The commit 9425aeaffb ("BUG/MAJOR: peers: Update peers section state from a
thread-safe manner") introduced regressions about state transitions of a
peer.
A peer may be in a connected, accepted or released state. Before, changes for
these states were performed synchronously. Since the commit above, changes
are mainly performed in the sync process task.
The first regression was about the released then accepted state transition,
called the renewed state. In reality the state was always crushed by the
accepted state. After some review, the state was just removed to always
perform the cleanup in the sync process task before acknowledging the
connected or accepted states.
Then, a wakeup of the peer applet was missing from the sync process task
after the ack of connected or accepted states, blocking the applet.
Finally, when a peer is in released, connected or accepted state, we must
take care to wait the sync process task wakeup before trying to receive or
send messages.
This patch must only be backported if the above commit is backported.
The bug was introduced by commit 9425aeaffb ("BUG/MAJOR: peers: Update peers
section state from a thread-safe manner"). A peers flags was set on a peer
by error. Just remove it.
This patch must only be backported if the above commit is backported.
Now that peers entirely rely on peer->srv for connection settings, and
that it was confirmed that it works properly thanks to previous commit,
let's finish what we started in f6ae258 ("MINOR: peers: rely on srv->addr
and remove peer->addr") and stop using srv->addr to find out peers port
and instead rely on srv->svc_port as it's already done for other proxy
types.
When a peer applet is pushing updates, we limit the number of update sent at
once via a global parameter to not spend too much time in the applet. On
interrupt, we claimed for more room to be woken up quickly. However, this
statement is only true if something was pushed in the buffer. Otherwise,
with an empty buffer, if the stream itself is not woken up, the applet
remains also blocked because there is no send activity on the other side to
unblock it.
In this case, instead of requesting more room, it is sufficient to state the
applet have more data to send.
This patch must be backported as far as 2.6.
The last fixes on the peers to improve the locking mechanism introduced new
peer flags and the value of some old flags was changed. This was done in the
commit 9b78e33837 ("MINOR: peers: Add 2 peer flags about the peer learn
status"). But, to ease the debugging of the peers team, old values are
restored.
This patch must be backported with the commit above.
Thanks to all previous changes, it is now possible to stop locking all peers
at once in the resync process function. Peer are locked one after the
other. Wen a peer is locked, another one may be locked when all peer sharing
the same shard must be updated. Otherwise, at anytime, at most one peer is
locked. This should significantly improve the situation.
This patch depends on the following patchs:
* BUG/MAJOR: peers: Update peers section state from a thread-safe manner
* BUG/MINOR: peers: Report a resync was explicitly requested from a thread-safe manner
* MINOR: peers: Add functions to commit peer changes from the resync task
* MINOR: peers: sligthly adapt part processing the stopping signal
* MINOR: peers: Add flags to report the peer state to the resync task
* MINOR: peers: Add 2 peer flags about the peer learn status
* MINOR: peers: Split resync process function to separate running/stopping states
It may be good to backport it to 2.9. All the seris should fix the issue #2470.
It is the main part of this series. In the peer applet, only the peer flags
are updated. It is now the responsibility of the resync process function to
check changes on each peer to update the peers section state accordingly.
Concretly, changes on the connection state (accepted, connected, released or
renewed) are first reported at the peer level and then handled in
__process_peer_state() function.
In the same manner, when the learn status of a peer changes, the peers
section state is no longer updated immediately. The resync task is woken up
to deal with this changes.
Thanks to these changes, the peers should be now really thread-safe.
This patch relies on the following ones:
* BUG/MINOR: peers: Report a resync was explicitly requested from a thread-safe manner
* MINOR: peers: Add functions to commit peer changes from the resync task
* MINOR: peers: sligthly adapt part processing the stopping signal
* MINOR: peers: Add flags to report the peer state to the resync task
* MINOR: peers: Add 2 peer flags about the peer learn status
* MINOR: peers: Split resync process function to separate running/stopping states
No bug was reported about the thread-safety of peers. Only a performance
issue was encountered with a huge number of peers (> 50). So there is no
reason to backport all these patches further than 2.9.
Flags on the peers section state must be updated from a thread-safe manner.
It is not true today. With this patch we take care PEERS_F_RESYNC_REQUESTED
flag is only set by the resync task. To do so, a peer flag is used. This
flag is only set once and never removed. It is juste used for debugging
purpose. So it is enough to set it on a peer and be sure to report it on the
peers section when the sync task is executed.
