Since commit 365ee28 ("BUG/MINOR: hlua: prevent LJMP in hlua_traceback()")
we now use lua_pcall() to protect sensitive parts of hlua_traceback()
function, and this to prevent Lua from crashing the process in case of
unexpected Lua error.
This is still relevant, but an error was made, as lua_pcall() was given
the nresult argument '1' when _hlua_traceback() internal function
doesn't push any argument on the stack. Because of this, it seems Lua
API still tries to push garbage object on top of the stack before
returning. This may cause functions that leverage hlua_traceback() in
the middle of stack manipulation to end up having a corrupted stack when
continuing after the hlua_traceback().
There doesn't seem to be many places where this could be a problem, as
this was discovered using the reproducer documented in f535d3e
("BUG/MEDIUM: debug: only dump Lua state when panicking"). Indeed, when
hlua_traceback() was used from the signal handler while the thread was
previously executing Lua, when returning to Lua after the handler the
Lua stack would be corrupted.
To fix the issue, we emphasize on the fact that the _hlua_traceback()
function doesn't push anything on the stack, returns 0, thus lua_pcall()
is given 0 'nresult' argument to prevent anything from being pushed after
the execution, preserving the original stack state.
This should be backported to all stable versions (because 365ee28 was
backported there)
This patch changes the handling of named defaults sections. Prior to
this patch, every unreferenced defaults proxies were removed on post
parsing. Now by default, these sections are kept after postparsing and
only purged on deinit. The objective is to allow reusing them as base
configuration for dynamic backends.
To implement this, refcount of every still addressable named sections is
incremented by one after parsing. This ensures that they won't be
removed even if referencing proxies are removed at runtime. This is done
via the new function proxy_ref_all_defaults().
To ensure defaults instances are still properly removed on deinit, the
inverse operation is performed : refcount is decremented by one on every
defaults sections via proxy_unref_all_defaults().
The original behavior can still be used by using the new global keyword
tune.defaults.purge. This is useful for users using configuration with
large number of defaults and not interested in dynamic backends
creation.
Defaults section are indexed by their name in defproxy_by_name tree. For
named sections, there is no duplicate : if two instances have the same
name, the older one is removed from the tree. However, this was not the
case for unnamed defaults which are all stored inconditionnally in
defproxy_by_name.
This commit introduces a new approach for unnamed defaults. Now, these
instances are never inserted in the defproxy_by_name tree. Indeed, this
is not needed as no tree lookup is performed with empty names. This may
optimize slightly config parsing with a huge number of named and unnamed
defaults sections, as the first ones won't fill up the tree needlessly.
However, defproxy_by_name tree is also used to purge unreferenced
defaults instances, both on postparsing and deinit. Thus, a new approach
is needed for unnamed sections cleanup. Now, each time a new defaults is
parsed, if the previous instance is unnamed, it is freed unless if
referenced by a proxy. When config parsing is ended, a similar operation
is performed to ensure the last unnamed defaults section won't stay in
memory. To implement this, last_defproxy static variable is now set to
global. Unnamed sections which cannot be removed due to proxies
referencing proxies will still be removed when such proxies are freed
themselves, at runtime or on deinit.
Defaults proxies instance are stored in a global name tree. When there
is a name conflict and the older entry cannot be simply discarded as it
is already referenced, the older entry is instead removed from the name
tree and inserted into the orphaned list.
The purpose of the orphaned list was to guarantee that any remaining
unreferenced defaults are purged either on postparsing or deinit.
However, this is in fact completely useless. Indeed on postparsing,
orphaned entries are always referenced. On deinit instead, defaults are
already freed along the cleanup of all frontend/backend instances clean
up, thanks to their refcounting.
This patch streamlines this by removing orphaned list. Instead, a
defaults section is inserted into a new global defaults_list during
their whole lifetime. This is not strictly necessary but it ensures that
defaults instances can still be accessed easily in the future if needed
even if not present in the name tree. On deinit, a BUG_ON() is added to
ensure that defaults_list is indeed emptied.
Another benefit from this patch is to simplify the defaults deletion
procedure. Orphaned simple list is replaced by a proper double linked
list implementation, so a single LIST_DELETE() is now performed. This
will be notably useful as defaults may be removed at runtime in the
future if backends deletion at runtime is implemented.
