apache/include/util_filter.h

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2000 The Apache Software Foundation.  All rights
 * reserved.
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 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
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#ifndef AP_FILTER_H
#define AP_FILTER_H

#ifdef __cplusplus
extern "C" {
#endif

#ifdef APR_HAVE_STDARG_H
#include <stdarg.h>
#endif

#include "httpd.h"
#include "apr.h"
#include "ap_buckets.h"

/**
 * @package Apache filter library
 */

#define AP_NOBODY_WROTE         -1
#define AP_NOBODY_READ          -2

/* Input filtering macros */
#define AP_GET_LINE                0  /* Get one line from the next filter */
#define AP_GET_ANY_AMOUNT         -1  /* Get as much data as the next filter
                                       * is willing to give up.
                                       */

/*
 * FILTER CHAIN
 *
 * Filters operate using a "chaining" mechanism. The filters are chained
 * together into a sequence. When output is generated, it is passed through
 * each of the filters on this chain, until it reaches the end (or "bottom")
 * and is placed onto the network.
 *
 * The top of the chain, the code generating the output, is typically called
 * a "content generator." The content generator's output is fed into the
 * filter chain using the standard Apache output mechanisms: ap_rputs(),
 * ap_rprintf(), ap_rwrite(), etc.
 *
 * Each filter is defined by a callback. This callback takes the output from
 * the previous filter (or the content generator if there is no previous
 * filter), operates on it, and passes the result to the next filter in the
 * chain. This pass-off is performed using the ap_fc_* functions, such as
 * ap_fc_puts(), ap_fc_printf(), ap_fc_write(), etc.
 *
 * When content generation is complete, the system will pass an "end of
 * stream" marker into the filter chain. The filters will use this to flush
 * out any internal state and to detect incomplete syntax (for example, an
 * unterminated SSI directive).
 */

/* forward declare the filter type */
typedef struct ap_filter_t ap_filter_t;

/*
 * ap_filter_func:
 *
 * This function type is used for filter callbacks. It will be passed a
 * pointer to "this" filter, and a "bucket" containing the content to be
 * filtered.
 *
 * In filter->ctx, the callback will find its context. This context is
 * provided here, so that a filter may be installed multiple times, each
 * receiving its own per-install context pointer.
 *
 * Callbacks are associated with a filter definition, which is specified
 * by name. See ap_register_input_filter() and ap_register_output_filter()
 * for setting the association between a name for a filter and its 
 * associated callback (and other information).
 *
 * The *bucket structure (and all those referenced by ->next and ->prev)
 * should be considered "const". The filter is allowed to modify the
 * next/prev to insert/remove/replace elements in the bucket list, but
 * the types and values of the individual buckets should not be altered.
 *
 * The return value of a filter should be an APR status value.
 */
typedef apr_status_t (*ap_out_filter_func)(ap_filter_t *f, ap_bucket_brigade *b);
typedef apr_status_t (*ap_in_filter_func)(ap_filter_t *f, ap_bucket_brigade *b, apr_ssize_t length);
typedef union ap_filter_func {
    ap_out_filter_func out_func;
    ap_in_filter_func in_func;
} ap_filter_func;

/*
 * ap_filter_type:
 *
 * Filters have different types/classifications. These are used to group
 * and sort the filters to properly sequence their operation.
 *
 * AP_FTYPE_CONTENT:
 *     These filters are used to alter the content that is passed through
 *     them. Examples are SSI or PHP.
 *
 * AP_FTYPE_CONNECTION:
 *     These filters will alter the content, but in ways that are more
 *     strongly associated with the output connection. Examples are
 *     compression, character recoding, or chunked transfer coding.
 *
 *     It is important to note that these types of filters are not allowed
 *     in a sub-request. A sub-requests output can certainly be filtered
 *     by AP_FTYPE_CONTENT filters, but all of the "final processing" is
 *     determined by the main request.
 *
 * The types have a particular sort order, which allows us to insert them
 * into the filter chain in a determistic order. Within a particular grouping,
 * the ordering is equivalent to the order of calls to ap_add_*_filter().
 */
typedef enum {
    AP_FTYPE_CONTENT,
    AP_FTYPE_CONNECTION
} ap_filter_type;

