                   -----------------------------------------
                          Filters Guide - version 1.7
                          ( Last update: 2016-04-18 )
                   ------------------------------------------
                          Author : Christopher Faulet
              Contact : christopher dot faulet at capflam dot org


ABSTRACT
--------

The filters support is a new feature of HAProxy 1.7. It is a way to extend
HAProxy without touching its core code and, in certain extent, without knowing
its internals. This feature will ease contributions, reducing impact of
changes. Another advantage will be to simplify HAProxy by replacing some parts
by filters. As we will see, and as an example, the HTTP compression is the first
feature moved in a filter.

This document describes how to write a filter and what you have to keep in mind
to do so. It also talks about the known limits and the pitfalls to avoid.

As said, filters are quite new for now. The API is not freezed and will be
updated/modified/improved/extended as needed.


SUMMARY
-------

  1.    Filters introduction
  2.    How to use filters
  3.    How to write a new filter
  3.1.      API Overview
  3.2.      Defining the filter name and its configuration
  3.3.      Managing the filter lifecycle
  3.4.      Handling the streams creation and desctruction
  3.5.      Analyzing the channels activity
  3.6.      Filtering the data exchanged
  4.    FAQ


1. FILTERS INTRODUCTION
-----------------------

First of all, to fully understand how filters work and how to create one, it is
best to know, at least from a distance, what is a proxy (frontend/backend), a
stream and a channel in HAProxy and how these entities are linked to each other.
doc/internals/entities.pdf is a good overview.

Then, to support filters, many callbacks has been added to HAProxy at different
places, mainly around channel analyzers. Their purpose is to allow filters to
be involved in the data processing, from the stream creation/destruction to
the data forwarding. Depending of what it should do, a filter can implement all
or part of these callbacks. For now, existing callbacks are focused on
streams. But futur improvements could enlarge filters scope. For example, it
could be useful to handle events at the connection level.

In HAProxy configuration file, a filter is declared in a proxy section, except
default. So the configuration corresponding to a filter declaration is attached
to a specific proxy, and will be shared by all its instances. it is opaque from
the HAProxy point of view, this is the filter responsibility to manage it. For
each filter declaration matches a uniq configuration. Several declarations of
the same filter in the same proxy will be handle as different filters by
HAProxy.

A filter instance is represented by a partially opaque context (or a state)
attached to a stream and passed as arguments to callbacks. Through this context,
filter instances are stateful. Depending the filter is declared in a frontend or
a backend section, its instances will be created, respectively, when a stream is
created or when a backend is selected. Their behaviors will also be
different. Only instances of filters declared in a frontend section will be
aware of the creation and the destruction of the stream, and will take part in
the channels analyzing before the backend is defined.

It is important to remember the configuration of a filter is shared by all its
instances, while the context of an instance is owned by a uniq stream.

Filters are designed to be chained. It is possible to declare several filters in
the same proxy section. The declaration order is important because filters will
be called one after the other respecting this order. Frontend and backend
filters are also chained, frontend ones called first. Even if the filters
processing is serialized, each filter will bahave as it was alone (unless it was
developed to be aware of other filters). For all that, some constraints are
imposed to filters, especially when data exchanged between the client and the
server are processed. We will dicuss again these contraints when we will tackle
the subject of writing a filter.


2. HOW TO USE FILTERS
---------------------

To use a filter, you must use the parameter 'filter' followed by the filter name
and, optionnaly, its configuration in the desired listen, frontend or backend
section. For example:

    listen test
        ...
        filter trace name TST
        ...


See doc/configuration.txt for a formal definition of the parameter 'filter'.
Note that additional parameters on the filter line must be parsed by the filter
itself.

The list of available filters is reported by 'haproxy -vv':

    $> haproxy -vv
    HA-Proxy version 1.7-dev2-3a1d4a-33 2016/03/21
    Copyright 2000-2016 Willy Tarreau <willy@haproxy.org>

    [...]

    Available filters :
            [COMP] compression
            [TRACE] trace


Multiple filter lines can be used in a proxy section to chain filters. Filters
will be called in the declaration order.

Some filters can support implicit declarartions in certain circumstances
(without the filter line). This is not recommanded for new features but are
useful for existing ones moved in a filter, for backward compatibility
reasons. Implicit declarartions are supported when there is only one filter used
on a proxy. When several filters are used, explicit declarartions are mandatory.
The HTTP compression filter is one of these filters. Alone, using 'compression'
keywords is enough to use it. But when at least a second filter is used, a
filter line must be added.

