When a backend has no LB algo specified and is not in dispatch, proxy
nor transparent mode, use "balance roundrobin" by default instead of
complaining. This will be particularly useful with stats and redirects.
When data are forwarded between socket, we must update the output
socket's write timeout. This was forgotten, causing sessions to
unexpectedly expire during long posts.
Released version 1.3.16-rc1 with the following main changes :
- appsessions: cleanup DEBUG_HASH and initialize request_counter
- [MINOR] acl: add new keyword "connslots"
- [MINOR] cfgparse: fix off-by 2 in error message size
- [BUILD] fix build with gcc 4.3
- [BUILD] fix MANDIR default location to match documentation
- [TESTS] add a debug patch to help trigger the stats bug
- [BUG] Flush buffers also where there are exactly 0 bytes left
- [MINOR] Allow to specify a domain for a cookie
- [BUG/CLEANUP] cookiedomain -> cookie_domain rename + free(p->cookie_domain)
- [MEDIUM] Fix memory freeing at exit
- [MEDIUM] Fix memory freeing at exit, part 2
- [BUG] Fix listen & more of 2 couples <ip>:<port>
- [DOC] remove buggy comment for use_backend
- [CRITICAL] fix server state tracking: it was O(n!) instead of O(n)
- [MEDIUM] add support for URI hash depth and length limits
- [MINOR] permit renaming of x-forwarded-for header
- [BUILD] fix Makefile.bsd and Makefile.osx for stream_interface
- [BUILD] Haproxy won't compile if DEBUG_FULL is defined
- [MEDIUM] upgrade to ebtree v4.0
- [DOC] update the README file with new build options
- [MEDIUM] reduce risk of event starvation in ev_sepoll
- [MEDIUM] detect streaming buffers and tag them as such
- [MEDIUM] add support for conditional HTTP redirection
- [BUILD] make install should depend on haproxy not "all"
- [DEBUG] add a TRACE macro to facilitate runtime data extraction
- [BUG] event pollers must not wait if a task exists in the run queue
- [BUG] queue management: wake oldest request in queues
- [BUG] log: reported queue position was offed-by-one
- [BUG] fix the dequeuing logic to ensure that all requests get served
- [DOC] documentation for the "retries" parameter was missing.
- [MEDIUM] implement a monotonic internal clock
- [MEDIUM] further improve monotonic clock by check forward jumps
- [OPTIM] add branch prediction hints in list manipulations
- [MAJOR] replace ultree with ebtree in wait-queues
- [BUG] we could segfault during exit while freeing uri_auths
- [BUG] wqueue: perform proper timeout comparisons with wrapping values
- [MINOR] introduce now_ms, the current date in milliseconds
- [BUG] disable buffer read timeout when reading stats
- [MEDIUM] rework the wait queue mechanism
- [BUILD] change declaration of base64tab to fix build with Intel C++
- [OPTIM] shrink wake_expired_tasks() by using task_wakeup()
- [MAJOR] use an ebtree instead of a list for the run queue
- [MEDIUM] introduce task->nice and boot access to statistics
- [OPTIM] task_queue: assume most consecutive timers are equal
- [BUILD] silent a warning in unlikely() with gcc 4.x
- [MAJOR] convert all expiration timers from timeval to ticks
- [BUG] use_backend would not correctly consider "unless"
- [TESTS] added test-acl.cfg to test some ACL combinations
- [MEDIUM] add support for configuration keyword registration
- [MEDIUM] modularize the global "stats" keyword configuration parser
- [MINOR] cfgparse: add support for warnings in external functions
- [MEDIUM] modularize the "timeout" keyword configuration parser
- [MAJOR] implement tcp request content inspection
- [MINOR] acl: add a new parsing function: parse_dotted_ver
- [MINOR] acl: add req_ssl_ver in TCP, to match an SSL version
- [CLEANUP] remove unused include/types/client.h
- [CLEANUP] remove many #include <types/xxx> from C files
- [CLEANUP] remove dependency on obsolete INTBITS macro
- [DOC] document the new "tcp-request" keyword and associated ACLs
- [MINOR] acl: add REQ_CONTENT to the list of default acls
- [MEDIUM] acl: permit fetch() functions to set the result themselves
- [MEDIUM] acl: get rid of dummy values in always_true/always_false
- [MINOR] acl: add the "wait_end" acl verb
- [MEDIUM] acl: enforce ACL type checking
- [MEDIUM] acl: set types on all currently known ACL verbs
