Maxconn is a bit of a misnomer when it comes to servers, as it doesn't
control the maximum number of connections we establish to a server, but
the maximum number of simultaneous requests. So add "strict-maxconn",
that will make it so we will never establish more connections than
maxconn.
It extends the meaning of the "restricted" setting of
tune.takeover-other-tg-connections, as it will also attempt to get idle
connections from other thread groups if strict-maxconn is set.
To be able to add more accurate termination events for each location, the
enum will be splitted by location. Indeed, there are at most 16 possbile
events. It will be pretty confusing to use same termination events for the
different locations. So the best is to split them.
In this patch, the termination events for the fd, hs and xprt locations are
introduced. For now some holes are added to keep similar events aligned
across enums. But this may change in future.
Termination events logs will be used to report the events that led to close
a connection. Unlike flags, that reflect a state, the idea here is to store
a log to preserve the order of the events. Most of time, when debugging an
issue, the order of the events is crucial to be able to understand the root
cause of the issue. The traces are trully heplful to do so. But it is not
always possible to active them because it is pretty verbose. On heavily
loaded platforms, it is not acceptable. We hope that the termination events
logs will help us in that situations.
One termination events log will be be store at each layer (connection, mux
connection, mux stream...) as a 32-bits integer. Each event will be store on
8 bits, 4 bits for the location and 4 bits for the type. So the first four
events will be stored only for each layer. It should be enough why a
connection is closed.
In this patch, the enums defining the termination event locations and types
are added. The macro to report a new event is also added and a function to
convert a termination events log to a string that could be display in log
messages for instance.
These functions return a symbolic error code such as ECONNRESET to keep
logs compact while making them human-readable. It's a good alternative
to the numeric code in that it's more expressive, and a good one to the
full message since it's shorter and more precise (some codes even match
errno names).
The doc was updated so that the symbolic names appear in the table. It
could be useful to backport this feature to help with troubleshooting
some issues, though backporting the doc might possibly be more annoying
in case users have local patches already, so maybe the table update does
not need to be backported in this case.
While we get reports of connection setup errors in fc_err/bc_err, we
don't have the equivalent for the recv/send/splice syscalls. Let's
add provisions for new codes that cover the common errno values that
recv/send/splice can return, i.e. ECONNREFUSED, ENOMEM, EBADF, EFAULT,
EINVAL, ENOTCONN, ENOTSOCK, ENOBUFS, EPIPE. We also add a special case
for when the poller reported the error itself. It's worth noting that
EBADF/EFAULT/EINVAL will generally indicate serious bugs in the code
and should not be reported.
The only thing is that it's quite hard to forcefully (and reliably)
trigger these errors in automated tests as the timing is critical.
Using iptables to manually reset established connections in the
middle of large transfers at least permits to see some ECONNRESET
and/or EPIPE, but the other ones are harder to trigger.
It was the only one prefixed with "CO_ERR_", making it harder to batch
process and to look up. It was added in 2.5 by commit 61944f7a73 ("MINOR:
ssl: Set connection error code in case of SSL read or write fatal failure")
so it can be backported as far as 2.6 if needed to help integrate other
patches.
When an abstract unix socket is bound by HAProxy (using "abns@" prefix),
NUL bytes are appended at the end of its path until sun_path is filled
(for a total of 108 characters).
Here we add an alternative to pass only the non-NUL length of that path
to connect/bind calls, such that the effective path of the socket's name
is as humanly written. This may be useful to interconnect with existing
softwares that implement abstract sockets with this logic instead of the
default haproxy one.
This is achieved by implementing the "abnsz" socket prefix (instead of
"abns"), which stands for "zero-terminated ABNS". "abnsz" prefix may be
used anywhere "abns" is. Internally, haproxy uses the custom socket
family (AF_CUST_ABNS vs AF_CUST_ABNSZ) to differentiate default abns
sockets from zero-terminated ones.
Documentation was updated and regtest was added.
Fixes GH issues #977 and #2479
Co-authored-by: Aurelien DARRAGON <adarragon@haproxy.com>
For now it's the same as abns. We'll need to modify sock_unix_addrcmp(),
and a few other ones to support effective path length when dealing with
the \0. Let's check with Tristan's patch for this (upcoming patch).