    # filter line is optionnal
    listen t1
        bind *:80
        compression algo gzip
        compression offload
        server srv x.x.x.x:80

    # filter line is mandatory for the compression filter
    listen t2
        bind *:81
        filter trace name T2
        filter compression
        compression algo gzip
        compression offload
        server srv x.x.x.x:80


3. HOW TO WRITE A NEW FILTER
----------------------------

If you want to write a filter, there are 2 header files that you must know:

  * include/types/filters.h: This is the main header file, containing all
                             important structures you will use. It represents
                             the filter API.
  * include/proto/filters.h: This header file contains helper functions that
                             you may need to use. It also contains the internal
                             API used by HAProxy to handle filters.

To ease the filters integration, it is better to follow some conventions:

  * Use 'flt_' prefix to name your filter (e.g: flt_http_comp or flt_trace).
  * Keep everything related to your filter in a same file.

The filter 'trace' can be used as a template to write your own filter. It is a
good start to see how filters really work.

3.1 API OVERVIEW
----------------

Writing a filter can be summarized to write functions and attach them to the
existing callbacks. Available callbacks are listed in the following structure:

    struct flt_ops {
        /*
         * Callbacks to manage the filter lifecycle
         */
        int  (*init)  (struct proxy *p, struct flt_conf *fconf);
        void (*deinit)(struct proxy *p, struct flt_conf *fconf);
        int  (*check) (struct proxy *p, struct flt_conf *fconf);

        /*
         * Stream callbacks
         */
        int  (*stream_start)     (struct stream *s, struct filter *f);
        void (*stream_stop)      (struct stream *s, struct filter *f);

        /*
         * Channel callbacks
         */
        int  (*channel_start_analyze)(struct stream *s, struct filter *f,
                      struct channel *chn);
        int  (*channel_analyze)      (struct stream *s, struct filter *f,
                                      struct channel *chn,
                                      unsigned int an_bit);
        int  (*channel_end_analyze)  (struct stream *s, struct filter *f,
                                      struct channel *chn);

        /*
         * HTTP callbacks
         */
        int  (*http_data)          (struct stream *s, struct filter *f,
                                    struct http_msg *msg);
        int  (*http_chunk_trailers)(struct stream *s, struct filter *f,
                                    struct http_msg *msg);
        int  (*http_end)           (struct stream *s, struct filter *f,
                                    struct http_msg *msg);
        int  (*http_forward_data)  (struct stream *s, struct filter *f,
                                    struct http_msg *msg,
                                    unsigned int len);

        void (*http_reset)         (struct stream *s, struct filter *f,
                                    struct http_msg *msg);
        void (*http_reply)         (struct stream *s, struct filter *f,
                                    short status,
                                    const struct chunk *msg);

        /*
         * TCP callbacks
         */
        int  (*tcp_data)        (struct stream *s, struct filter *f,
                                 struct channel *chn);
        int  (*tcp_forward_data)(struct stream *s, struct filter *f,
                                 struct channel *chn,
                                 unsigned int len);
    };


We will explain in following parts when these callbacks are called and what they
should do.

Filters are declared in proxy sections. So each proxy have an ordered list of
filters, possibly empty if no filter is used. When the configuration of a proxy
is parsed, each filter line represents an entry in this list. In the structure
'proxy', the filters configurations are stored in the field 'filter_configs',
each one of type 'struct flt_conf *':

    /*
     * Structure representing the filter configuration, attached to a proxy and
     * accessible from a filter when instantiated in a stream
     */
    struct flt_conf {
        const char     *id;   /* The filter id */
        struct flt_ops *ops;  /* The filter callbacks */
        void           *conf; /* The filter configuration */
        struct list     list; /* Next filter for the same proxy */
    };

  * 'flt_conf.id' is an identifier, defined by the filter. It can be
    NULL. HAProxy does not use this field. Filters can use it in log messages or
    as a uniq identifier to check multiple declarations. It is the filter
    responsibility to free it, if necessary.

  * 'flt_conf.conf' is opaque. It is the internal configuration of a filter,
    generally allocated and filled by its parsing function (See § 3.2). It is
    the filter responsibility to free it.

  * 'flt_conf.ops' references the callbacks implemented by the filter. This
    field must be set during the parsing phase (See § 3.2) and can be refine
    during the initialization phase (See § 3.3). If it is dynamically allocated,
    it is the filter responsibility to free it.