This patch relies on previous ones:
* MINOR: peers: Add functions to commit peer changes from the resync task
* MINOR: peers: sligthly adapt part processing the stopping signal
* MINOR: peers: Add flags to report the peer state to the resync task
* MINOR: peers: Add 2 peer flags about the peer learn status
* MINOR: peers: Split resync process function to separate running/stopping states
For now, nothing is done in these functions. It is only a patch to prepare
the huge part of the refactoring about the locking mechanism of the peers.
These functions will be responsible to check peers state and their learn
status to update the peers section flags accordingly.
The signal and the PEERS_F_DONOTSTOP flag are now handled in the loop on peers
to force sessions shutdown. We will need to loop on all peers to update their
state. It is easier this way.
As the previous patch, this patch is also part of the refactoring of peer
locking mechanisme. Here we add flags to represent a transitional state for
a peer. It will be the resync task responsibility to update the peers state
accordingly.
A peer may be in 4 transitional states:
* accepted : a connection was accepted from a peer
* connected: a connection to a peer was established
* release : a peer session was released
* renewed : a peer session was released because it was replaced by a new
one. Concretly, this will be equivalent to released+accepted
If none of these flags is set, it means the transition, if any, was
processed by the resync task, or no transition happened.
PEER_F_LEARN_PROCESS and PEER_F_LEARN_FINISHED flags are added to help to
fix locking issue about peers. Indeed, a peer is able to update the peers
"section" state under its own lock. Because the resync task locks all peers
at once, there is no conflict at this level. But there is nothing to prevent
2 peers to update the peers state in same time. So it seems there is no real
issue here, but there is a theorical thread-safety issue here. And it means
the locking mechanism of the peers must be reviewed.
In this context, the 2 flags above will help to move all update of the peers
state in the scope of resync task. Each peer will be able to update its own
state and the resync task will be responsible to update the peers state
accordingly.
The function responsible to deal with resynchro between all peers is now split
in two subfunctions. The first one is used when HAProxy is running while the
other one is used in soft-stop case.
This patch is required to be able to refactor locking mechanism of the peers.
Sending "trace peers event" on the CLI crashes because the event list
in the peers is not finished. This was introduced in 2.4 by commit
d865935f32 ("MINOR: peers: Add traces to peer_treat_updatemsg().")
so this must be backported to 2.4.
Right now we're taking the stick-tables update lock for reads just for
the sake of checking if the update index is past it or not. That's
costly because even taking the read lock is sufficient to provoke a
cache line write, while when under load or attack it's frequent that
the update has not yet been propagated and wouldn't require anything.
This commit brings a new field to the stksess, "seen", which is zeroed
when the entry is updated, and set to one as soon as at least one peer
starts to consult it. This way it will reflect that the entry must be
updated again so that this peer can see it. Otherwise no update will
be necessary. For now the flag is only set/reset but not exploited.
A great care is taken to avoid writes whenever possible.
In peer_send_msg(), we take a lock before calling
peer_send_teach_process_msgs because of the check on the flags and update
indexes, and the function then drops it then takes it again just to resume
in the same situation, so that on return we can drop it again! Not only
this is absurd because it doubles the costs of taking the lock, it's also
totally inefficient because it takes a write lock while the only usage
that is done with it is to read the indexes! Let's drop the lock from
peer_send_teach_process_msgs() and move it explicitly in its only caller
around the condition, and turn it into a read lock only.
In 2.9, the stick-tables' locking was split between the lock used to
manipulate the contents (->lock) and the lock used to manipulate the
list of updates and the update indexes (->updt_lock). This was done
with commit 87e072eea5 ("MEDIUM: stick-table: use a distinct lock for
the updates tree"). However a part was overlooked in the peers code,
the parts that consult (and update) the indexes use the table's lock
instead of the update lock. It's surprising that it hasn't caused more
trouble. It's likely due to the fact that the tree nodes are not often
immediately freed and that their memory area remains connected to valid
nodes in the tree during peer_stksess_lookup(), while other parts only
check or update indexes, thus are not that critical.
This needs to be backported wherever the commit above is, thus logically
2.9.
These both flags are set after releasing the applet, in
appctx_shut(). Concretly, it means the applet is shutdown for reads and
writes. Once set, the applet's I/O handler was no longer called. Tests on
these flags are useless. There is no chance to match them.