Co-authored-by: Aurelien DARRAGON <adarragon@haproxy.com>
Now that we can easily distinguish regular UNIX socket from ABNS sockets
by simply looking at the address family, stop looking at the first byte
from addr->sun_path to guess if the socket is an ABNS one or not. Looking
at the family is straightforward and will allow to differentiate between
upcoming ABNSZ and ABNS (where looking at the first byte from path won't
help anymore).
This is a pre-requisite to adding the abnsz socket address family:
in this patch we make use of protocol API rework started by 732913f
("MINOR: protocol: properly assign the sock_domain and sock_family") in
order to implement a dedicated address family for ABNS sockets (based on
UNIX parent family).
Thanks to this, it will become trivial to implement a new ABNSZ (for abns
zero) family which is essentially the same as ABNS but with a slight
difference when it comes to path handling (ABNS uses the whole sun_path
length, while ABNSZ's path is zero terminated and evaluation stops at 0)
It was verified that this patch doesn't break reg-tests and behaves
properly (tests performed on the CLI with show sess and show fd).
Anywhere relevant, AF_CUST_ABNS is handled alongside AF_UNIX. If no
distinction needs to be made, real_family() is used to fetch the proper
real family type to handle it properly.
Both stream and dgram were converted, so no functional change should be
expected for this "internal" rework, except that proto will be displayed
as "abns_{stream,dgram}" instead of "unix_{stream,dgram}".
Before ("show sess" output):
0x64c35528aab0: proto=unix_stream src=unix:1 fe=GLOBAL be=<NONE> srv=<none> ts=00 epoch=0 age=0s calls=1 rate=0 cpu=0 lat=0 rq[f=848000h,i=0,an=00h,ax=] rp[f=80008000h,i=0,an=00h,ax=] scf=[8,0h,fd=21,rex=10s,wex=] scb=[8,1h,fd=-1,rex=,wex=] exp=10s rc=0 c_exp=
After:
0x619da7ad74c0: proto=abns_stream src=unix:1 fe=GLOBAL be=<NONE> srv=<none> ts=00 epoch=0 age=0s calls=1 rate=0 cpu=0 lat=0 rq[f=848000h,i=0,an=00h,ax=] rp[f=80008000h,i=0,an=00h,ax=] scf=[8,0h,fd=22,rex=10s,wex=] scb=[8,1h,fd=-1,rex=,wex=] exp=10s rc=0 c_exp=
Co-authored-by: Aurelien DARRAGON <adarragon@haproxy.com>
As described in GH #2765, there were situations where http connections
would be re-used for requests to different endpoints, which is obviously
unexpected. In GH #2765, this occured with httpclient and UNIX socket
combination, but later code analysis revealed that while disabling http
reuse on httpclient proxy helped, it didn't fix the underlying issue since
it was found that conn_calculate_hash_sockaddr() didn't take into account
families such as AF_UNIX or AF_CUST_SOCKPAIR, and because of that the
sock_addr part of the connection wasn't hashed.
To properly fix the issue, let's explicly handle UNIX (both regular and
ABNS) and AF_CUST_SOCKPAIR families, so that the destination address is
properly hashed. To prevent this bug from re-appearing: when the family
isn't known, instead of doing nothing like before, let's fall back to a
generic (unoptimal) hashing which hashes the whole sockaddr_storage struct
As a workaround, http-reuse may be disabled on impacted proxies.
(unfortunately this doesn't help for httpclient since reuse policy
defaults to safe and cannot be modified from the config)
It should be backported to all stable versions.
Shout out to @christopherhibbert for having reported the issue and
provided a trivial reproducer.
[ada: prior to 3.0, ctx adjt is required because conn_hash_update()'s
prototype is slightly different]
The SPOE was significantly lightened. It is now possible to refactor it to
use a dedicated multiplexer. The first step is to add a SPOP mode for
proxies. The corresponding multiplexer mode is also added.
For now, there is no SPOP multiplexer, so it is only declarative. But at the
end, the SPOP multiplexer will be automatically selected for servers inside
a SPOP backend.
The related issue is #2502.
Since the following commit, session is initialized early for rhttp
preconnect.
12c40c25a9
MEDIUM: rhttp: create session for active preconnect
Session origin member was not set. However, this prevents several
session fetches to not work as expected. Worst, this caused a regression
as previously session was created after reversal with origin member
defined. This was reported by user William Manley on the mailing-list
which rely on set-dst.
One possible fix would be to set origin on session_new(). However, as
this is done before reversal, some session members may be incorrectly
initialized, in particular source and destination address.
Thus, session origin is only set after reversal is completed. This
ensures that session fetches have the same behavior on standard
connections and reversable ones.