The filter configuration is global and shared by all its instances. A filter
instance is created in the context of a stream and attached to this stream. in
the structure 'stream', the field 'strm_flt' is the state of all filter
instances attached to a stream:

    /*
     * Structure reprensenting the "global" state of filters attached to a
     * stream.
     */
    struct strm_flt {
        struct list    filters;             /* List of filters attached to a stream */
        struct filter *current[2];          /* From which filter resume processing, for a specific channel.
                                             * This is used for resumable callbacks only,
                                             * If NULL, we start from the first filter.
                                             * 0: request channel, 1: response channel */
        unsigned short flags;               /* STRM_FL_* */
        unsigned char  nb_req_data_filters; /* Number of data filters registerd on the request channel */
        unsigned char  nb_rsp_data_filters; /* Number of data filters registerd on the response channel */
    };


Filter instances attached to a stream are stored in the field
'strm_flt.filters', each instance is of type 'struct filter *':

    /*
     * Structure reprensenting a filter instance attached to a stream
     *
     * 2D-Array fields are used to store info per channel. The first index
     * stands for the request channel, and the second one for the response
     * channel.  Especially, <next> and <fwd> are offets representing amount of
     * data that the filter are, respectively, parsed and forwarded on a
     * channel. Filters can access these values using FLT_NXT and FLT_FWD
     * macros.
     */
    struct filter {
        struct flt_conf *config; /* the filter's configuration */
        void           *ctx;     /* The filter context (opaque) */
        unsigned short  flags;   /* FLT_FL_* */
        unsigned int    next[2]; /* Offset, relative to buf->p, to the next
                                  * byte to parse for a specific channel
                                  * 0: request channel, 1: response channel */
        unsigned int    fwd[2];  /* Offset, relative to buf->p, to the next
                                  * byte to forward for a specific channel
                                  * 0: request channel, 1: response channel */
        struct list     list;    /* Next filter for the same proxy/stream */
    };

  * 'filter.config' is the filter configuration previously described. All
    instances of a filter share it.

  * 'filter.ctx' is an opaque context. It is managed by the filter, so it is its
    responsibility to free it.

  * 'filter.next' and 'filter.fwd' will be described later (See § 3.6).


3.2. DEFINING THE FILTER NAME AND ITS CONFIGURATION
---------------------------------------------------

When you write a filter, the first thing to do is to add it in the supported
filters. To do so, you must register its name as a valid keyword on the filter
line:

    /* Declare the filter parser for "my_filter" keyword */
    static struct flt_kw_list flt_kws = { "MY_FILTER_SCOPE", { }, {
            { "my_filter", parse_my_filter_cfg },
            { NULL, NULL },
        }
    };

    __attribute__((constructor))
    static void
    __my_filter_init(void)
    {
        flt_register_keywords(&flt_kws);
    }


Then you must define the internal configuration your filter will use. For
example:

    struct my_filter_config {
        struct proxy *proxy;
        char         *name;
        /* ... */
    };


You also must list all callbacks implemented by your filter. Here, we use a
global variable:

    struct flt_ops my_filter_ops {
        .init   = my_filter_init,
        .deinit = my_filter_deinit,
        .check  = my_filter_config_check,

        /* ... */
     };


Finally, you must define the function to parse your filter configuration, here
'parse_my_filter_cfg'. This function must parse all remaining keywords on the
filter line:

    /* Return -1 on error, else 0 */
    static int
    parse_my_filter_cfg(char **args, int *cur_arg, struct proxy *px,
                        struct flt_conf *flt_conf, char **err)
    {
        struct my_filter_config *my_conf;
        int pos = *cur_arg;

        /* Allocate the internal configuration used by the filter */
        my_conf = calloc(1, sizeof(*my_conf));
        if (!my_conf) {
            memprintf(err, "%s: out of memory", args[*cur_arg]);
            return -1;
        }
        my_conf->proxy = px;

        /* ... */

        /* Parse all keywords supported by the filter and fill the internal
         * configuration */
        pos++; /* Skip the filter name */
        while (*args[pos]) {
            if (!strcmp(args[pos], "name")) {
                if (!*args[pos + 1]) {
                    memprintf(err, "'%s' : '%s' option without value",
                              args[*cur_arg], args[pos]);
                              goto error;
                }
                my_conf->name = strdup(args[pos + 1]);
                if (!my_conf->name) {
                    memprintf(err, "%s: out of memory", args[*cur_arg]);
                    goto error;
                }
                pos += 2;
            }

            /* ... parse other keywords ... */
        }
        *cur_arg = pos;

        /* Set callbacks supported by the filter */
        flt_conf->ops  = &my_filter_ops;

        /* Last, save the internal configuration */
        flt_conf->conf = my_conf;
        return 0;

      error:
         if (my_conf->name)
            free(my_conf->name);
        free(my_conf);
        return -1;
    }


WARNING: In your parsing function, you must define 'flt_conf->ops'. You must
         also parse all arguments on the filter line. This is mandatory.