Similarly to the previous commit, we get rid of unused peer member.
peer->addr was only used to save a copy of the sever's addr at parsing
time. But instead of relying on an intermediate variable, we can actually
use server's address directly when initiating the peer session.
As with other streams created from server's settings (tcp/http, log, ring),
we should rely on srv->svc_port for the port part of the address. This
shouldn't change anything for peers since the address is fully resolved
at parsing time and runtime changes are not supported, but this should
help to make the code future-proof.
peer_recv_msg() may return because the message is incomplete without
checking if a shutdown is pending for the SC. The function relies on
co_getblk() to detect shutdowns. However, the message length decoding may be
interrupted if the multi-bytes integer is incomplete. In this case, the SC
is not check for shutdowns.
When this happens, this leads to an appctx spinning loop.
This patch should fix the issue #2373. It must be backported to 2.8.
In this patch, we add the possibility to declare on a table definition
("table" in peer section, or "stick-table" in proxy section) that we
want the remote/peer updates on that table to be pushed on a local
haproxy table in addition to the source table.
Consider this example:
|peers mypeers
| peer local 127.0.0.1:3334
| peer clust 127.0.0.1:3333
| table t1.local type string size 10m store server_id,server_key expire 30s
| table t1.clust type string size 10m store server_id,server_key write-to mypeers/t1.local expire 30s
With this setup, we consider haproxy uses t1.local as cache/local table
for read and write operations, and that t1.clust is a remote table
containing datas processed from t1.local and similar tables from other
haproxy peers in a cluster setup. The t1.clust table will be used to
refresh the local/cache one via the "write-to" statement.
What will happen, is that every time haproxy will see entry updates for
the t1.clust table: it will overwrite t1.local table with fresh data and
will update the entry expiration timer. If t1.local entry doesn't exist
yet (key doesn't exist), it will automatically create it. Note that only
types that cannot be used for arithmetic ops will be handled, and this
to prevent processed values from the remote table from interfering with
computations based on values from the local table. (ie: prevent
cumulative counters from growing indefinitely).
"write-to" will only push supported types if they both exist in the source
and the target table. Be careful with server_id and server_key storage
because they are often declared implicitly when referencing a table in
sticking rules but it is required to declare them explicitly for them to
be pushed between a remote and a local table through "write-to" option.
Also note that the "write-to" target table should have the same type as
the source one, and that the key length should be strictly equal,
otherwise haproxy will raise an error due to the tables being
incompatibles. A table that is already being written to cannot be used
as a source table for a "write-to" target.
Thanks to this patch, it will now be possible to use sticking rules in
peer cluster context by using a local table as a local cache which
will be automatically refreshed by one or multiple remote table(s).
This commit depends on:
- "MINOR: stktable: stktable_init() sets err_msg on error"
- "MINOR: stktable: check if a type should be used as-is"
The number of updates sent at once was limited to not loop too long to emit
updates when the buffer size is huge or when the number of sync tables is
huge. The limit can be configured and is set to 200 by default. However,
this fix introduced a bug. It is impossible to syncrhonize two peers if the
number of tables is higher than this limit. Thus by default, it is not
possible to sync two peers if there are more than 200 tables to sync.
Technically speacking, a teaching process is finished if we loop on all tables
with no new update messages sent. Because we are limited at each call, the loop
is splitted on several calls. However the restart point for the next loop is
always the last table for which we emitted an update message. Thus with more
tables than the limit, the loop never reachs the end point.
Worse, in conjunction with the bug fixed by "BUG/MEDIUM: peers: Be sure to
always refresh recconnect timer in sync task", it is possible to trigger the
watchdog because the applets may be woken up in loop and leave requesting
more room while its buffer is empty.
To fix the issue, restart conditions for a teaching loop were changed. If
the teach process is interrupted, we now save the restart point, called
stop_local_table. It is the last evaluated table on the previous loop. This
restart point is reset when the teach process is finished.
In additionn, the updates_sent variable in peer_send_msgs() was renamed to
updates to avoid ambiguities. Indeed, the variable is incremented, whether
messages were sent or not.
This patch must be backported as far as 2.6.
A sync task used to manage reconnect, sessions creation or shutdown and data
synchronization is responsible to refresh reconnect and heartbeat timers for
each remote peers and trigger applets wakeup. These timers are used to
refresh the sync task timeer itself. Thus it is important to take care to
always properly refresh them.
However, when there are some data to push, the reconnect timer is not
checked. It may be expired and not refreshed. In this case, an expired timer
may be used to the sync task, leading to a storm of wakeups. The sync task
is woken up in loop because its timer is in the past, waking up Peer applets
at each time.