This does not need to be backported.
Implement pool-conn-name support for idle connection reuse. It replaces
SNI as arbitrary identifier for connections in the idle pool. Thus,
every SNI reference in this context have been replaced.
Main change occurs in connect_server() where pool-conn-name sample fetch
is now prehash to generate idle connection identifier. SNI is now solely
used in the context of SSL for ssl_sock_set_servername().
Update API for PROXY protocol header encoding. Previously, it requires
stream parameter to be set. Change make_proxy_line() and associated
functions to add an extra session parameter. This is useful in context
where no stream is instantiated. For example, this is the case for rhttp
preconnect.
This change allows to extend PROXY v2 TLV encoding. Replace
build_logline() which requires a stream instance and call directly
sess_build_logline().
Note that stream parameter is kept as it is necessary for unique ID
encoding.
This change has no functional change for standard connections. However,
it is necessary to support TLV encoding on rhttp preconnect.
TASK_WOKEN_ANY was incorrectly used as argument to task_wakeup() for
rhttp preconnect task. This value is used as a flag. Replace it by
proper individual values. This is labelled as a bug but it has no known
impact.
This should be backported up to 2.9.
conn_recv_proxy() is responsible to parse PROXY protocol header. For v2
of the protocol, TLVs parsing is implemented. However, this step was
only done inside 'PROXY' command label. TLVs were never extracted for
'LOCAL' command mode.
Fix this by extracting TLV parsing loop outside of the switch case. Of
notable importance, tlv_offset is updated on LOCAL label to point to
first TLV location.
This bug should be backported up to 2.9 at least. It should even
probably be backported to every stable versions. Note however that this
code has changed much over time. It may be useful to use option
'--ignore-all-space' to have a clearer overview of the git diff.
Thanks to the previous fix, it is now possible to get the number of opened
streams for a connection and the negociated limit. Here, corresponding
sample feches are added, in fc_ and bc_ scopes.
On frontend side, the limit of streams is imposed by HAProxy. But on the
backend side, the limit is defined by the server. it may be useful for
debugging purpose because it may explain slow-downs on some processing.
log format expressions are broadly used within the code: once they are
parsed from input string, they are converted to a linked list of
logformat nodes.
We're starting to face some limitations because we're simply storing the
converted expression as a generic logformat_node list.
The first issue we're facing is that storing logformat expressions that
way doesn't allow us to add metadata alongside the list, which is part
of the prerequites for implementing log-profiles.
Another issue with storing logformat expressions as generic lists of
logformat_node elements is that it's starting to become really hard to
tell when we rely on logformat expressions or not in the code given that
there isn't always a comment near the list declaration or manipulation
to indicate that it's relying on logformat expressions under the hood,
so this adds some complexity for code maintenance.
This patch looks quite impressive due to changes in a lot of header and
source files (since logformat expressions are broadly used), but it does
a simple thing: it defines the lf_expr structure which itself holds a
generic list of logformat nodes, and then declares some helpers to
manipulate lf_expr elements and fixes the code so that we now exclusively
manipulate logformat_node lists as lf_expr elements outside of log.c.
For now, lf_expr struct only contains the list of logformat nodes (no
additional metadata), but now that we have dedicated type and helpers,
doing so in the future won't be problematic at all and won't require
extensive code changes.
Several places reuse the same code to ensure a connection is properly
freed, either via its MUX or by calling the proper set of functions.
Factorize all of this in a new function conn_release().
This new function is now called via session_free() and
session_accept_fd(). It will also be reused on delete server to
proactively close idle connections.
By default, backend connections are accounted by the server. This allows
to determine the number of idle connections to keep. A backend
connection can also be marked as private to prevent its reuse. It is
thus removed from server lists into the session list. As such, a private
connection is not accounted into server : conn_set_private() uses
srv_release_conn() to ensure this.
When using HTTP/2 on backend side with default http-reuse safe, the
above principle are mixed. Indeed, when a connection is first used, or
switches from idle to used, it is moved into the session list but it is
not flagged as private. This is done to prevent its sharing by different
clients to prevent head-of-line blocking issue. When all streams are
closed, the connection becomes idle again and is reinserted in the
server list. This has been introduced by the following patch :
0d21deaded
MEDIUM: backend: add reused conn to sess if mux marked as HOL blocking
When freeing a backend connection, special care is taken to ensure
server used counter is decremented. This is implemented into
conn_backend_deinit(). However, this function does this only if the
connection is not present in a session list. This is valid for private
connections. However, if a connection is non-private and present only
temporarily into a session list, the decrement operation won't be
executed despite the connection being accounted by the server.
