the HTTP request processing, which is
fine.

When the next HTTP request arrives (on the same connection, since
keep-alive is used), u.hdr.ah is re-allocated and partially
re-initialized (by lws_allocate_header_table(), however, the
u.hdr.ues field is not re-initialized, so any previous (overlapping)
value from u.http remains. In the failure case, it appears to be
URIES_SEEN_PERCENT rather than the expected URIES_IDLE
(perhaps 'request_version = HTTP_VERSION_1_1' in lws_handshake_server()).
This confuses the parser, causing it to insert a '%' before the
URI value.

No failure occurs in the first request on the connection since
the u.hdr.ues is zeroed when initially allocated and URIES_IDLE
is 0.

As a workaround, I added:
wsi->u.hdr.ues = URIES_IDLE;
to lws_allocate_header_table(), but perhaps the full u.hdr structure
should be re-initialized.

This has only been tested on Windows, so it could be a remnant
of union handling there (Visual Studio 10, Windows 7, 64-bit
platform but compiled in 32-bit mode with the native 'cl' compiler)
or tickled by the particular test/datastream used.

Regards, Carl






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