As discussed in GH #3051, default-path is not taken into account when
loading files using lua-load-per-thread. In fact, the initial
hlua_load_state() (performed on first thread which parses the config)
is successful, but other threads run hlua_load_state() later based
on config hints which were saved by the first thread, and those config
hints only contain the file path provided on the lua-load-per-thread
config line, not the absolute one. Indeed, `default-path` directive
changes the current working directory only for the thread parsing the
configuration.
To fix the issue, when storing config hints under hlua_load_per_thread()
we now make sure to save the absolute file path for `lua-load-per-thread'
argument.
Thanks to GH user @zhanhb for having reported the issue
It may be backported to all stable versions.
Following commit 75e480d10 ("MEDIUM: stats: avoid 1 indirection by storing
the shared stats directly in counters struct"), in order to minimize the
impact of the recent sharded counters work, we try to push things a bit
further in this patch by storing and using "fast" pointers at the session
and stream levels when available to avoid costly indirections and
systematic "tgid" resolution (which can not be cached by the CPU due to
its THREAD-local nature).
Indeed, we know that a session/stream is tied to a given CPU, thanks to
this we know that the tgid for a given session/stream will never change.
Given that, we are able to store sharded frontend and listener counters
pointer at the session level (namely sess->fe_tgcounters and
sess->li_tgcounters), and once the backend and the server are selected,
we are also able to store backend and server sharded counters
pointer at the stream level (namely s->be_tgcounters and s->sv_tgcounters)
Everywhere we rely on these counters and the stream or session context is
available, we use the fast pointers it instead of the indirect pointers
path to make the pointer resolution a bit faster.
This optimization proved to bring a few percents back, and together with
the previous 75e480d10 commit we now fixed the performance regression (we
are back to back with 3.2 stats performance)
Between 3.2 and 3.3-dev we noticed a noticeable performance regression
due to stats handling. After bisecting, Willy found out that recent
work to split stats computing accross multiple thread groups (stats
sharding) was responsible for that performance regression. We're looking
at roughly 20% performance loss.
More precisely, it is the added indirections, multiplied by the number
of statistics that are updated for each request, which in the end causes
a significant amount of time being spent resolving pointers.
We noticed that the fe_counters_shared and be_counters_shared structures
which are currently allocated in dedicated memory since a0dcab5c
("MAJOR: counters: add shared counters base infrastructure")
are no longer huge since 16eb0fab31 ("MAJOR: counters: dispatch counters
over thread groups") because they now essentially hold flags plus the
per-thread group id pointer mapping, not the counters themselves.
As such we decided to try merging fe_counters_shared and
be_counters_shared in their parent structures. The cost is slight memory
overhead for the parent structure, but it allows to get rid of one
pointer indirection. This patch alone yields visible performance gains
and almost restores 3.2 stats performance.
counters_fe_shared_get() was renamed to counters_fe_shared_prepare() and
now returns either failure or success instead of a pointer because we
don't need to retrieve a shared pointer anymore, the function takes care
of initializing existing pointer.
A new field was added in the applet structure to be able to set flags on the
applets The first one is related to the new API. APPLET_FL_NEW_API is set
for applets based on the new API. It was set on all HAProxy's applets.
Thanks to this patch, the lua cosocket applet is now using its own
buffers. .rcv_buf and .snd_buf callback functions are now defined to use the
default raw functions. Functions to receive and send data have also been
updated to use the applet API and to remove any dependencies on the
stream-connectors and the channels.
It is a fix similar to the previous one ("BUG/MEDIUM: hlua: Report to SC
when data were consumed on a lua socket"), but for the write side. The
writer must notify the cosocket it needs more space in the request buffer to
produce more data by calling sc_need_room(). Otherwise, there is nothing to
prevent to wake the cosocket applet up again and again.
This patch must be backported as far as 2.8, and maybe to 2.6 too.
The lua cosocket are quite strange. There is an applet used to handle the
connection and writer and readers subscribed on it to write or read
data. Writers and readers are tasks woken up by the cosocket applet when
data can be consumed or produced, depending on the channels buffers
state. Then the cosocket applet is woken up by writers and readers when read
or write events were performed.
It means the cosocket applet has only few information on what was produced
or consumed. It is the writers and readers responsibility to notify any
blocking. Among other things, the readers must take care to notify the
stream on top of the cosocket applet that some data was consumed. Otherwise,
it may remain blocked, waiting for a write event (a write event from the
stream point of view is a read event from the cosocket point of view).