- [MEDIUM] acl: when possible, report the name and requirements of ACLs in warnings
- [CLEANUP] remove 65 useless NULL checks before free
- [MEDIUM] memory: update pool_free2() to support NULL pointers
- [MEDIUM] buffers: ensure buffer_shut* are properly called upon shutdowns
- [MEDIUM] process_srv: rely on buffer flags for client shutdown
- [MEDIUM] process_srv: don't rely at all on client state
- [MEDIUM] process_cli: don't rely at all on server state
- [BUG] fix segfault with url_param + check_post
- [BUG] server timeout was not considered in some circumstances
- [BUG] client timeout incorrectly rearmed while waiting for server
- [MAJOR] kill CL_STINSPECT and CL_STHEADERS (step 1)
- [MAJOR] get rid of SV_STANALYZE (step 2)
- [MEDIUM] simplify and centralize request timeout cancellation and request forwarding
- [MAJOR] completely separate HTTP and TCP states on the request path
- [BUG] fix recently introduced loop when client closes early
- [MAJOR] get rid of the SV_STHEADERS state
- [MAJOR] better separation of response processing and server state
- [MAJOR] clearly separate HTTP response processing from TCP server state
- [MEDIUM] remove unused references to {CL|SV}_STSHUT*
- [MINOR] term_trace: add better instrumentations to trace the code
- [BUG] ev_sepoll: closed file descriptors could persist in the spec list
- [BUG] process_response must not enable the read FD
- [BUG] buffers: remove BF_MAY_CONNECT and fix forwarding issue
- [BUG] process_response: do not touch srv_state
- [BUG] maintain_proxies must not disable backends
- [CLEANUP] get rid of BF_SHUT*_PENDING
- [MEDIUM] buffers: add BF_EMPTY and BF_FULL to remove dependency on req/rep->l
- [MAJOR] process_session: rely only on buffer flags
- [MEDIUM] use buffer->wex instead of buffer->cex for connect timeout
- [MEDIUM] centralize buffer timeout checks at the top of process_session
- [MINOR] ensure the termination flags are set by process_xxx
- [MEDIUM] session: move the analysis bit field to the buffer
- [OPTIM] process_cli/process_srv: reduce the number of tests
- [BUG] regparm is broken on gcc < 3
- [BUILD] fix warning in proto_tcp.c with gcc >= 4
- [MEDIUM] merge inspect_exp and txn->exp into request buffer
- [BUG] process_cli/process_srv: don't call shutdown when already done
- [BUG] process_request: HTTP body analysis must return zero if missing data
- [TESTS] test-fsm: 22 regression tests for state machines
- [BUG] Fix empty X-Forwarded-For header name when set in defaults section
- [BUG] fix harmless but wrong fd insertion sequence
- [MEDIUM] make it possible for analysers to follow the whole session
- [MAJOR] rework of the server FSM
- [OPTIM] remove useless fd_set(read) upon shutdown(write)
- [MEDIUM] massive cleanup of process_srv()
- [MEDIUM] second level of code cleanup for process_srv_data
- [MEDIUM] third cleanup and optimization of process_srv_data()
- [MEDIUM] process_srv_data: ensure that we always correctly re-arm timeouts
- [MEDIUM] stream_sock_process_data moved to stream_sock.c
- [MAJOR] make the client side use stream_sock_process_data()
- [MEDIUM] split stream_sock_process_data
- [OPTIM] stream_sock_read must check for null-reads more often
- [MINOR] only call flow analysers when their read side is connected.
- [MEDIUM] reintroduce BF_HIJACK with produce_content
- [MINOR] re-arrange buffer flags and rename some of them
- [MINOR] do not check for BF_SHUTR when computing write timeout
- [OPTIM] ev_sepoll: detect newly created FDs and check them once
- [OPTIM] reduce the number of calls to task_wakeup()
- [OPTIM] force inlining of large functions with gcc >= 3
- [MEDIUM] indicate a reason for a task wakeup
- [MINOR] change type of fdtab[]->owner to void*
- [MAJOR] make stream sockets aware of the stream interface
- [MEDIUM] stream interface: add the ->shutw method as well as in and out buffers
- [MEDIUM] buffers: add BF_READ_ATTACHED and BF_ANA_TIMEOUT
- [MEDIUM] process_session: make use of the new buffer flags
- [CLEANUP] process_session: move debug outputs out of the critical loop
- [MEDIUM] move QUEUE and TAR timers to stream interfaces
- [OPTIM] add compiler hints in tick_is_expired()
- [MINOR] add buffer_check_timeouts() to check what timeouts have fired.