This patch renames functions which deal with defaults section. A common
"defaults_px_" prefix is defined. This serves as a marker to identify
functions which can only be used with proxies defaults capability. New
BUG_ON() are enforced to ensure this is valid.
Also, older proxy_unref_or_destroy_defaults() is renamed
defaults_px_detach().
Function proxy_preset_defaults() purpose has evolved over time.
Originally, it was only used to initialize defaults proxies instances.
Until today, it was extended so that all proxies use it. Its objective
is to initialize settings to common default values.
To remove the confusion, this function is now removed. Its content is
integrated directly into init_new_proxy().
For a long time, we've tried to show the Lua state and backtrace when
dumping threads so as to be able to figure is (and which) Lua code was
misbehaving, e.g. by performing expensive library calls. Since 3.1 with
commit 365ee28510 ("BUG/MINOR: hlua: prevent LJMP in hlua_traceback()"),
it appears that the approach is more fragile (though that fix addressed
a real issue about out-of-memory), and it's possible to occasionally
observe crashes or CPU loops with "show threads" while running Lua
heavily. While users of "show threads" are rare, the watchdog warnings,
which were also enabled on 3.1, also trigger these issues, which is
even more of a concern.
This patch goes the simple way to address this for now: since the purpose
of the Lua backtrace was to help locate Lua call places upon a panic,
let's only call the backtrace on panic but not in other situations. After
a panic we obviously don't care that the Lua stack might be corrupted
since it's never going to be resumed anyway. This may be relaxed in the
future if a solution is found to reliably produce harmless Lua backtraces.
The commit above was backported to all stable branches, so this patch
will be needed everywhere. However, TAINTED_PANIC only appeared in 2.8,
and given the rarety of this bug before 3.1, it's probably not needed
to make any extra effort to go beyond 2.8.
It's easy enough to test a version for being subject to this issue,
by running the following Lua code:
local function stress(txn)
for _, backend in pairs(core.backends) do
for _, server in pairs(backend.servers) do
local stats = server:get_stats()
end
end
end
core.register_fetches("stress", stress)
in the following config file:
global
stats socket /tmp/haproxy.stat level admin mode 666
tune.lua.bool-sample-conversion normal
lua-load-per-thread "stress.lua"
listen stress
bind :8001
mode http
timeout client 5s
timeout server 5s
timeout connect 5s
http-request return status 200 content-type text/plain lf-string %[lua.stress()]
server s1 127.0.0.1:8000
and stressing port 8001 with 100+ connections requesting / in loop, then
issuing "show threads" on the CLI using socat in loops as well. Normally
it instantly segfaults (sometimes during the first "show").
A defaults proxy instance may be move into the orphaned list when it is
replaced by a newer section with the same name. This is attached via
<next> member as a single linked list entry. However, proxy free does
not clear <next> attach point.
This causes a crash on deinit if orphaned list is not empty. First, all
frontend/backend instances are freed. This triggers the release of every
referenced defaults instances as their refcount reach zero, but orphaned
list is not clean up. A loop is then conducted on orphaned list via
proxy_destroy_all_unref_defaults(). This causes a segfault due to access
on already freed entries.
To fix this, this patch extends proxy_destroy_defaults(). If orphaned
list is not empty, a loop is performed to remove a possible entry of the
currently released defaults instance. This ensures that loop over
orphaned list won't be able to access to already freed entries.
This bug is pretty rare as it requires to have duplicate name in
defaults sections, and also to use settings which forces defaults
referencing, such as TCP/HTTP rules. This can be reproduced with the
minimal config here :
defaults def
http-request return status 200
frontend fe
bind :20080
defaults def
Note that in fact orphaned list looping is not strictly necessary, as
defaults instances are automatically removed via refcounting. This will
be the purpose of a future patch. However, to limit the risk of
regression on stable releases during backport, this patch uses the more
direct approach for now.
This must be backported up to 3.1.
When a QCS instance is fully closed on qcs_close_remote() invokation, it
is moved into purg_list for later cleanup. This reuses <el_send> list
element, so a BUG_ON() ensures that QCS is not already present in
send_list.
This code is safe for bidirectional streams, as local channel is only
closed after FIN or RESET_STREAM emission completion, so such QCS won't
be present in the send_list on full closure.