Thie patch must be backported as far as 2.8, and maybe to 2.6 too.
Thanks to this patch, the lua HTTP applet is now using its own buffers.
.rcv_buf and .snd_buf callback functions are now defined to use the default
HTX functions. Functions to receive and send data have also been updated to
use the applet API and to remove any dependencies on the stream-connectors
and the channels.
hlua_http_get_headers() function was using the HTTP message from the stream
TXN to retrieve headers from a message. However, this will be an issue to
update the lua HTTP applet to use its own buffers. Indeed, in that case,
information from the channels will be unavailable. So now,
hlua_http_get_headers() is now using a buffer containing an HTX message. It
is just an API change bacause, internally, the function was already
manipulation an HTX message.
When a lua HTTP applet is created, a "request" object is created, filled
with the request information (method, path, headers...), to be able to
easily retrieve these information from the script. However, this was done
when thee appctx was created, retrieving the info from the request channel.
To be ale to update the applet to use its own buffer, it is now performed on
the first applet run. Indead, when the applet is created, the info are not
forwarded yet and should not be accessed. Note that for now, information are
still retrieved from the channel.
Thanks to this patch, the lua TCP applet is now using its own buffers.
.rcv_buf and .snd_buf callback functions are now defined to use the default
raw functions. Other changes are quite light. Mainly, end of stream and
errors are reported on the appctx instead of the stream-endpoint descriptor.
When an HTTP applet tries to retrieve data, the request headers are still in
the buffer. But, instead of being silently removed, their size is removed
from the amount of data retrieved. When the request payload is fully
retrieved, it is not an issue. But it is a problem when a length is
specified. The data are shorten from the headers size.
So now, we take care to silently remove headers.
This patch must be backported to all stable versions.
It was already forbidden to use HTTP sample fetch functions from lua
services. An error is triggered if it happens. However, the error must be
extended to any L6/L7 sample fetch functions.
Indeed, a lua service is an applet. It totally unexepected for an applet to
access to input data in a channel's buffer. These data have not been
analyzed yet and are still subject to any change. An applet, lua or not,
must never access to "not forwarded" data. Only output data are
available. For now, if a lua applet relies on any L6/L7 sampel fetch
functions, the behavior is undefined and not consistent.
So to fix the issue, hlua flag HLUA_F_MAY_USE_HTTP is renamed to
HLUA_F_MAY_USE_CHANNELS_DATA. This flag is used to prevent any lua applet to
use L6/L7 sample fetch functions.
This patch could be backported to all stable versions.
Now that native mailers configuration is only usable with Lua mailers,
Willy noticed that we lack a way to warn the user if mailers were
previously configured on an older version but Lua mailers were not loaded,
which could trick the user into thinking mailers keep working when
transitionning to 3.2 while it is not.
In this patch we add the 'core.use_native_mailers_config()' Lua function
which should be called in Lua script body before making use of
'Proxy:get_mailers()' function to retrieve legacy mailers configuration
from haproxy main config. This way haproxy effectively knows that the
native mailers config is actually being used from Lua (which indicates
user correctly migrated from native mailers to Lua mailers), else if
mailers are configured but not used from Lua then haproxy warns the user
about the fact that they will be ignored unless they are used from Lua.
(e.g.: using the provided 'examples/lua/mailers.lua' to ease transition)
As mentioned in 2.8 announce on the mailing list [1] and on the wiki [2]
native mailers were deprecated and planned for removal in 3.3. Now is
the time to drop the legacy code for native mailers which is based on a
tcpcheck "hack" and cannot be maintained. Lua mailers should be used as
a drop in replacement. Indeed, "mailers" and associated config directives
are preserved because mailers config is exposed to Lua, which helps smoothing
the transition from native mailers to Lua based ones.
As a reminder, to keep mailers configuration working as before without
making changes to the config file, simply add the line below to the global
section:
lua-load examples/lua/mailers.lua
mailers.lua script (provided in the git repository, adjust path as needed)
may be customized by users familiar with Lua, by default it emulates the
behavior of the native (now removed) mailers.