In the previous example, we expect to read a filter line as follows:

    filter my_filter name MY_NAME ...


Optionnaly, by implementing the 'flt_ops.check' callback, you add a step to
check the internal configuration of your filter after the parsing phase, when
the HAProxy configuration is fully defined. For example:

    /* Check configuration of a trace filter for a specified proxy.
     * Return 1 on error, else 0. */
    static int
    my_filter_config_check(struct proxy *px, struct flt_conf *my_conf)
    {
        if (px->mode != PR_MODE_HTTP) {
            Alert("The filter 'my_filter' cannot be used in non-HTTP mode.\n");
            return 1;
        }

        /* ... */

        return 0;
    }


3.3. MANAGING THE FILTER LIFECYCLE
----------------------------------

Once the configuration parsed and checked, filters are ready to by used. There
are two callbacks to manage the filter lifecycle:

  * 'flt_ops.init': It initializes the filter for a proxy. You may define this
                    callback if you need to complete your filter configuration.

  * 'flt_ops.deinit': It cleans up what the parsing function and the init
                      callback have done. This callback is useful to release
                      memory allocated for the filter configuration.

Here is an example:

    /* Initialize the filter. Returns -1 on error, else 0. */
    static int
    my_filter_init(struct proxy *px, struct flt_conf *fconf)
    {
        struct my_filter_config *my_conf = fconf->conf;

        /* ... */

        return 0;
    }

    /* Free ressources allocated by the trace filter. */
    static void
    my_filter_deinit(struct proxy *px, struct flt_conf *fconf)
    {
        struct my_filter_config *my_conf = fconf->conf;

        if (my_conf) {
            free(my_conf->name);
            /* ... */
            free(my_conf);
        }
        fconf->conf = NULL;
    }


TODO: Add callbacks to handle creation/destruction of filter instances. And
      document it.


3.4. HANDLING THE STREAMS CREATION AND DESCTRUCTION
---------------------------------------------------

You may be interessted to handle stream creation and destruction. If so, you
must define followings callbacks:

  * 'flt_ops.stream_start': It is called when a stream is started. This callback
                            can fail by returning a negative value. It will be
                            considered as a critical error by HAProxy which
                            disabled the listener for a short time.

  * 'flt_ops.stream_stop': It is called when a stream is stopped. This callback
                           always succeed. Anyway, it is too late to return an
                           error.

For example:

    /* Called when a stream is created. Returns -1 on error, else 0. */
    static int
    my_filter_stream_start(struct stream *s, struct filter *filter)
    {
         struct my_filter_config *my_conf = FLT_CONF(filter);

         /* ... */

         return 0;
    }

    /* Called when a stream is destroyed */
    static void
    my_filter_stream_stop(struct stream *s, struct filter *filter)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);

         /* ... */
    }


WARNING: Handling the streams creation and destuction is only possible for
         filters defined on proxies with the frontend capability.


3.5. ANALYZING THE CHANNELS ACTIVITY
------------------------------------

The main purpose of filters is to take part in the channels analyzing. To do so,
there is a callback, 'flt_ops.channel_analyze', called before each analyzer
attached to a channel, execpt analyzers responsible for the data
parsing/forwarding (TCP data or HTTP body). Concretely, on the request channel,
'flt_ops.channel_analyze' could be called before following analyzers:

  * tcp_inspect_request        (AN_REQ_INSPECT_FE and AN_REQ_INSPECT_BE)
  * http_wait_for_request      (AN_REQ_WAIT_HTTP)
  * http_wait_for_request_body (AN_REQ_HTTP_BODY)
  * http_process_req_common    (AN_REQ_HTTP_PROCESS_FE)
  * process_switching_rules    (AN_REQ_SWITCHING_RULES)
  * http_process_req_ common   (AN_REQ_HTTP_PROCESS_BE)
  * http_process_tarpit        (AN_REQ_HTTP_TARPIT)
  * process_server_rules       (AN_REQ_SRV_RULES)
  * http_process_request       (AN_REQ_HTTP_INNER)
  * tcp_persist_rdp_cookie     (AN_REQ_PRST_RDP_COOKIE)
  * process_sticking_rules     (AN_REQ_STICKING_RULES)
  * flt_analyze_http_headers   (AN_FLT_HTTP_HDRS)