To fix the issue, the peer's reconnect timer is now refresh to the default
reconnect timeout, if necessary, when there are some data to push.
This patch must be backported to all stable versions.
The function drops the lock very early, and the only operations that
are performed on the entry code are updating the current peer's
last_local_table, which doesn't need to be protected. Thus it's
easier to drop the lock before entering the function and it further
limits its scope.
This has raised the peak RPS from 2050 to 2355k/s with a peers section on
the 80-core machine.
Updating an entry in the updates tree is currently performed under the
table's write lock, which causes huge contention with other accesses
such as lookups and free. Aside the updates tree, the update,
localupdate and commitupdate variables, nothing is manipulated, so
let's create a distinct lock (updt_lock) to protect these together
to remove this contention. It required to add an extra lock in the
few places where we delete the update (though only if we're really
going to delete it) to protect the tree. This is very convenient
because now peer_send_teachmsgs() only needs to take this read lock,
and there is very little contention left on the stick-table.
With this alone, the performance jumped from 614k to 1140k/s on a
80-thread machine with a peers section! Stick-table updates with
no peers however now has to stand two locks and slightly regressed
from 4.0-4.1M/s to 3.9-4.0. This is fairly minimal compared to the
significant unlocking of the peers updates and considered totally
acceptable.
This function doesn't need to be write-locked. It performs a lookup
of the next update at its index, atomically updates the ref_cnt on
the stksess, updates some shared_table fields on the local thread,
and updates the table's commitupdate. Now that this update is atomic
we don't need to keep the write lock during that period. In addition
this function's callers do not rely on the write lock to be held
either since it was droped during peer_send_updatemsg() anyway.
Now, when the function is entered with a write lock, it's downgraded
to a read lock, otherwise a read lock is grabbed. Updates are looked
up under the read lock and the message is sent without the lock. The
commitupdate is still performed under the read lock (so as not to
break the code too much), and the write lock is re-acquired when
leaving if needed. This allows multiple peers to look up updates in
parallel and to avoid stalling stick-table lookups.
This function maintains the write lock for a while. In practice it does
not need to hold it that long, and some parts could be performed under a
read lock. This patch first drops then re-acquires the write lock at the
function's entry. The purpose is simply to break the end-to-end atomicity
to prove that it has no impact in case something needs to be bisected
later. In fact the write lock is already dropped while calling
peer_send_updatemsg().
The ->commitupdate index doesn't need to be kept consistent with other
operations, it only needs to be correct and to reflect the last known
value. Right now it's updated under the stick-table lock, which is
expensive and maintains this lock longer than needed. Let's move it
outside of the lock, and update it using a CAS. This patch simply
replaces the assignment with a CAS and makes sure all reads are atomic.
On failed CAS we use a simple cpu_relax(), no need for more as there
should not be that much contention here (updates are not that fast).
Due to the ts->ref_cnt being manipulated and checked inside wrlocks,
we continue to have it updated under plenty of read locks, which have
an important cost on many-thread machines.
This patch turns them all to atomic ops and carefully moves them outside
of locks every time this is possible:
- the ref_cnt is incremented before write-unlocking on creation otherwise
the element could vanish before we can do it
- the ref_cnt is decremented after write-locking on release
- for all other cases it's updated out of locks since it's guaranteed by
the sequence that it cannot vanish
- checks are done before locking every time it's used to decide
whether we're going to release the element (saves several write locks)
- expiration tests are just done using atomic loads, since there's no
particular ordering constraint there, we just want consistent values.
For Lua, the loop that is used to dump stick-tables could switch to read
locks only, but this was not done.
For peers, the loop that builds updates in peer_send_teachmsgs is extremely
expensive in write locks and it doesn't seem this is really needed since
the only updated variables are last_pushed and commitupdate, the first
one being on the shared table (thus not used by other threads) and the
commitupdate could likely be changed using a CAS. Thus all of this could
theoretically move under a read lock, but that was not done here.
On a 80-thread machine with a peers section enabled, the request rate
increased from 415 to 520k rps.
sc_need_room() now takes the required free space to receive more data as
parameter. All calls to this function are updated accordingly. For now, this
value is set but not used. When we are waiting for a buffer, 0 is used. So
we expect to be unblocked ASAP. However this must be reviewed because
SC_FL_NEED_BUF is probably enough in this case and this flag is already set
if the input buffer allocation fails.