[1]: https://www.mail-archive.com/haproxy@formilux.org/msg43600.html
[2]: https://github.com/haproxy/wiki/wiki/Breaking-changes
As reported by Chris Staite in GH #3002, trying to yield from a Lua
action during a client disconnect causes the script to be interrupted
(which is expected) and an alert to be emitted with the error:
"Lua function '%s': yield not allowed".
While this error is well suited for cases where the yield is not expected
at all (ie: when context doesn't allow it) and results from a yield misuse
in the Lua script, it isn't the case when the yield is exceptionnally not
available due to an abort or error in the request/response processing.
Because of that we raise an alert but the user cannot do anything about it
(the script is correct), so it is confusing and polluting the logs.
In this patch we introduce the ACT_OPT_FINAL_EARLY flag which is a
complementary flag to ACT_OPT_FIRST. This flag is set when the
ACT_OPT_FIRST is set earlier than normal (due to error/abort).
hlua_action() then checks for this flag to decide whether an error (alert)
or a simple log message should be emitted when the yield is not available.
It should solve GH #3002. Thanks to Chris Staite (@chrisstaite-menlo) for
having reported the issue and suggested a solution.
It was still failing on Ubuntu-24.04 with GCC+ASAN. So, instead of
understand the code path the compiler followed to report uninitialized
variables, let's init them now.
No backport needed.
In hlua_applet_tcp_recv_try() and hlua_applet_tcp_getline_yield(), GCC 14.2
reports warnings about 'blk2' variable that may be used uninitialized. It is
a bit strange because the code is pretty similar than before. But to make it
happy and to avoid bugs if the API change in future, 'blk2' is now used only
when its length is greater than 0.
No need to backport.
In hlua_applet_tcp_getline_yield(), the function may yield if there is no
data available. However we must take care to add a return statement just
after the call to hlua_yieldk(). I don't know the details of the LUA API,
but at least, this return statement fix a build error about uninitialized
variables that may be used.
It is a 3.3-specific issue. No backport needed.
The functions responsible to extract data from the applet input buffer or to
push data into the applet output buffer are now relying on the newly added
functions in the applet API. This simplifies a bit the code.
Most fe and be counters are good candidates for being shared between
processes. They are now grouped inside "shared" struct sub member under
be_counters and fe_counters.
Now they are properly identified, they would greatly benefit from being
shared over thread groups to reduce the cost of atomic operations when
updating them. For this, we take the current tgid into account so each
thread group only updates its own counters. For this to work, it is
mandatory that the "shared" member from {fe,be}_counters is initialized
AFTER global.nbtgroups is known, because each shared counter causes the stat
to be allocated lobal.nbtgroups times. When updating a counter without
concurrency, the first counter from the array may be updated.
To consult the shared counters (which requires aggregation of per-tgid
individual counters), some helper functions were added to counter.h to
ease code maintenance and avoid computing errors.
Shareable counters are not tagged as shared counters and are dynamically
allocated in separate memory area as a prerequisite for being stored
in shared memory area. For now, GUID and threads groups are not taken into
account, this is only a first step.
also we ensure all counters are now manipulated using atomic operations,
namely, "last_change" counter is now read from and written to using atomic
ops.
Despite the numerous changes caused by the counters being moved away from
counters struct, no change of behavior should be expected.
while new_server() takes the parent proxy as argument and even assigns
srv->proxy to the parent proxy, it didn't actually inserted the server
to the parent proxy server list on success.
The result is that sometimes we add the server to the list after
new_server() is called, and sometimes we don't.
This is really error-prone and because of that hooks such as
REGISTER_POST_SERVER_CHECK() which as run for all servers listed in
all proxies may not be relied upon for servers which are not actually
inserted in their parent proxy server list. Plus it feels very strange
to have a server that points to a proxy, but then the proxy doesn't know
about it because it cannot find it in its server list.
To prevent errors and make proxy->srv list reliable, we move the insertion
logic directly under new_server(). This requires to know if we are called
during parsing or during runtime to either insert or append the server to
the parent proxy list. For that we use PR_FL_CHECKED flag from the parent
proxy (if the flag is set, then the proxy was checked so we are past the
init phase, thus we assume we are called during runtime)
This implies that during startup if new_server() has to be cancelled on
error paths we need to call srv_detach() (which is now exposed in server.h)
before srv_drop().