This bug has several impacts. The server used counter won't be able to
reach its initial null value, even when all its connections are closed.
This can result in a wrong estimation of necessary idle connections,
which may cause unnecessary new connection usage. Also, this will
prevent definitely the server from being removed via "delete server" CLI
command.
This should be backported up to 2.4. Note that conn_backend_deinit() was
introduced in 2.9. For lesser versions, the change should be done
directly into conn_free().
By default, backend connections are attached to a server instance. This
allows to implement connection reuse. However, in some particular cases,
connection cannot be shared accross several clients. These connections
are considered and private and are attached to the session instance
instead.
These private connections are also indexed by the target server to not
mix them. All of this is implemented via a dedicated structure
previously named struct sess_srv_list.
Rename it to better reflect its usage to struct sess_priv_conns. Also
rename its internal members and all of the associated functions.
This commit is only a renaming, thus no functional impact is expected.
set-bc-{mark,tos} actions are pretty similar to set-fc-{mark,tos} to set
mark/tos on packets sent from haproxy to server: set-bc-{mark,tos} actions
act on the whole backend/srv connection: from connect() to connection
teardown, thus they may only be used before the connection to the server
is instantiated, meaning that they are only relevant for request-oriented
rules such as tcp-request or http-request rules. For now their use is
limited to content request rules, because tos and mark informations are
stored directly within the stream, thus it is required that the stream
already exists.
stream flags are used in combination with dedicated stream struct members
variables to pass 'tos' and 'mark' informations so that they are correctly
considered during stream connection assignment logic (prior to connecting
to actually connecting to the server)
'tos' and 'mark' fd sockopts are taken into account in conn hash
parameters for connection reuse mechanism.
The documentation was updated accordingly.
Some CPU time is needlessly wasted in conn_calculate_hash(), because all
params are first copied into a temporary buffer before computing the
hash on the whole buffer. Instead, let's leverage the XXH progressive
hash update functions to avoid expensive memcpys.
Now with fc_glitches and bc_glitches we can retrieve the number of
detected glitches on a front or back connection. On the backend it
can indicate a bug in a server that may induce frequent reconnections
hence CPU usage in TLS reconnections, and on the frontend it may
indicate an abusive client that may be trying to attack the stack
or to fingerprint it. Small non-zero values are definitely expected
and can be caused by network glitches for example, as well as rare
bugs in the other component (or maybe even in haproxy). These should
never be considered as alarming as long as they remain low (i.e.
much less than one per request). A reg-test is provided.
Add a new member <nb_rhttp_conns> in thread_ctx structure. Its purpose
is to count the current number of opened reverse HTTP connections
regarding from their listeners membership.
This patch will be useful to support multi-thread for active reverse
HTTP, in order to select the less loaded thread.
Note that despite access to <nb_rhttp_conns> are only done by the
current thread, atomic operations are used. This is because once
multi-thread support will be added, external threads will also retrieve
values from others.
Previous commit renames 'proto_reverse_connect' module to 'proto_rhttp'.
This commits follows this by replacing various custom prefix by 'rhttp_'
to make the code uniform.
Note that 'reverse_' prefix was kept in connection module. This is
because if a new reversable protocol not based on HTTP is implemented,
it may be necessary to reused the same connection function which are
protocol agnostic.
Change the flags used for reversed connection :
* CO_FL_REVERSED is now put after reversal for passive connect. For
active connect, it is delayed when accept is completed after reversal.
* CO_FL_ACT_REVERSING replace the old CO_FL_REVERSED. It is put only for
active connect on reversal and removes once accept is done.
This allows to identify a connection as reversed during its whole
lifetime. This should be useful to extend reverse connect.
To follow-up the implementation of the new set-proxy-v2-tlv-fmt
keyword in the server, the connection is updated to use the previously
allocated TLVs. If no value was specified, we send out an empty TLV.
As the feature is fully working with this commit, documentation and a
test for the server and default-server are added as well.
Define a new function srv_add_to_avail_list(). This function is used to
centralize connection insertion in available tree. It reuses a BUG_ON()
statement to ensure the connection is not present in the idle list.