- [MEDIUM] use buffer_check_timeouts instead of stream_sock_check_timeouts()
- [MINOR] add an expiration flag to the stream_sock_interface
- [MAJOR] migrate the connection logic to stream interface
- [MAJOR] add a connection error state to the stream_interface
- [MEDIUM] add the SN_CURR_SESS flag to the session to track open sessions
- [MEDIUM] continue layering cleanups.
- [MEDIUM] stream_interface: added a DISconnected state between CON/EST and CLO
- [MEDIUM] remove stream_sock_update_data()
- [MINOR] maintain a global session list in order to ease debugging
- [BUG] shutw must imply close during a connect
- [MEDIUM] process shutw during connection attempt
- [MEDIUM] make the stream interface control the SHUT{R,W} bits
- [MAJOR] complete layer4/7 separation
- [CLEANUP] move the session-related functions to session.c
- [MINOR] call session->do_log() for logging
- [MINOR] replace the ambiguous client_return function by stream_int_return
- [MINOR] replace client_retnclose() with stream_int_retnclose()
- [MINOR] replace srv_close_with_err() with http_server_error()
- [MEDIUM] make the http server error function a pointer in the session
- [CLEANUP] session.c: removed some migration left-overs in sess_establish()
- [MINOR] stream_sock_data_finish() should not expose fd
- [MEDIUM] extract TCP request processing from HTTP
- [MEDIUM] extract the HTTP tarpit code from process_request().
- [MEDIUM] move the HTTP request body analyser out of process_request().
- [MEDIUM] rename process_request to http_process_request
- [BUG] fix forgotten server session counter
- [MINOR] declare process_session in session.h, not proto_http.h
- [MEDIUM] first pass of lifting to proto_uxst.c:uxst_event_accept()
- [MINOR] add an analyser code for UNIX stats request
- [MINOR] pre-set analyser flags on the listener at registration time
- [BUG] do not forward close from cons to prod with analysers
- [MEDIUM] ensure that sock->shutw() also closes read for init states
- [MINOR] add an analyser state in struct session
- [MAJOR] make unix sockets work again with stats
- [MEDIUM] remove cli_fd, srv_fd, cli_state and srv_state from the session
- [MINOR] move the listener reference from fd to session
- [MEDIUM] reference the current hijack function in the buffer itself
- [MINOR] slightly rebalance stats_dump_{raw,http}
- [MINOR] add a new back-reference type : struct bref
- [MINOR] add back-references to sessions for later use by a dumper.
- [MEDIUM] add support for "show sess" in unix stats socket
- [BUG] do not release the connection slot during a retry
- [BUG] dynamic connection throttling could return a max of zero conns
- [BUG] do not try to pause backends during reload
- [BUG] ensure that listeners from disabled proxies are correctly unbound.
- [BUG] acl-related keywords are not allowed in defaults sections
- [BUG] cookie capture is declared in the frontend but checked on the backend
- [BUG] critical errors should be reported even in daemon mode
- [MINOR] redirect: add support for the "drop-query" option
- [MINOR] redirect: add support for "set-cookie" and "clear-cookie"
- [MINOR] redirect: in prefix mode a "/" means not to change the URI
- [BUG] do not dequeue requests on a dead server
- [BUG] do not dequeue the backend's pending connections on a dead server
- [MINOR] stats: indicate if a task is running in "show sess"
- [BUG] check timeout must not be changed if timeout.check is not set
- [BUG] "option transparent" is for backend, not frontend !