However, things are different for remote uni streams. As such streams do
not have any local channel, qcs_close_remote() will always proceed to
full closure. Most of the time this is fine, but the aformentionned
BUG_ON() could be triggered if emission is required on a remote uni
stream : this only happens after read was aborted and a STOP_SENDING
frame is prepared.
Fix this by adding an extra operation in qcs_close_remote() : on full
close, STOP_SENDING is cancelled if it was prepared and the QCS instance
is removed from send_list. This is safe as STOP_SENDING is unnecessary
after the remote channel is closed. This operation is performed before
purg_list insertion which prevents the BUG_ON() crash issue.
This patch must be backported up to 3.1.
The problem is that the certificate is generated with a CN greater than
64 bytes when the SNI is too long, which is not suppose to be supported,
and will end up with a handshake failure.
The patch fixes the issue by avoiding to add a CN when the SNI is longer than
64 bytes. Indeed this is not a mandatory field anymore and was deprecated more
than 20 years ago. The SAN DNS is enough for this case.
Must be backported in every stable branches.
It was reported by Przemyslaw Bromber that using the "generate-certificates"
option combined with AWS-LC would crash HAProxy when a request is done with a
SNI longer than 64 bytes.
The problem is that the certificate is generated with a CN greater than 64
bytes which results in ssl_sock_do_create_cert() returning NULL. This
NULL value being passed to SSL_set_SSL_CTX.
With OpenSSL, passing a NULL SSL_CTX does not seem to be an issue as it
would just ignore it.
With AWS_LC, passing a NULL seems to crash the function. This was
reported to upstream AWS-LC and fixed in patch 7487ad1dcd8
https://github.com/aws/aws-lc/pull/2946.
This must be backported in every branches.
This patch implements dump_all_vars([scope],[prefix]) sample fetch
function that dumps all variables in a given scope, optionally
filtered by name prefix.
Output format: var1=value1, var2=value2, ...
- String values are quoted and escaped (", , \r, \n, \b, \0)
- All sample types are supported via sample_convert()
- Scope can be: sess, txn, req, res, proc
- Prefix filtering is optional
Example usage:
http-request return string %[dump_all_vars(txn)]
http-request return string %[dump_all_vars(txn,user)]
This addresses GitHub issue #1623.
Currently, variable names are only used during parsing and are not
stored at runtime. This makes it impossible to iterate through
variables and retrieve their names.
This patch adds infrastructure to store variable names:
- Add 'name' and 'name_len' fields to var_desc structure
- Add 'name' field to var structure
- Add VDF_NAME_ALLOCATED flag to track memory ownership
- Store names in vars_fill_desc(), var_set(), vars_check_arg(),
and parse_store()
- Free names in var_clear() and release_store_rule()
- Add ARGT_VAR handling in release_sample_arg() to free the
allocated name when the flag is set
This prepares the ground for implementing dump_all_vars() in the
next commit.
Tested with:
- ASAN-enabled build on Linux (TARGET=linux-glibc USE_OPENSSL=1
ARCH_FLAGS="-g -fsanitize=address")
- Regression tests: reg-tests/sample_fetches/vars.vtc
- Regression tests: reg-tests/startup/default_rules.vtc
This function takes a string appends it to a buffer in a format
compatible with most languages (double-quoted, with special characters
escaped). It handles standard escape sequences like \n, \r, \", \\.
This generic utility is desined to be used for logging or debugging
purposes where arbitrary string data needs to be safely emitted without
breaking the output format. It will be primarily used by the upcoming
dump_all_vars() sample fetch to dump variable contents safely.
This patch move the peers section parsing code from src/cfgparse.c to a
dedicated src/cfgparse-peers.c file. This seperation improves code
organization and prepares for further refactoring of the "peers" keyword
registration system.
No functional changes in this patch - the code is moved as-is with only
the necessary adjustments for compliation (adding SPDX header and
updating Makefile for build).
This is the first patch in a series to address issue #3221, which
reports that "peers" section keywords are not displayed with -dKall.
As reported by GH user @HiggsTeilchen on #3250, the use of "option
dont-parse-log" may result in segmentation fault when parsing the
configuration. In fact, "option assume-rfc6587-ntf" is also affected.