And on the response channel:

  * tcp_inspect_response     (AN_RES_INSPECT)
  * http_wait_for_response   (AN_RES_WAIT_HTTP)
  * process_store_rules      (AN_RES_STORE_RULES)
  * http_process_res_common  (AN_RES_HTTP_PROCESS_BE)
  * flt_analyze_http_headers (AN_FLT_HTTP_HDRS)

Note that 'flt_analyze_http_headers' (AN_FLT_HTTP_HDRS) is a new analyzer. It
has been added to let filters analyze HTTP headers after all processing, just
before the data parsing/forwarding.

Unlike the other callbacks previously seen before, 'flt_ops.channel_analyze' can
interrupt the stream processing. So a filter can decide to not execute the
analyzer that follows and wait the next iteration. If there are more than one
filter, following ones are skipped. On the next iteration, the filtering resumes
where it was stopped, i.e. on the filter that has previously stopped the
processing. So it is possible for a filter to stop the stream processing for a
while before continuing. For example:

    /* Called before a processing happens on a given channel.
     * Returns a negative value if an error occurs, 0 if it needs to wait,
     * any other value otherwise. */
    static int
    my_filter_chn_analyze(struct stream *s, struct filter *filter,
                          struct channel *chn, unsigned an_bit)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);

        switch (an_bit) {
            case AN_REQ_WAIT_HTTP:
                if (/* wait that a condition is verified before continuing */)
                    return 0;
                break;
            /* ... * /
        }
        return 1;
    }

  * 'an_bit' is the analyzer id. All analyzers are listed in
    'include/types/channels.h'.

  * 'chn' is the channel on which the analyzing is done. You can know if it is
    the request or the response channel by testing if CF_ISRESP flag is set:

      │ ((chn->flags & CF_ISRESP) == CF_ISRESP)


In previous example, the stream processing is blocked before receipt of the HTTP
request until a condition is verified.

To surround activity of a filter during the channel analyzing, two new analyzers
has been added:

  * 'flt_start_analyze' (AN_FLT_START_FE/AN_FLT_START_BE): For a specific
    filter, this analyzer is called before any call to the 'channel_analyze'
    callback. From the filter point of view, it calls the
    'flt_ops.channel_start_analyze' callback.

  * 'flt_end_analyze' (AN_FLT_END): For a specific filter, this analyzer is
    called when all other analyzers have finished their processing. From the
    filter point of view, it calls the 'flt_ops.channel_end_analyze' callback.

For TCP streams, these analyzers are called only once. For HTTP streams, if the
client connection is kept alive, this happens at each request/response roundtip.

'flt_ops.channel_start_analyze' and 'flt_ops.channel_end_analyze' callbacks can
interrupt the stream processing, as 'flt_ops.channel_analyze'. Here is an
example:

    /* Called when analyze starts for a given channel
     * Returns a negative value if an error occurs, 0 if it needs to wait,
     * any other value otherwise. */
    static int
    my_filter_chn_start_analyze(struct stream *s, struct filter *filter,
                                struct channel *chn)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);

        /* ... TODO ... */

        return 1;
    }

    /* Called when analyze ends for a given channel
     * Returns a negative value if an error occurs, 0 if it needs to wait,
     * any other value otherwise. */
    static int
    my_filter_chn_end_analyze(struct stream *s, struct filter *filter,
                              struct channel *chn)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);

        /* ... TODO ... */

        return 1;
    }


Workflow on channels can be summarized as following:

                |
     +----------+-----------+
     | flt_ops.stream_start |
     +----------+-----------+
                |
               ...
                |
                +-<-- [1]                 +------->---------+
                |                 --+     |                 |                 --+
                +------<----------+ |     |                 +--------<--------+ |
                |                 | |     |                 |                 | |
                V                 | |     |                 V                 | |
+-------------------------------+ | |     | +-------------------------------+ | |
|      flt_start_analyze        +-+ |     | |      flt_start_analyze        +-+ |
|(flt_ops.channel_start_analyze)|   | F   | |(flt_ops.channel_start_analyze)|   |
+---------------+---------------+   | R   | +---------------+---------------+   |
                |                   | O   |                 |                   |
                +------<--------+   | N   ^                 +--------<-------+  | B
                |               |   | T   |                 |                |  | A
+---------------+----------+    |   | E   | +---------------+----------+     |  | C
|+--------------V-----------+   |   | N   | |+--------------V-----------+    |  | K
||+--------------------------+  |   | D   | ||+--------------------------+   |  | E
|||  flt_ops.channel_analyze |  |   |     | |||  flt_ops.channel_analyze |   |  | N
+||             V            +--+   |     | +||             V            +---+  | D
 +|          analyzer        |      |     |  +|          analyzer        |      |
  +-------------+------------+      |     |   +-------------+------------+      |
                |                 --+     |                 |                   |
                +------------>------------+                ...                  |
                                                            |                   |
                                                [ data filtering (see below) ]  |
                                                            |                   |
                                                           ...                  |
                                                            |                   |
                                                            +--------<--------+ |
                                                            |                 | |
                                                            V                 | |
                                            +-------------------------------+ | |
                                            |       flt_end_analyze         +-+ |
                                            | (flt_ops.channel_end_analyze) |   |
                                            +---------------+---------------+   |
                                                            |                 --+
                        If HTTP stream, go back to [1] --<--+
                                                            |
                                                           ...
                                                            |
                                                 +----------+-----------+
                                                 | flt_ops.stream_stop  |
                                                 +----------+-----------+
                                                            |
                                                            V


TODO: Add pre/post analyzer callbacks with a mask. So, this part will be
      massively refactored very soon.


 3.6. FILTERING THE DATA EXCHANGED
-----------------------------------

WARNING: To fully understand this part, you must be aware on how the buffers
         work in HAProxy. In particular, you must be comfortable with the idea
         of circular buffers. See doc/internals/buffer-operations.txt and
         doc/internals/buffer-ops.fig for details.
         doc/internals/body-parsing.txt could also be useful.

An extended feature of the filters is the data filtering. By default a filter
does not look into data exchanged between the client and the server because it
is expensive. Indeed, instead of forwarding data without any processing, each
byte need to be buffered.

So, to enable the data filtering on a channel, at any time, in one of previous
callbacks, you should call 'register_data_filter' function. And conversely, to
disable it, you should call 'unregister_data_filter' function. For example:

    my_filter_chn_analyze(struct stream *s, struct filter *filter,
                          struct channel *chn, unsigned an_bit)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);

        /* 'chn' must be the request channel */
        if (!(chn->flags & CF_ISRESP) && an_bit == AN_FLT_HTTP_HDRS) {
            struct http_txn *txn = s->txn;
            struct http_msg *msg = &txn->req;
            struct buffer   *req = msg->chn->buf;
            struct hdr_ctx   ctx;

            /* Enable the data filtering for the request if 'X-Filter' header
             * is set to 'true'. */
            if (http_find_header2("X-Filter", 8, req->p, &txn->hdr_idx, &ctx) &&
                ctx.vlen >= 3 && memcmp(ctx.line + ctx.val, "true", 4) == 0)
                register_data_filter(s, chn_filter);
        }

        return 1;
    }

Here, the data filtering is enabled if the HTTP header 'X-Filter' is found and
set to 'true'.

If several filters are declared, the evaluation order remains the same,
regardless the order of the registrations to the data filtering.

Depending on the stream type, TCP or HTTP, the way to handle data filtering will
be slightly different. Among other things, for HTTP streams, there are more
callbacks to help you to fully handle all steps of an HTTP transaction. But the
basis is the same. The data filtering is done in 2 stages:

  * The data parsing: At this stage, filters will analyze input data on a
    channel. Once a filter has parsed some data, it cannot parse it again. At
    any time, a filter can choose to not parse all available data. So, it is
    possible for a filter to retain data for a while. Because filters are
    chained, a filter cannot parse more data than its predecessors. Thus only
    data considered as parsed by the last filter will be available to the next
    stage, the data forwarding.

  * The data forwarding: At this stage, filters will decide how much data
    HAProxy can forward among those considered as parsed at the previous
    stage. Once a filter has marked data as forwardable, it cannot analyze it
    anymore. At any time, a filter can choose to not forward all parsed
    data. So, it is possible for a filter to retain data for a while. Because
    filters are chained, a filter cannot forward more data than its
    predecessors. Thus only data marked as forwardable by the last filter will
    be actually forwarded by HAProxy.