The peers applet now use the applet API to send message instead of the
channel API. This way, it does not need to take care to request more room if
it fails to put data into the channel's buffer.
This puts an end to the occasional confusion between the "now" date
that is internal, monotonic and not synchronized with the system's
date, and "date" which is the system's date and not necessarily
monotonic. Variable "now" was removed and replaced with a 64-bit
integer "now_ns" which is a counter of nanoseconds. It wraps every
585 years, so if all goes well (i.e. if humanity does not need
haproxy anymore in 500 years), it will just never wrap. This implies
that now_ns is never nul and that the zero value can reliably be used
as "not set yet" for a timestamp if needed. This will also simplify
date checks where it becomes possible again to do "date1<date2".
All occurrences of "tv_to_ns(&now)" were simply replaced by "now_ns".
Due to the intricacies between now, global_now and now_offset, all 3
had to be turned to nanoseconds at once. It's not a problem since all
of them were solely used in 3 functions in clock.c, but they make the
patch look bigger than it really is.
The clock_update_local_date() and clock_update_global_date() functions
are now much simpler as there's no need anymore to perform conversions
nor to round the timeval up or down.
The wrapping continues to happen by presetting the internal offset in
the short future so that the 32-bit now_ms continues to wrap 20 seconds
after boot.
The start_time used to calculate uptime can still be turned to
nanoseconds now. One interrogation concerns global_now_ms which is used
only for the freq counters. It's unclear whether there's more value in
using two variables that need to be synchronized sequentially like today
or to just use global_now_ns divided by 1 million. Both approaches will
work equally well on modern systems, the difference might come from
smaller ones. Better not change anyhting for now.
One benefit of the new approach is that we now have an internal date
with a resolution of the nanosecond and the precision of the microsecond,
which can be useful to extend some measurements given that timestamps
also have this resolution.
Instead we're using ns_to_sec(tv_to_ns(&now)) which allows the tv_sec
part to disappear. At this point, "now" is only used as a timeval in
clock.c where it is updated.
Just like for other applets, we now use the SE descriptor instead of the
channel to report error and end-of-stream. We must just be sure to consume
request data when we are waiting the applet to be released.
It was done by hand by callers when a shutdown for read or write was
performed. It is now always handled by the functions performing the
shutdown. This way the callers don't take care of it. This will avoid some
bugs.
Since 2.6 with commit 34e4085f8 ("MEDIUM: peers: Balance applets across
threads") the initialization of a peers appctx may be postponed with a
wakeup, causing some partially initialized appctx to be visible. The
"show peers" command used to only care about peers without appctx, but
now it must also take care of those with no stconn, otherwise it can
occasionally crash while dumping them.
This fix must be backported to 2.6. Thanks to Patrick Hemmer for
reporting the problem.
CF_READ_NULL flag is not really useful and used. It is a transient event
used to wakeup the stream. As we will see, all read events on a channel may
be resumed to only one and are all used to wake up the stream.
In this patch, we introduce CF_READ_EVENT flag as a replacement to
CF_READ_NULL. There is no breaking change for now, it is just a
rename. Gradually, other read events will be merged with this one.
In process_peer_sync: a check was performed to know whether the peers section
handler should kill itself if the corresponding proxy was not started on the
current process.
This logic was initially implemented in early 1.6 development to prevent
some issues when peers where used in conjunction with nbproc > 1:
f83d3fe00a MEDIUM: init: stop any peers section not bound to the correct process
46dc1ca MEDIUM: peers: unregister peers that were never started
But later in 1.6 dev, a new commit has been introduced:
47c8c029db MEDIUM: init: completely deallocate unused peers
With the latter, the check implemented in 46dc1ca ("MEDIUM: peers: unregister
peers that were never started") will never succeed: it is dead code.
Since nbproc support has been dropped in 2.5, things have changed a bit:
f83d3fe00a logic was moved in mworker_cleanlisteners, but as in 46dc1ca :
peers task is safely destroyed before peers_fe is set to NULL.
Conversely, peers_fe is first set by init_peers_frontend() before peers task
is scheduled by peers_init_sync() in check_config_validity().
Again, it is safe to say that we will never reach !peers->peers_fe
in process_peer_sync(): this self-killing mechanism is not relevant anymore.
--
To cut a long story short: I stumbled on this while tracking down
current signal api usage.