The reason behind this is that cfg_parse_log_forward() leverages the
cfg_parse_listen_match_option() function to check for generic proxy
options that are relevant in the PR_MODE_SYSLOG context. And while it
is not documented, this function assumes that the currently evaluated
proxy is stored in the global variable 'curproxy', which
cfg_parse_log_forward() doesn't offer.
cfg_parse_listen_match_option() uses curproxy to check the currently
evaluated proxy's capabilities is compatible with the option, so if a
proxy with the frontend capability was defined earlier in the config,
parsing would succeed, if curproxy points to proxy without the frontend
capabilty (ie: backend), a warning would be emitted to tell that the
option would be ignored while it is perfectly valid for the log-forward
proxy, and if no proxy was defined earlier in the config a segfault would
be triggered.
To fix the issue, we explicitly make "curproxy" global variable point to
the log-forward proxy being parsed in cfg_parse_log_forward() before
leveraging cfg_parse_listen_match_option() to check for compatible
options.
It must be backported with 834e9af8 ("MINOR: log: add options eval for
log-forward"), which was introduced in 3.2 precisely.
There remained some cases (on error paths) were a server could be freed
while still attached on the parent proxy server list. In 3.3 this can be
problematic because new_server() automatically adds the server to the
parent proxy list.
The bug is insignificant because it is on errors paths during init and
often haproxy exits right after. But let's fix that to ensure no UAF or
undefined behavior occurs because of that.
This patch depends on ("MINOR: cli: use srv_drop() when server was created using new_server()")
It must be backported in 3.3 with the above mentioned patch.
Now that new_server() is becoming more and more complex, we need to
take care that servers created using new_server() must be released
using the corresponding release function srv_drop() which takes care
of properly de-initing the server and its members.
Fix the left shift of args when "default" prefix matches. The cause of the
bug was the absence of zeroing of the right element during the shift. The
same bug for "no" prefix was fixed by commit 0f99e3497, but missed for
"default".
The shift of ("default", "option", "dontlog-normal")
produced ("option", "dontlog-normal", "dontlog-normal")
instead of ("option", "dontlog-normal", "")
As an example, a valid config line:
default option dontlog-normal
caused a parse error:
[ALERT] (32914) : config : parsing [bug-default-prefix.cfg:22] : 'option dontlog-normal' cannot handle unexpected argument 'dontlog-normal'.
The patch should be backported to all stable versions, since the absence of
zeroing was introduced with "default" keyword.
This converter checks the validity and decrypts the content of a JWE
token that has an asymetric "alg" algorithm (RSA). In such a case, we
must provide a path to an already loaded certificate and private key
that has the "jwt" option set to "on".
This converter checks the validity and decrypts the content of a JWE
token that has a symetric "alg" algorithm. In such a case, we only
require a secret as parameter in order to decrypt the token.
Those converters allow to encrypt or decrypt data with AES in Cipher
Block Chaining mode. They work the same way as the already existing
aes_gcm_enc/_dec ones apart from the AEAD tag notion which is not
supported in CBC mode.
The parameter parsing and processing and the actual crypto part of the
aes_gcm converter are interleaved. This patch puts the crypto parts in a
dedicated function for better reuse in the upcoming JWE processing.
A recent patch has introduced a new state for proxies : unpublished
backends. Such backends won't be eligilible for traffic, thus
use_backend/default_backend rules which target them won't match and
content switching rules processing will continue.
This patch defines a new frontend keywords 'force-be-switch'. This
keyword allows to ignore unpublished or disabled state. Thus,
use_backend/default_backend will match even if the target backend is
unpublished or disabled. This is useful to be able to test a backend
instance before exposing it outside.
This new keyword is converted into a persist rule of new type
PERSIST_TYPE_BE_SWITCH, stored in persist_rules list proxy member. This
is the only persist rule applicable to frontend side. Prior to this
commit, pure frontend proxies persist_rules list were always empty.
This new features requires adjustment in process_switching_rules(). Now,
when a use_backend/default_backend rule matches with an non eligible
backend, frontend persist_rules are inspected to detect if a
force-be-switch is present so that the backend may be selected.
Utility function warnif_cond_conflicts() is used when parsing an ACL.
Previously, the function directly calls ha_warning() to report an error.
Change the function so that it now takes the error message as argument.
Caller can then output it as wanted.
This change is necessary to use the function when parsing a keyword
registered as cfg_kw_list. The next patch will reuse it.
A previous patch defines a new proxy status : unpublished backends. This
patch extends this by changing proxy status reported in stats. If
unpublished is set, an extra "(UNPUB)" is added to the field.