Internally, filters own 2 offsets, relatively to 'buf->p', representing the
number of bytes already parsed in the available input data and the number of
bytes considered as forwarded. We will call these offsets, respectively, 'nxt'
and 'fwd'. Following macros reference these offsets:

  * FLT_NXT(flt, chn), flt_req_nxt(flt) and flt_rsp_nxt(flt)

  * FLT_FWD(flt, chn), flt_req_fwd(flt) and flt_rsp_fwd(flt)

where 'flt' is the 'struct filter' passed as argument in all callbacks and 'chn'
is the considered channel.

Using these offsets, following operations on buffers are possible:

    chn->buf->p + FLT_NXT(flt, chn) // the pointer on parsable data for
                                    // the filter 'flt' on the channel 'chn'.
    // Everything between chn->buf->p and 'nxt' offset was already parsed
    // by the filter.

    chn->buf->i - FLT_NXT(flt, chn) // the number of bytes of parsable data for
                                    // the filter 'flt' on the channel 'chn'.

    chn->buf->p + FLT_FWD(flt, chn) // the pointer on forwardable data for
                                    // the filter 'flt' on the channel 'chn'.
    // Everything between chn->buf->p and 'fwd' offset was already forwarded
    // by the filter.


Note that at any time, for a filter, 'nxt' offset is always greater or equal to
'fwd' offset.

TODO: Add schema with buffer states when there is 2 filters that analyze data.


3.6.1 FILTERING DATA ON TCP STREAMS
-----------------------------------

The TCP data filtering is the easy case, because HAProxy do not parse these
data. So you have only two callbacks that you need to consider:

  * 'flt_ops.tcp_data': This callback is called when unparsed data are
    available. If not defined, all available data will be considered as parsed
    for the filter.

  * 'flt_ops.tcp_forward_data': This callback is called when parsed data are
    available.  If not defined, all parsed data will be considered as forwarded
    for the filter.

Here is an example:

    /* Returns a negative value if an error occurs, else the number of
     * consumed bytes. */
    static int
    my_filter_tcp_data(struct stream *s, struct filter *filter,
                       struct channel *chn)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);
        int avail = chn->buf->i - FLT_NXT(filter, chn);
        int ret   = avail;

        /* Do not parse more than 'my_conf->max_parse' bytes at a time */
        if (my_conf->max_parse != 0 && ret > my_conf->max_parse)
            ret = my_conf->max_parse;

        /* if available data are not completely parsed, wake up the stream to
         * be sure to not freeze it. */
        if (ret != avail)
            task_wakeup(s->task, TASK_WOKEN_MSG);
        return ret;
    }


    /* Returns a negative value if an error occurs, else * or the number of
     * forwarded bytes. */
    static int
    my_filter_tcp_forward_data(struct stream *s, struct filter *filter,
                               struct channel *chn, unsigned int len)
    {
        struct my_filter_config *my_conf = FLT_CONF(filter);
        int ret = len;

        /* Do not forward more than 'my_conf->max_forward' bytes at a time */
        if (my_conf->max_forward != 0 && ret > my_conf->max_forward)
            ret = my_conf->max_forward;

        /* if parsed data are not completely forwarded, wake up the stream to
         * be sure to not freeze it. */
        if (ret != len)
            task_wakeup(s->task, TASK_WOKEN_MSG);
        return ret;
    }


3.6.2 FILTERING DATA ON HTTP STREAMS
------------------------------------

The HTTP data filtering is a bit tricky because HAProxy will parse the body
structure, especially chunked body. So basically there is the HTTP counterpart
to the previous callbacks:

  * 'flt_ops.http_data': This callback is called when unparsed data are
    available. If not defined, all available data will be considered as parsed
    for the filter.

  * 'flt_ops.http_forward_data': This callback is called when parsed data are
    available.  If not defined, all parsed data will be considered as forwarded
    for the filter.

But the prototype for these callbacks is slightly different. Instead of having
the channel as parameter, we have the HTTP message (struct http_msg). You need
to be careful when you use 'http_msg.chunk_len' size. This value is the number
of bytes remaining to parse in the HTTP body (or the chunk for chunked
messages). The HTTP parser of HAProxy uses it to have the number of bytes that
it could consume:

    /* Available input data in the current chunk from the HAProxy point of view.
     * msg->next bytes were already parsed. Without data filtering, HAProxy
     * will consume all of it. */
    Bytes = MIN(msg->chunk_len, chn->buf->i - msg->next);


But in your filter, you need to recompute it:

    /* Available input data in the current chunk from the filter point of view.
     * 'nxt' bytes were already parsed. */
    Bytes = MIN(msg->chunk_len + msg->next, chn->buf->i) - FLT_NXT(flt, chn);