- [MINOR] transfer errors were not reported anymore in data phase
- [MEDIUM] add a send limit to a buffer
- [MEDIUM] don't report buffer timeout when there is I/O activity
- [MEDIUM] indicate when we don't care about read timeout
- [MINOR] add flags to indicate when a stream interface is waiting for space/data
- [MEDIUM] enable inter-stream_interface wakeup calls
- [MAJOR] implement autonomous inter-socket forwarding
- [MINOR] add the splice_len member to the buffer struct in preparation of splice support
- [MEDIUM] stream_sock: factor out the return path in case of no-writes
- [MEDIUM] i/o: rework ->to_forward and ->send_max
- [OPTIM] stream_sock: do not ask for polling on EAGAIN if we have read
- [OPTIM] buffer: replace rlim by max_len
- [OPTIM] stream_sock: factor out the buffer full handling out of the loop
- [CLEANUP] replace a few occurrences of (flags & X) && !(flags & Y)
- [CLEANUP] stream_sock: move the write-nothing condition out of the loop
- [MEDIUM] split stream_sock_write() into callback and core functions
- [MEDIUM] stream_sock_read: call ->chk_snd whenever there are data pending
- [MINOR] stream_sock: fix a few wrong empty calculations
- [MEDIUM] stream_sock: try to send pending data on chk_snd()
- [MINOR] global.maxpipes: add the ability to reserve file descriptors for pipes
- [MEDIUM] splice: add configuration options and set global.maxpipes
- [MINOR] introduce structures required to support Linux kernel splicing
- [MEDIUM] add definitions for Linux kernel splicing
- [MAJOR] complete support for linux 2.6 kernel splicing
- [BUG] reserve some pipes for backends with splice enabled
- [MEDIUM] splice: add hints to support older buggy kernels
- [MEDIUM] introduce pipe pools
- [MEDIUM] splice: make use of pipe pools
- [STATS] report pipe usage in the statistics
- [OPTIM] make global.maxpipes default to global.maxconn/4 when not specified
- [BUILD] fix snapshot date extraction with negative timezones
- [MEDIUM] move global tuning options to the global structure
- [MEDIUM] splice: add the global "nosplice" option
- [BUILD] add USE_LINUX_SPLICE to enable LINUX_SPLICE on linux 2.6
- [BUG] we must not exit if protocol binding only returns a warning
- [MINOR] add support for bind interface name
- [BUG] inform the user when root is expected but not set
- [MEDIUM] add support for source interface binding
- [MEDIUM] add support for source interface binding at the server level
- [MEDIUM] implement bind-process to limit service presence by process
- [DOC] document maxpipes, nosplice, option splice-{auto,request,response}
- [DOC] filled the logging section of the configuration manual
- [DOC] document HTTP status codes
- [DOC] document a few missing info about errorfile
- [BUG] fix random memory corruption using "show sess"
- [BUG] fix unix socket processing of interrupted output
- [DOC] add diagrams of queuing and future ACL design
- [BUILD] proto_http did not build on gcc-2.95
- [BUG] the "source" keyword must first clear optional settings
- [BUG] global.tune.maxaccept must be limited even in mono-process mode
- [MINOR] ensure that http_msg_analyzer updates pointer to invalid char
- [MEDIUM] store a complete dump of request and response errors in proxies
- [MEDIUM] implement error dump on unix socket with "show errors"
- [DOC] document "show errors"
- [MINOR] errors dump must use user-visible date, not internal date.
- [MINOR] time: add __usec_to_1024th to convert usecs to 1024th of second
- [MINOR] add curr_sec_ms and curr_sec_ms_scaled for current second.
- [MEDIUM] measure and report session rate on frontend, backends and servers
- [BUG] the "connslots" keyword was matched as "connlots"
- [MINOR] acl: add 2 new verbs: fe_sess_rate and be_sess_rate
- [MEDIUM] implement "rate-limit sessions" for the frontend
- [BUG] interface binding: length must include the trailing zero
- [BUG] typo in timeout error reporting : report *res and not *err
- [OPTIM] maintain_proxies: only wake up when the frontend will be ready
- [OPTIM] rate-limit: cleaner behaviour on low rates and reduce consumption
- [BUG] switch server-side stream interface to close in case of abort
- [CLEANUP] remove last references to term_trace
- [OPTIM] freq_ctr: do not rotate the counters when reading
- [BUG] disable any analysers for monitoring requests
- [BUG] rate-limit in defaults section was ignored
- [BUG] task: fix handling of duplicate keys
- [OPTIM] task: don't unlink a task from a wait queue when waking it up
- [OPTIM] displace tasks in the wait queue only if absolutely needed
- [MEDIUM] minor update to the task api: let the scheduler queue itself
- [BUG] event_accept() must always wake the task up, even in health mode
- [CLEANUP] task: distinguish between clock ticks and timers
- [OPTIM] task: reduce the number of calls to task_queue()
- [OPTIM] do not re-check req buffer when only response has changed
- [CLEANUP] don't enable kernel splicing when socket is closed
- [CLEANUP] buffer_flush() was misleading, rename it as buffer_erase
- [MINOR] buffers: implement buffer_flush()
- [MEDIUM] rearrange forwarding condition to enable splice during analysis
- [BUILD] build fixes for Solaris
- [BUILD] proto_http did not build on gcc-2.95 (again)
- [CONTRIB] halog: fast log parser for haproxy
- [CONTRIB] halog: faster fgets() and add support for percentile reporting
The forwarding condition was not very clear. We would only enable
forwarding when send_max is zero, and we would only splice when no
analyser is installed. In fact we want to enable forward when there
is no analyser and we want to splice at soon as there is data to
forward, regardless of the analysers.