The consequence of this commit is that REGISTER_POST_SERVER_CHECK() should
not run reliably on all servers created using new_server() (without having
to manually loop on global servers_list)
It is not expected/supported to reuse an httpclient instance to process
several requests. A new instance must be created for each request. However,
in lua, there is nothing to prevent a user to create an httpclient object
and use it in a loop to process requests.
That's unfortunate because this will apparently work, the requests will be
sent and a response will be received and processed. However internally some
ressources will be allocated and never released. When the next response is
processed, the ressources allocated for the previous one are definitively
lost.
In this patch we take care to check that the httpclient object was never
used when a request is sent from a lua script by checking
HTTPCLIENT_FS_STARTED flags. This flag is set when a httpclient applet is
spawned to process a request and never removed after that. In lua, the
httpclient applet is created when the request is sent. So, it is the right
place to do this test.
This patch should fix the issue #2986. It should be backported as far as
2.6.
An optional timeout was added to AppletTCP.receive() to interrupt calls after a
delay. It was mandatory to be able to implement interactive applets (like
trisdemo). However, this broke the API and it made impossible to differentiate
the shutdowns from the delays expirations. Indeed, in both cases, an empty
string was returned.
Because historically an empty string was used to notify a connection shutdown,
it should not be changed. So now, 'nil' value is returned when no data was
available before the delay expiration.
The new AppletTCP:try_receive() function was also affected. To fix it, instead
of stating there is no delay when a receive is tried, an expired delay is
set. Concretely TICK_ETERNITY was replaced by now_ms.
Finally, AppletTCP:getline() function is not concerned for now because there
is no way to interrupt it after some delay.
The documentation and trisdemo lua script were updated accordingly.
This patch depends on "BUG/MEDIUM: hlua: Properly detect shudowns for TCP
applets based on the new API". However, it is a 3.2-specific issue, so no
backport is needed.
The commit e5e36ce09 ("BUG/MEDIUM: hlua/cli: Fix lua CLI commands to work
with applet's buffers") fixed the TCP applets API to work with applets using
its own buffers. Howver the getline() function was not updated. It could be
an issue for anyone registering a CLI commands reading lines.
This patch should be backported as far as 3.0.
The internal function responsible to receive data for TCP applets with
internal buffers is buggy. Indeed, for these applets, the buffer API is used
to get data. So there is no tests on the SE to properly detect connection
shutdowns. So, it must be performed by hand after the call to b_getblk_nc().
This patch must be backported as far as 3.0.
It is especially a problem with Lua filters, but it is important to disable
the 0-copy forwarding if a filter alters the payload, or at least to be able
to disable it. While the filter is registered on the data filtering, it is
not an issue (and it is the common case) because, there is now way to
fast-forward data at all. But it may be an issue if a filter decides to
alter the payload and to unregister from data filtering. In that case, the
0-copy forwarding can be re-enabled in a hardly precdictable state.
To fix the issue, a SC flags was added to do so. The HTTP compression filter
set it and lua filters too if the body length is changed (via
HTTPMessage.set_body_len()).
Note that it is an issue because of a bad design about the HTX. Many info
about the message are stored in the HTX structure itself. It must be
refactored to move several info to the stream-endpoint descriptor. This
should ease modifications at the stream level, from filter or a TCP/HTTP
rules.
This should be backported as far as 3.0. If necessary, it may be backported
on lower versions, as far as 2.6. In that case, it must be reviewed and
adapted.
There was no function for a lua filter to change the body length of an HTTP
Message. But it is mandatory to be able to alter the message payload. It is
not possible update to directly update the message headers because the
internal state of the message must also be updated accordingly.
It is the purpose of HTTPMessage.set_body_len() function. The new body
length myst be passed as argument. If it is an integer, the right
"Content-Length" header is set. If the "chunked" string is used, it forces
the message to be chunked-encoded and in that case the "Transfer-Encoding"
header.
This patch should fix the issue #2837. It could be backported as far as 2.6.
When the channel API was revisted, the both functions above was added. An
offset can be passed as argument. However, this parameter could be reported
to be out of range if there was not enough input data was received yet. It
is an issue, especially with a tcp rule, because more data could be
received. If an error is reported too early, this prevent the rule to be
reevaluated later. In fact, an error should only be reported if the offset
is part of the output data.
Another issue is about the conditions to report 'nil' instead of an empty
string. 'nil' was reported when no data was found. But it is not aligned
with the documentation. 'nil' must only be returned if no more data cannot
be received and there is no input data at all.