Also, HTML stats is also slightly updated. If a backend is up but
unpublished, its status will be reported in orange color.
Define a new set of CLI commands publish/unpublish backend <be>. The
objective is to be able to change the status of a backend to
unpublished. Such a backend is considered ineligible to traffic : this
allows to skip use_backend rules which target it.
Note that contrary to disabled/stopped proxies, an unpublished backend
still has server checks running on it.
Internally, a new proxy flags PR_FL_BE_UNPUBLISHED is defined. CLI
commands handler "publish backend" and "unpublish backend" are executed
under thread isolation. This guarantees that the flag can safely be set
or remove in the CLI handlers, and read during content-switching
processing.
A proxy can be marked as disabled using the keyword with the same name.
The doc mentions that it won't process any traffic. However, this is not
really the case for backends as they may still be selected via switching
rules during stream processing.
In fact, currently access to disabled backends will be conducted up to
assign_server(). However, no eligible server is found at this stage,
resulting in a connection closure or an HTTP 503, which is expected. So
in the end, servers in disabled backends won't receive any traffic. But
this is only because post-parsing steps are not performed on such
backends. Thus, this can be considered as functional but only via
side-effects.
This patch clarifies the handling of disable backends, so that they are
never selected via switching rules. Now, process_switching_rules() will
ignore disable backends and continue rules evaluation.
As this is a behavior change, this patch is labelled as medium. The
documentation manuel for use_backend is updated accordingly.
This commit rewrites process_switching_rules() function. The objective
is to simplify backend selection so that a single unified
stream_set_backend() call is kept, both for regular and default backends
case.
This patch will be useful to add new capabilities on backends, in the
context of dynamic backend support implementation.
force-persist proxy keyword is converted into a persist_rule, stored in
proxy persist_rules list member. Each new rule is dynamically allocated
during parsing.
This commit fixes the memory leak on deinit due to a missing free on
persist_rules list entries. This is done via deinit_proxy()
modification. Each rule in the list is freed, along with its associated
ACL condition type.
This can be backported to every stable version.
If we want to be able to have more than 64 thread groups, we can no
longer use thread group masks as long.
One remaining place where it is done is in struct thread_set. However,
it is not really used as a mask anywhere, all we want is a thread group
counter, so convert that mask to a counter.
Fix the arithmetic when pre-filling non_empty_tgids when we still have
more than 32/64 thread groups left, to get the right index, we of course
have to divide the number of thread groups by the number of bits in a
long.
This bug was introduced by commit
7e1fed4b7a, but hopefully was not hit
because it requires to have at least as much thread groups as there are
bits in a long, which is impossible on 64bits machines, as MAX_TGROUPS
is still 32.
Now that it is unused, eliminate all_tgroups_mask, as we can't 64bits
masks to represent thread groups, if we want to be able to have more
than 64 thread groups.
In order to be able to have more than 64 thread groups, turn
non_empty_tgids into a long array, so that we have enough bits to
represent everty thread group, and manipulate it with the ha_bit_*
functions.
As reported by GH user @Lzq-001 on issue #3245, the config below would
cause haproxy to SEGFAULT after having reported an error:
frontend 0000000
http-request set-map %[hdr(0000)0_
Root cause is simple, in parse_http_set_map(), we define the release
function (which is responsible to clear lf_expr expressions used by the
action), prior to initializing the expressions, while the release
function assumes the expressions are always initialized.
For all similar actions, we already perform the init prior to setting
the related release function, but this was not the case for
parse_http_set_map(). We fix the bug by initializing the expressions
earlier.
Thanks to @Lzq-001 for having reported the issue and provided a simple
reproducer.
It should be backported to all stable versions, note for versions prior to
3.0, lf_expr_init() should be replace by LIST_INIT(), see
6810c41 ("MEDIUM: tree-wide: add logformat expressions wrapper")
Contrarily to what was previously believed, there are corner cases where
the counters may not be allocated, and we may want to make them optional
at a later date, so we have to check if those counters are there.
However, just checking that shared.tg is non-NULL is enough, we can then
assume that shared.tg[tgid - 1] has properly been allocated too.
Also modify the various COUNTER_SHARED_* macros to make sure they check
for that too.