In process_session(), we used to re-run through all the evaluation
loop when only the response had changed. Now we carefully check in
this order :
- changes to the stream interfaces (only SI_ST_DIS)
- changes to the request buffer flags
- changes to the response buffer flags
And we branch to the appropriate section. This saves significant
CPU cycles, which is important since process_session() is one of
the major CPU eaters.
The same changes have been applied to uxst_process_session().
Most of the time, task_queue() will immediately return. By extracting
the preliminary checks and putting them in an inline function, we can
significantly reduce the number of calls to the function itself, and
most of the tests can be optimized away due to the caller's context.
Another minor improvement in process_runnable_tasks() consisted in
taking benefit from the processor's branch prediction unit by making
a special case of the process_session() callback which is by far the
most common one.
All this improved performance by about 1%, mainly during the call
from process_runnable_tasks().
Timers are unsigned and used as tree positions. Ticks are signed and
used as absolute date within current time frame. While the two are
normally equal (except zero), it's important not to confuse them in
the code as they are not interchangeable.
We add two inline functions to turn each one into the other.
The comments have also been moved to the proper location, as it was
not easy to understand what was a tick and what was a timer unit.
All the tasks callbacks had to requeue the task themselves, and update
a global timeout. This was not convenient at all. Now the API has been
simplified. The tasks callbacks only have to update their expire timer,
and return either a pointer to the task or NULL if the task has been
deleted. The scheduler will take care of requeuing the task at the
proper place in the wait queue.
We don't need to remove then add tasks in the wait queue every time we
update a timeout. We only need to do that when the new timeout is earlier
than previous one. We can rely on wake_expired_tasks() to perform the
proper checks and bounce the misplaced tasks in the rare case where this
happens. The motivation behind this is that we very rarely hit timeouts,
so we save a lot of CPU cycles by moving the tasks very rarely. This now
means we can also find tasks with expiration date set to eternity in the
queue, and that is not a problem.
In many situations, we wake a task on an I/O event, then queue it
exactly where it was. This is a real waste because we delete/insert
tasks into the wait queue for nothing. The only reason for this is
that there was only one tree node in the task struct.
By adding another tree node, we can have one tree for the timers
(wait queue) and one tree for the priority (run queue). That way,
we can have a task both in the run queue and wait queue at the
same time. The wait queue now really holds timers, which is what
it was designed for.
The net gain is at least 1 delete/insert cycle per session, and up
to 2-3 depending on the workload, since we save one cycle each time
the expiration date is not changed during a wake up.
A bug was introduced with the ebtree-based scheduler. It seldom causes
some timeouts to last longer than required if they hit an expiration
date which is the same as the last queued date, is also part of a
duplicate tree without being the top of the tree. In this case, the
task will not be expired until after the duplicate tree has been
flushed.
It is easier to reproduce by setting a very short client timeout (1s)
and sending connections and waiting for them to expire with the 408
status. Then in parallel, inject at about 1kh/s. The bug causes the
connections to sometimes wait longer than 1s before timing out.
The cause was the use of eb_insert_dup() on wrong nodes, as this
function is designed to work only on the top of the dup tree. The
solution consists in updating last_timer only when its bit is -1,
and using it only if its bit is still -1 (top of a dup tree).
The fix has not reduced performance because it only fixes the case
where this bug could fire, which is extremely rare.