This patch should fix the issue #2716. It should be backported as far as 2.6.
tune.lua.bool-sample-conversion must be set before any lua-load or
lua-load-per-thread is used for it to be considered. Indeed, lua-load
directives are parsed on the fly and will cause some parts of the scripts
to be executed during init already (script body/init contexts).
As such, we cannot afford to have "tune.lua.bool-sample-conversion" set
after some Lua code was loaded, because it would mean that the setting
would be handled differently for Lua's code executed during or after
config parsing.
To avoid ambiguities, the documentation now states that the setting must
be set before any lua-load(-per-thread) directive, and if the setting
is met after some Lua was already loaded, the directive is ignored and
a warning informs about that.
It should fix GH #2957
It may be backported with 29b6d8af16 ("MINOR: hlua: rename
"tune.lua.preserve-smp-bool" to "tune.lua.bool-sample-conversion"")
When called, this function will try to enforce a yield (if available) as
soon as possible. Indeed, automatic yield is already enforced every X
Lua instructions. However, there may be some cases where we know after
running heavy operation that we should yield already to avoid taking too
much CPU at once.
This is what this function offers, instead of asking the user to manually
yield using "core.yield()" from Lua itself after using an expensive
Lua method offered by haproxy, we can directly enforce the yield without
the need to do it in the Lua script.
When a service is initialized, the "use-service" rule that was executed is
now saved in the stream, using "current_rule" field, instead of saving it
into the applet context. It is safe to do so becaues this field is unused at
this stage. To avoid any issue, it is reset after the service
initialization. Doing so, it is no longer necessary to save it in the applet
context. It was the last usage of the rule pointer in the applet context.
The init functions for TCP and HTTP lua services were updated accordingly.
The lua function name was used in error messages of HTTP/TCP lua services
while the applet name can be used. Concretely, this will not change
anything, because when a lua service is regiestered, the lua function name
is used to name the applet. But it is easier, cleaner and more logicial
because it is really the applet name that should be displayed in these error
messages.
Thanks to the CLI refactoring ("MAJOR: cli: Refacor parsing and execution of
pipelined commands"), it is possible to fix the I/O handler function used by
lua CLI commands to no longer use the SC.
When the applet API was refactored to no longer manipulate the channels or
the stream-connectors, this part was missed. However, without the patch
above, it could not be fixed. It is now possible so let's do it.
This patch must not be backported becaues it depends on refactoring of the
CLI applet.
Till now, h1_parse_cont_len_header() was used during the H1 message parsing and
by the lua HTTP applets to parse the content-length header value. But a more
generic function was added some years ago doing exactly the same operations. So
let's use it instead.
Jacques Heunis from bloomberg reported on the mailing list [1] that
with haproxy 2.8 up to master, yielding from a Lua tcp service while
data was still buffered inside haproxy would eat some data which was
definitely lost.
He provided the reproducer below which turned out to be really helpful:
global
log stdout format raw local0 info
lua-load haproxy_yieldtest.lua
defaults
log global
timeout connect 10s
timeout client 1m
timeout server 1m
listen echo
bind *:9090
mode tcp
tcp-request content use-service lua.print_input
haproxy_yieldtest.lua:
core.register_service("print_input", "tcp", function(applet)
core.Info("Start printing input...")
while true do
local inputs = applet:getline()
if inputs == nil or string.len(inputs) == 0 then
core.Info("closing input connection")
return
end
core.Info("Received line: "..inputs)
core.yield()
end
end)
And the script below:
#!/usr/bin/bash
for i in $(seq 1 9999); do
for j in $(seq 1 50); do
echo "${i}_foo_${j}"
done
sleep 2
done
Using it like this:
./test_seq.sh | netcat localhost 9090
We can clearly see the missing data for every "foo" burst (every 2
seconds), as they are holes in the numbering.
Thanks to the reproducer, it was quickly found that only versions
>= 2.8 were affected, and that in fact this regression was introduced
by commit 31572229e ("MEDIUM: hlua/applet: Use the sedesc to report and
detect end of processing")
In fact in 31572229e 2 mistakes were made during the refaco.
Indeed, both in hlua_applet_tcp_fct() (which is involved in the reproducer
above) and hlua_applet_http_fct(), the request (buffer) is now
systematically consumed when returning from the function, which wasn't the
case prior to this commit: when HLUA_E_AGAIN is returned, it means a
yield was requested and that the processing is not done yet, thus we
should not consume any data, like we did prior to the refacto.