Since commit 5afcb686b ("MAJOR: connection: purge idle conn by last usage")
in 2.9-dev4, the test on conn->toremove_list added to conn_get_idle_flag()
in 2.8 by commit 3a7b539b1 ("BUG/MEDIUM: connection: Preserve flags when a
conn is removed from an idle list") becomes misleading. Indeed, now both
toremove_list and idle_list are shared by a union since the presence in
these lists is mutually exclusive. However, in conn_get_idle_flag() we
check for the presence in the toremove_list to decide whether or not to
delete the connection from the tree. This test now fails because instead
it sees the presence in the idle or safe list via the union, and concludes
the element must not be removed. Thus the element remains in the tree and
can be found later after the connection is released, causing crashes that
Tristan reported in issue #2292.
The following config is sufficient to reproduce it with 2 threads:
defaults
mode http
timeout client 5s
timeout server 5s
timeout connect 1s
listen front
bind :8001
server next 127.0.0.1:8002
frontend next
bind :8002
timeout http-keep-alive 1
http-request redirect location /
Sending traffic with a few concurrent connections and some short timeouts
suffices to instantly crash it after ~10k reqs:
$ h2load -t 4 -c 16 -n 10000 -m 1 -w 1 http://0:8001/
With Amaury we analyzed the conditions in which the function is called
in order to figure a better condition for the test and concluded that
->toremove_list is never filled there so we can safely remove that part
from the test and just move the flag retrieval back to what it was prior
to the 2.8 patch above. Note that the patch is not reverted though, as
the parts that would drop the unexpected flags removal are unchanged.
This patch must NOT be backported. The code in 2.8 works correctly, it's
only the change in 2.9 that makes it misbehave.
In conn_delete_from_tree() there remains a cast of the toremove_list
to struct list while the introduction of the union precisely was to
avoid this cast. It's a leftover from the first version of patch
5afcb686b ("MAJOR: connection: purge idle conn by last usage") merged
into in 2.9-dev4, let's fix that.
No backport is needed.
Add a new MUX flag MX_FL_REVERSABLE. This value is used to indicate that
MUX instance supports connection reversal. For the moment, only HTTP/2
multiplexer is flagged with it.
This allows to dynamically check if reversal can be completed during MUX
installation. This will allow to relax requirement on config writing for
'tcp-request session attach-srv' which currently cannot be used mixed
with non-http/2 listener instances, even if used conditionnally with an
ACL.
Define a new error code for connection CO_ER_REVERSE. This will be used
to report an issue which happens on a connection targetted for reversal
before reverse process is completed.
Add reporting using send_log() for preconnect operation. This is minimal
to ensure we understand the current status of listener in active reverse
connect.
To limit logging quantity, only important transition are considered.
This requires to implement a minimal state machine as a new field in
receiver structure.
Here are the logs produced :
* Initiating : first time preconnect is enabled on a listener
* Error : last preconnect attempt interrupted on a connection error
* Reaching maxconn : all necessary connections were reversed and are
operational on a listener
When a connection is freed during preconnect before reversal, the error
must be notified to the listener to remove any connection reference and
rearm a new preconnect attempt. Currently, this can occur through 2 code
paths :
* conn_free() called directly by H2 mux
* error during conn_create_mux(). For this case, connection is flagged
with CO_FL_ERROR and reverse_connect task is woken up. The process
task handler is then responsible to call conn_free() for such
connection.
Duplicated steps where done both in conn_free() and process task
handler. These are now removed. To facilitate code maintenance,
dedicated operation have been centralized in a new function
rev_notify_preconn_err() which is called by conn_free().
In commit fecc573da ("MEDIUM: connection: Generic, list-based allocation
and look-up of PPv2 TLVs") there was a tiny mistake, elements of length
<= 128 are allocated from pool_pp_128 but only those of length < 128 are
released to this pool, other ones go to pool_pp_256. Because of this,
elements of size exactly 128 are allocated from 128 and released to 256.
It can be reproduced a few times by running sample_fetches/tlvs.vtc 1000
times with -DDEBUG_DONT_SHARE_POOLS -DDEBUG_MEMORY_POOLS -DDEBUG_EXPR
-DDEBUG_STRICT=2 -DDEBUG_POOL_INTEGRITY -DDEBUG_POOL_TRACING
-DDEBUG_NO_POOLS. Not sure why it doesn't reproduce more often though.
No backport is needed. This should address github issues #2275 and #2274.
The fetch logic is redundant and can be simplified by simply
calling the generic fetch with the correct TLV ID set as an
argument, similar to fc_pp_authority.