It's easier to take the counter's age into account when consulting it
than to rotate it first. It also saves some CPU cycles and avoids the
multiply for outdated counters, finally saving CPU cycles here too
when multiple operations need to read the same counter.
The freq_ctr code has also shrinked by one third consecutively to these
optimizations.
term_trace was very useful while reworking the lower layers but has almost
completely been removed from every place it was referenced. Even the few
remaining ones were not accurate, so it's better to completely remove those
references and re-add them from scratch later if needed.
In pure TCP mode, there is no response analyser to switch the server-side
stream interface from INI to CLO when the output has been closed after an
abort. This caused sessions to remain indefinitely active when they were
aborted by the client during a TCP content analysis.
The proper action is to switch the stream interface to the CLO state from
INI when we have write enable and shutdown write.
The rate-limit was applied to the smoothed value which does a special
case for frequencies below 2 events per period. This caused irregular
limitations when set to 1 session per second.
The proper way to handle this is to compute the number of remaining
events that can occur without reaching the limit. This is what has
been added. It also has the benefit that the frequency calculation
is now done once when entering event_accept(), before the accept()
loop, and not once per accept() loop anymore, thus saving a few CPU
cycles during very high loads.
With this fix, rate limits of 1/s are perfectly respected.
The new "rate-limit sessions" statement sets a limit on the number of
new connections per second on the frontend. As it is extremely accurate
(about 0.1%), it is efficient at limiting resource abuse or DoS.
These new ACLs match frontend session rate and backend session rate.
Examples are provided in the doc to explain how to use that in order
to limit abuse of service.
With this change, all frontends, backends, and servers maintain a session
counter and a timer to compute a session rate over the last second. This
value will be very useful because it varies instantly and can be used to
check thresholds. This value is also reported in the stats in a new "rate"
column.
Several algorithms will need to know the millisecond value within
the current second. Instead of doing a divide every time it is needed,
it's better to compute it when it changes, which is when now and now_ms
are recomputed.
curr_sec_ms_scaled is the same multiplied by 2^32/1000, which will be
useful to compute some ratios based on the position within last second.
The new "show errors" command sent on a unix socket will dump
all captured request and response errors for all proxies. It is
also possible to bound the log to frontends and backends whose
ID is passed as an optional parameter.
The output provides information about frontend, backend, server,
session ID, source address, error type, and error position along
with a complete dump of the request or response which has caused
the error.
If a new error scratches the one currently being reported, then
the dump is aborted with a warning message, and processing goes
on to next error.
Each proxy instance, either frontend or backend, now has some room
dedicated to storing a complete dated request or response in case
of parsing error. This will make it possible to consult errors in
order to find the exact cause, which is particularly important for
troubleshooting faulty applications.
If an invalid character is encountered while parsing an HTTP message, we
want to get buf->lr updated to reflect it.
Along this change, a few useless __label__ declarations have been removed
because they caused gcc to consume stack space without putting anything
there.
On overloaded systems, it sometimes happens that hundreds or thousands
of incoming connections are queued in the system's backlog, and all get
dequeued at once. The problem is that when haproxy processes them and
does not apply any limit, this can take some time and the internal date
does not progress, resulting in wrong timer measures for all sessions.
The most common effect of this is that all of these sessions report a
large request time (around several hundreds of ms) which is in fact
caused by the time spent accepting other connections. This might happen
on shared systems when the machine swaps.
For this reason, we finally apply a reasonable limit even in mono-process
mode. Accepting 100 connections at once is fast enough for extreme cases
and will not cause that much of a trouble when the system is saturated.
Problem reported by John Lauro. When "source ... usesrc ..." is
set in the defaults section, it is not possible anymore to remove
the "usesrc" part when declaring a more precise "source" in a
backend. The only workaround was to declare it by server.
We need to clear optional settings when declaring a new "source".
The problem was the same with the "interface" declaration.
Unix socket processing was still quite buggy. It did not properly
handle interrupted output due to a full response buffer. The fix
mainly consists in not trying to prematurely enable write on the
response buffer, just like the standard session works. This also
gets the unix socket code closer to the standard session code
handling.
Commit 8a5c626e73 introduced the sessions
dump on the unix socket. This implementation is buggy because it may try
to link to the sessions list's head after the last session is removed
with a backref. Also, for the LIST_ISEMPTY test to succeed, we have to
proceed with LIST_INIT after LIST_DEL.