Big thanks to Jacques who did a great job reproducing and reporting this
issue on the mailing list.
[1]: https://www.mail-archive.com/haproxy@formilux.org/msg45778.html
It should be backported up to 2.8 with commit 31572229e
errmsg is used with memprintf and friends, thus it must be NULL
initialized before being passed to memprintf, else invalid read will
occur.
However in hlua_init() the errmsg value isn't initialized, let's fix that
This is really minor because it would only cause issue on error paths,
yet it may be backported to all stable versions, just in case.
Some notification_* functions were not thread safe by default as they
assumed only one producer would emit events for registered tasks.
While this suited well with the Lua sockets use-case, this proved to
be a limitation with some other event sources (ie: lua Queue class)
instead of having to deal with both the non thread safe and thread
safe variants (_mt suffix), which is error prone, let's make the
entire API thread safe regarding the event list.
Pruning functions still require that only one thread executes them,
with Lua this is always the case because there is one cleanup list
per context.
This is the non-blocking variant for AppletTCP:receive(). It doesn't
take any argument, instead it tries to read as much data as available
at once. If no data is available, empty string is returned.
Lua documentation was updated.
core.wait() now accepts optional delay parameter in ms. Passed this delay
the task is woken up if no event woke the task before.
Lua documentation was updated.
Similar to core.yield(), except that the task is not woken up
automatically, instead it waits for events to trigger the task
wakeup.
Lua documentation was updated.
Recent commit e5e36ce09 ("BUG/MEDIUM: hlua/cli: Fix lua CLI commands
to work with applet's buffers") revealed a bug in hlua cli applet handling
Indeed, playing with Willy's lua tetris script on the cli, a segfault
would be encountered when forcefully closing the session by sending a
CTRL+C on the terminal.
In fact the crash was caused by a UAF: while the cli applet was already
freed, the lua task responsible for waking it up would still point to it.
Thus hlua_applet_wakeup() could be called even if the applet didn't exist
anymore.
To fix the issue, in hlua_cli_io_release_fct() we must also free the hlua
task linked to the applet, like we already do for
hlua_applet_tcp_release() and hlua_applet_http_release().
While this bug exists on stable versions (where it should be backported
too for precaution), it only seems to be triggered starting with 3.0.
Baptiste reported that using the new optional timeout argument introduced
in 19e48f2 ("MINOR: hlua: add an optional timeout to AppletTCP:receive()")
the following error would occur at some point:
runtime error: file.lua:lineno: bad argument #-2 to 'receive' (number
expected, got light userdata) from [C]: in method 'receive...
In fact this is caused by exp_date being retrieved using relative index -1
instead of absolute index 3. Indeed, while using relative index is fine
most of the time when we trust the stack, when combined with yielding the
top of the stack when resuming from yielding is not necessarily the same
as when the function was first called (ie: if some data was pushed to the
stack in the yieldable function itself). As such, it is safer to use
explicit index to access exp_date variable at position 3 on the stack.
It was confirmed that doing so addresses the issue.
No backport needed unless 19e48f2 is.
TCP services might want to be interactive, and without a timeout on
receive(), the possibilities are a bit limited. Let's add an optional
timeout in the 3rd argument to possibly limit the wait time. In this
case if the timeout strikes before the requested size is complete,
a possibly incomplete block will be returned.
A better name was found for the option implemented in ec74438
("MINOR: hlua: add option to preserve bool type from smp to lua")
Indeed, "tune.lua.preserve-smp-bool {on | off}" wasn't explicit enough
nor did it encourage the adoption of the new "fixed" behavior (vs
historical behavior which is now considered as a bug).
Thus it becomes "tune.lua.bool-sample-conversion { normal | pre-3.1-bug }"
which actively encourage users to switch the new behavior after having
patched in-use Lua script if needed. From a technical point of view,
the logic remains the same, as the option currently defaults to
"pre-3.1-bug" to prevent script breakage, and a warning is emitted if
the option isn't set explicily and Lua is used.
Documentation and regtests were updated.
Must be backported in 3.1 with ec74438 and f2838f5 ("REGTESTS: fix
lua-based regtests using tune.lua.smp-preserve-bool")