This led me to a signal_unregister_handler() call performed in the
aforementionned dead code. To me this code was potentially unsafe because
signal_unregister_handler() is not thread safe and here it was used within a
task initialized via task_new_anywhere(). So I decided to check how bad this
could be (ie: conditions to be met for this code to run).. and here we are.
If shards are in use, we must fill the shard number on incoming updates,
otherwise some entries are assigned shard number zero, and may be broadcast
everywhere once updated, instead of being sent only to the peers having the
same shard number.
This fixes commit 36d156564 ("MINOR: peers: Support for peer shards"). No
backport is needed.
In peer_treat_updatemsg(), the lower layers of the stick-table code are
reimplemented, and the key length is never really known for an entry
being processed, it depends on the type being parsed and the moment
where it's done. This makes it quite difficult to stuff some shard
number calculation there.
This patch adds a keylen local variable that is always set to the length
of the current key depending on its type. It takes this opportunity for
reducing redudant expressions involving this length and always using the
new variable instead, limiting the risk of errors. Arguably that code
would have been way simpler by creating a dummy stktable_key and passing
it to stksess_new() as done anywhere else, but let's not change all that
a few days before the release.
In issue #1939, Ilya mentions that cppchecks warned about use of "%d" to
report the status state that's locally stored as an unsigned int. While
technically valid, this will never cause any trouble since in the end
what we store there are the applet's states (just a few enum values).
Better use %u anyway to silence this warning.
Since 0909f62266 ("BUG/MEDIUM: peers: messages about unkown tables not
correctly ignored"), the 'sc' variable is no longer used in
peer_treat_updatemsg() and peer_treat_definemsg() functions. So, we must
remove them to avoid compilation warning.
This patch must be backported with the commit above.
Table defintion's messages and update messages are not correctly
ignored if the table is not configured on the local peer.
It is a bug because, receiving those messages, the parser
returns an error and the upper layer considers that the state of the
peer's connection is modified (as it is done in the case of protocol
error) and switch immediatly the automate to process the new state.
But, even if message is silently ignored because the connection's
state doesn't change and we continue to process the next message, some
processing remains not performed: for instance the ALIVE flag is not set
on the peer's connection as it should be done after receiving any valid
messages. This results in a shutdown of the connection when timeout
is elapsed as if no message has been received during this delay.
This patch fix the behavior, those messages are now silently ignored
and the upper layer continue the processing as it is done for any valid
messages.
This bug appears with the code re-work of the peers on 2.0 so
it should be backported until this version.
We considered the resync process is finished if a full resync request
is ended receiving the "resync-finish" message. But in the case of
"shards" each node declared with a "shard" has only a partial view
of the table. And the resync process is ended whereas the original
peer tables content contains only a "shard" of the full content.
This patch allow to retrieve the entire tables requesting a resync
from all different "shards".
To do so we don't commit the end of a resync process receiving a
"resync-finish" if the node is part of "shard", we only flag this
peer and all peers using the same shard as "notup2date" as if we
received a "resync-partial" message, and we re-schedule a request
of a resync as it is done receiving a "resync-partial" message.
Doing this the peers flagged "notup2date" won't be addressed for the
next resync request round and the next resync request will be send to
a shard not yet requested.
Receving a "resync-finish" message we also check if all peers using
"shards" are flagged "notup2date". It meens that all peers have been
addressed and we can considered the resync process is now finished.
Note also that the "resync request" scheduler already handle a timeout
and if we are not able to retrieve a full resync after a delay. The
resync process is ended.
This patch should be backported in all versions handling "shard"
on peer lines.
Add "shards" new keyword for "peers" section to configure the number
of peer shards attached to such secions. This impact all the stick-tables
attached to the section.
Add "shard" new "server" parameter to configure the peers which participate to
all the stick-tables contents distribution. Each peer receive the stick-tables updates
only for keys with this shard value as distribution hash. The "shard" value
is stored in ->shard new server struct member.
cfg_parse_peers() which is the function which is called to parse all
the lines of a "peers" section is modified to parse the "shards" parameter
stored in ->nb_shards new peers struct member.
Add srv_parse_shard() new callback into server.c to pare the "shard"
parameter.
Implement stksess_getkey_hash() to compute the distribution hash for a
stick-table key as the 64-bits xxhash of the key concatenated to the stick-table
name. This function is called by stksess_setkey_shard(), itself
called by the already implemented function which create a new stick-table
key (stksess_new()).
Add ->idlen new stktable struct member to store the stick-table name length
to not have to compute it each time a stick-table key hash is computed.