ACK frames are either of type 0x02 or 0x03. The latter is an indication
that it contains extra ECN related fields. In haproxy QUIC stack, this
is considered as a different frame type, set to QUIC_FT_ACK_ECN, with
its own set of builder/parser functions.
This patch fixes ACK ECN parsing function. Indeed, the latter suffered
from two issues. First, 'first ACK range' and 'ACK ranges' were
inverted. Then, the three remaining ECN fields were simply ignored by
the parsing function.
This issue can cause desynchronization in the frames parsing code, which
may result in various result. Most of the time, the connection will be
aborted by haproxy due to an invalid frame content read.
Note that this issue was not detected earlier as most clients do not
enable ECN support if the peer is not able to emit ACK ECN frame first,
which haproxy currently never sends. Nevertheless, this is not the case
for every client implementation, thus proper ACK ECN parsing is
mandatory for a proper QUIC stack support.
Fix this by adjusting quic_parse_ack_ecn_frame() function. The remaining
ECN fields are parsed to ensure correct packet parsing. Currently, they
are not used by the congestion controller.
This must be backported up to 2.6.
The code to parse the "thread" keyword on bind lines was changed to
check if the thread numbers were correct against the value provided with
max-threads-per-group, if any were provided, however, at the time those
thread keywords have been set, it may not yet have been set, and that
breaks the feature, so revert to check against MAX_THREADS_PER_GROUP instead,
it should have no major impact.
Before updating counters, a few tests are made to check if the counters
exits. but those counters should always exist at this point, so just
remmove them.
This commit should have no impact, but can easily be reverted with no
functional impact if various crashes appear.
Instead of statically allocating the per-thread group counters,
based on the max number of thread groups available, allocate
them dynamically, based on the number of thread groups actually
used. That way we can increase the maximum number of thread
groups without using an unreasonable amount of memory.
The IPv6 header contains a payload length that excludes the 40 bytes of
IPv6 packet header, which differs from IPv4's total length which includes
it. As a result, the parser was wrong and would only see the IP part and
not the TCP one unless sufficient options were present tocover it.
This issue came in 3.4-dev2 with recent commit e88e03a6e4 ("MINOR:
net_helper: add ip.fp() to build a simplified fingerprint of a SYN"),
so no backport is needed.
As reported by GH user @kanashimia in GH #3241, providing anything else
than a table to Patref:add_bulk() method could cause a segfault because
we were calling lua_next() with the lua object without ensuring it
actually is a table.
Let's add the missing lua_istable() check on the stack object before
calling lua_next() function on it.
It should be backported up to 3.2 with 884dc62 ("MINOR: hlua_fcn:
add Patref:add_bulk()")
In GH #3241, GH user @kanashimia reported that the Patref:add_bulk()
method would raise a Lua exception when called with more than 101
elements at once.
As identified by @kanashimia there was an error in the way the
add_bulk() method was forced to yield after 101 elements precisely.
The yield is there to ensure Lua doesn't eat too much ressources at
once and doesn't impact haproxy's core responsiveness, but the check
for the yield was misplaced resulting in improper stack content upon
resume.
Thanks to user @kanashimia who even provided a reproducer which helped
a lot to troubleshoot the issue.
This fix should be backported up to 3.2 with 884dc62 ("MINOR: hlua_fcn:
add Patref:add_bulk()") where the bug was introduced.
Now that the tgid stored in the stats file has been increased to 16bits
by commit 022cb3ab7f, don't forget to
increase the variable size when reading it from the file, too.
This should have no impact given the maximum thread group limit is still
32.
Instead of always allocating MAX_TGROUPS members, allocate them
dynamically, using the number of thread groups we'll use, so that
increasing MAX_TGROUPS will not have a huge impact on the structure
size.
The fc_xxx info that are retrieved over tcp_info could currently not
be accessed before a stream is created due to a test that verified the
existence of a stream. The rationale here was that the function works
both for frontend and backend. Let's always retrieve these info from
the session for the frontend case so that it now becomes possible to
set variables at connection/session time. The doc did not mention this
limitation so this could almost be considered as a bug.
Some timers, like the handshake timer, are stored in the session and are
only copied to the logs struct when a stream is created. But this means
we can't measure it without a stream, nor store it once for all in a
variable at session creation time. Let's extend the sample fetch function
to retrieve it from the session when no stream is present. The doc did not
mention this limitation so this could almost be considered as a bug.