In addition to these callbacks, there are two other:

  * 'flt_ops.http_end': This callback is called when the whole HTTP
    request/response is processed. It can interrupt the stream processing. So,
    it could be used to synchronize the HTTP request with the HTTP response, for
    example:

        /* Returns a negative value if an error occurs, 0 if it needs to wait,
         * any other value otherwise. */
         static int
         my_filter_http_end(struct stream *s, struct filter *filter,
                            struct http_msg *msg)
         {
             struct my_filter_ctx *my_ctx = filter->ctx;


            if (!(msg->chn->flags & CF_ISRESP)) /* The request */
                my_ctx->end_of_req = 1;
            else /* The response */
                my_ctx->end_of_rsp = 1;

            /* Both the request and the response are finished */
            if (my_ctx->end_of_req == 1 && my_ctx->end_of_rsp == 1)
                return 1;

            /* Wait */
            return 0;
        }


  * 'flt_ops.http_chunk_trailers': This callback is called for chunked HTTP
    messages only when all chunks were parsed. HTTP trailers can be parsed into
    several passes. This callback will be called each time. The number of bytes
    parsed by HAProxy at each iteration is stored in 'msg->sol'.

Then, to finish, there are 2 informational callbacks:

  * 'flt_ops.http_reset': This callback is called when a HTTP message is
    reset. This only happens when a '100-continue' response is received. It
    could be useful to reset the filter context before receiving the true
    response.

  * 'flt_ops.http_reply': This callback is called when, at any time, HAProxy
    decides to stop the processing on a HTTP message and to send an internal
    response to the client. This mainly happens when an error or a redirect
    occurs.


3.6.3 REWRITING DATA
--------------------

The last part, and the trickiest one about the data filtering, is about the data
rewriting. For now, the filter API does not offer a lot of functions to handle
it. There are only functions to notify HAProxy that the data size has changed to
let it update internal state of filters. This is your responsibility to update
data itself, i.e. the buffer offsets. For a HTTP message, you also must update
'msg->next' and 'msg->chunk_len' values accordingly:

  * 'flt_change_next_size': This function must be called when a filter alter
    incoming data. It updates 'nxt' offset value of all its predecessors. Do not
    call this function when a filter change the size of incoming data leads to
    an undefined behavior.

        unsigned int avail = MIN(msg->chunk_len + msg->next, chn->buf->i) -
            flt_rsp_next(filter);

        if (avail > 10 and /* ...Some condition... */) {
            /* Move the buffer forward to have buf->p pointing on unparsed
             * data */
            b_adv(msg->chn->buf, flt_rsp_nxt(filter));

            /* Skip first 10 bytes. To simplify this example, we consider a
             * non-wrapping buffer  */
            memmove(buf->p + 10, buf->p, avail - 10);

            /* Restore buf->p value */
            b_rew(msg->chn->buf, flt_rsp_nxt(filter));

            /* Now update other filters */
            flt_change_next_size(filter, msg->chn, -10);

            /* Update the buffer state */
            buf->i -= 10;

            /* And update the HTTP message state */
            msg->chunk_len -= 10;

            return (avail - 10);
        }
        else
            return 0; /* Wait for more data */


  * 'flt_change_forward_size': This function must be called when a filter alter
    parsed data. It updates offset values ('nxt' and 'fwd') of all filters. Do
    not call this function when a filter change the size of parsed data leads to
    an undefined behavior.

        /* len is the number of bytes of forwardable data */
        if (len > 10 and /* ...Some condition... */) {
            /* Move the buffer forward to have buf->p pointing on non-forwarded
             * data */
            b_adv(msg->chn->buf, flt_rsp_fwd(filter));

            /* Skip first 10 bytes. To simplify this example, we consider a
             * non-wrapping buffer  */
            memmove(buf->p + 10, buf->p, len - 10);

            /* Restore buf->p value */
            b_rew(msg->chn->buf, flt_rsp_fwd(filter));

            /* Now update other filters */
            flt_change_forward_size(filter, msg->chn, -10);

            /* Update the buffer state */
            buf->i -= 10;

            /* And update the HTTP message state */
            msg->next -= 10;

            return (len - 10);
        }
        else
            return 0; /* Wait for more data */


TODO: implement all the stuff to easily rewrite data. For HTTP messages, this
      requires to have a chunked message. Else the size of data cannot be
      changed.


4. FAQ
------

4.1. Detect multiple declarations of the same filter
----------------------------------------------------

TODO