We already have a call that can retreive an TLV with any value.
Therefore, the fetch logic is redundant and can be simplified
by simply calling the generic fetch with the correct TLV ID
set as an argument.
Based on the new, generic allocation infrastructure, a new sample
fetch fc_pp_tlv is introduced. It is an abstraction for existing
PPv2 TLV sample fetches. It takes any valid TLV ID as argument and
returns the value as a string, similar to fc_pp_authority and
fc_pp_unique_id.
In order to be able to implement fetches in the future that allow
retrieval of any TLVs, a new generic data structure for TLVs is introduced.
Existing TLV fetches for PP2_TYPE_AUTHORITY and PP2_TYPE_UNIQUE_ID are
migrated to use this new data structure. TLV related pools are updated
to not rely on type, but only on size. Pools accomodate the TLV list
element with their associated value. For now, two pools for 128 B and
256 B values are introduced. More fine-grained solutions are possible
in the future, if necessary.
This patch improves readability by scoping HA proxy related PPv2 constants
with a 'HA" prefix. Besides, a new constant for the length of a CRC32C
TLV is introduced. The length is derived from the PPv2 spec, so 32 Bit.
Backend idle connections are purged on a recurring occurence during the
process lifetime. An estimated number of needed connections is
calculated and the excess is removed periodically.
Before this patch, purge was done directly using the idle then the safe
connection tree of a server instance. This has a major drawback to take
no account of a specific ordre and it may removed functional connections
while leaving ones which will fail on the next reuse.
The problem can be worse when using criteria to differentiate idle
connections such as the SSL SNI. In this case, purge may remove
connections with a high rate of reusing while leaving connections with
criteria never matched once, thus reducing drastically the reuse rate.
To improve this, introduce an alternative storage for idle connection
used in parallel of the idle/safe trees. Now, each connection inserted
in one of this tree is also inserted in the new list at
`srv_per_thread.idle_conn_list`. This guarantees that recently used
connection is present at the end of the list.
During the purge, use this list instead of idle/safe trees. Remove first
connection in front of the list which were not reused recently. This
will ensure that connection that are frequently reused are not purged
and should increase the reuse rate, particularily if distinct idle
connection criterias are in used.
Define a new function _srv_add_idle(). This is a simple wrapper to
insert a connection in the server idle tree. This is reserved for simple
usage and require to idle_conns lock. In most cases,
srv_add_to_idle_list() should be used.
This patch does not have any functional change. However, it will help
with the next patch as idle connection will be always inserted in a list
as secondary storage along with idle/safe trees.
Small change of API for conn_delete_from_tree(). Now the connection
instance is taken as argument instead of its inner node.
No functional change introduced with this commit. This simplifies
slightly invocation of conn_delete_from_tree(). The most useful changes
is that this function will be extended in the next patch to be able to
remove the connection from its new idle list at the same time as in its
idle tree.
An error can occured on a reverse connection before accept is completed.
In this case, no parent session can be notified. Instead, wake up the
receiver task on conn_create_mux().
As a counterpart to this, receiver task is extended to match CO_FL_ERROR
flag on pending connection. In this case, the onnection is freed. The
task is then requeued with a 1 second delay to start a new reverse
connection attempt.
Implement active reverse connection initialization. This is done through
a new task stored in the receiver structure. This task is instantiated
via bind callback and first woken up via enable callback.
Task handler is separated into two halves. On the first step, a new
connection is allocated and stored in <pend_conn> member of the
receiver. This new client connection will proceed to connect using the
server instance referenced in the bind_conf.
When connect has successfully been executed and HTTP/2 connection is
ready for exchange after SETTINGS, reverse_connect task is woken up. As
<pend_conn> is still set, the second halve is executed which only
execute listener_accept(). This will in turn execute accept_conn
callback which is defined to return the pending connection.
The task is automatically requeued inside accept_conn callback if bind
maxconn is not yet reached. This allows to specify how many connection
should be opened. Each connection is instantiated and reversed serially
one by one until maxconn is reached.
conn_free() has been modified to handle failure if a reverse connection
fails before being accepted. In this case, no session exists to notify
about the failure. Instead, reverse_connect task is requeud with a 1
second delay, giving time to fix a possible network issue. This will
allow to attempt a new connection reverse.
Note that for the moment connection rebinding after accept is disabled
for simplicity. Extra operations are required to migrate an existing
connection and its stack to a new thread which will be implemented
later.
