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This bug is pretty pernicious and have serious consequences : In 2.1, an
infinite loop in process_stream() because the backend stream-interface remains
in the ready state (SI_ST_RDY). In 2.0, a call in loop to process_stream()
because the stream-interface remains blocked in the connect state
(SI_ST_CON). In both cases, it happens after a connection retry attempt. In 1.9,
it seems to not happen. But it may be just by chance or just because it is
harder to get right conditions to trigger the bug. However, reading the code,
the bug seems to exist too.
Here is how the bug happens in 2.1. When we try to establish a new connection to
a server, the corresponding stream-interface is first set to the connect state
(SI_ST_CON). When the underlying connection is known to be connected (the flag
CO_FL_CONNECTED set), the stream-interface is switched to the ready state
(SI_ST_RDY). It is a transient state between the connect state (SI_ST_CON) and
the established state (SI_ST_EST). It must be handled on the next call to
process_stream(), which is responsible to operate the transition. During all
this time, errors can occur. A connection error or a client abort. The transient
state SI_ST_RDY was introduced to let a chance to process_stream() to catch
these errors before considering the connection as fully established.
Unfortunatly, if a read0 is catched in states SI_ST_CON or SI_ST_RDY, it is
possible to have a shutdown without transition to SI_ST_DIS (in fact, here,
SI_ST_CON is swichted to SI_ST_RDY). This happens if the request was fully
received and analyzed. In this case, the flag SI_FL_NOHALF is set on the backend
stream-interface. If an error is also reported during the connect, the behavior
is undefined because an error is returned to the client and a connection retry
is performed. So on the next connection attempt to the server, if another error
is reported, a client abort is detected. But the shutdown for writes was already
done. So the transition to the state SI_ST_DIS is impossible. We stay in the
state SI_ST_RDY. Because it is a transient state, we loop in process_stream() to
perform the transition.
It is hard to understand how the bug happens reading the code and even harder to
explain. But there is a trivial way to hit the bug by sending h2 requests to a
server only speaking h1. For instance, with the following config :
listen tst
bind *:80
server www 127.0.0.1:8000 proto h2 # in reality, it is a HTTP/1.1 server
It is a configuration error, but it is an easy way to observe the bug. Note it
may happen with a valid configuration.
So, after a careful analyzis, it appears that si_cs_recv() should never be
called for a not fully established stream-interface. This way the connection
retries will be performed before reporting an error to the client. Thus, if a
shutdown is performed because a read0 is handled, the stream-interface is
inconditionnaly set to the transient state SI_ST_DIS.
This patch must be backported to 2.0 and 1.9. However on these versions, this
patch reveals a design flaw about connections and a bad way to perform the
connection retries. We are working on it.
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)