/*
 * ap_filter_t:
 *
 * This is the request-time context structure for an installed filter (in
 * the output filter chain). It provides the callback to use for filtering,
 * the request this filter is associated with (which is important when
 * an output chain also includes sub-request filters), the context for this
 * installed filter, and the filter ordering/chaining fields.
 *
 * Filter callbacks are free to use ->ctx as they please, to store context
 * during the filter process. Generally, this is superior over associating
 * the state directly with the request. A callback should not change any of
 * the other fields.
 */

typedef struct ap_filter_rec_t ap_filter_rec_t;

/**
 * This structure is used for recording information about the
 * registered filters. It associates a name with the filter's callback
 * and filter type.
 *
 * At the moment, these are simply linked in a chain, so a ->next pointer
 * is available.
 */
struct ap_filter_rec_t {
    /** The registered name for this filter */
    const char *name;
    /** The function to call when this filter is invoked. */
    ap_filter_func filter_func;
    /** The type of filter, either AP_FTYPE_CONTENT or AP_FTYPE_CONNECTION.  
     * An AP_FTYPE_CONTENT filter modifies the data based on information 
     * found in the content.  An AP_FTYPE_CONNECTION filter modifies the 
     * data based on the type of connection.
     */
    ap_filter_type ftype;

    /** The next filter_rec in the list */
    struct ap_filter_rec_t *next;
};

/**
 * The representation of a filter chain.  Each request has a list
 * of these structures which are called in turn to filter the data.  Sub
 * requests get an exact copy of the main requests filter chain.
 */
struct ap_filter_t {
     /** The internal representation of this filter.  This includes
      *  the filter's name, type, and the actual function pointer.
     */
    ap_filter_rec_t *frec;

    /** A place to store any data associated with the current filter */
    void *ctx;

    /** The next filter in the chain */
    ap_filter_t *next;

    /** The request_rec associated with the current filter.  If a sub-request
     *  adds filters, then the sub-request is the request associated with the
     *  filter.
     */
    request_rec *r;

    /** The conn_rec associated with the current filter.  This is analogous
     *  to the request_rec, except that it is used for input filtering.
     */
    conn_rec *c;
};

/**
 * Get the current bucket brigade from the next filter on the filter
 * stack.  The filter should return an apr_status_t value.  If the bottom-most 
 * filter doesn't write to the network, then AP_NOBODY_READ is returned.
 * @param filter The next filter in the chain
 * @param bucket The current bucket brigade
 * @param length The maximum amount of data to be returned from the next
 *               lowest filter.  If filter a requests 15 bytes
 *               from the filter b, that doesn't stop the b
 *               from requesting 30 bytes from filter c.  It just
 *               stops b from returning more that 15 bytes to a.  The other
 *               15 must be stored by b.  A value of AP_GET_LINE (0) tells 
 *               the filter to only ever return a single line.  A value of
 *               AP_GET_ANY_AMOUNT (-1) tells a filter to return everything
 *               it has.
 * @return apr_status_t value
 * @deffunc apr_status_t ap_get_brigade(ap_filter_t *filter, ap_bucket_brigade *bucket, int length)
 */
API_EXPORT(apr_status_t) ap_get_brigade(ap_filter_t *filter, ap_bucket_brigade *bucket, int length);

/**
 * Pass the current bucket brigade down to the next filter on the filter
 * stack.  The filter should return an apr_status_t value.  If the bottom-most 
 * filter doesn't write to the network, then AP_NOBODY_WROTE is returned.
 * @param filter The next filter in the chain
 * @param bucket The current bucket brigade
 * @return apr_status_t value
 * @deffunc apr_status_t ap_pass_brigade(ap_filter_t *filter, ap_bucket_brigade *bucket)
 */
API_EXPORT(apr_status_t) ap_pass_brigade(ap_filter_t *filter, ap_bucket_brigade *bucket);

/*
 * ap_register_input_filter():
 *
 * This function is used to register an input filter with the system. 
 * After this registration is performed, then a filter may be added 
 * into the filter chain by using ap_add_input_filter() and simply 
 * specifying the name.
 *
 * The filter's callback and type should be passed.
 */
/**
 * Register an input filter for later use.  This allows modules to name their 
 * filter functions for later addition to a specific request
 * @param name The name to attach to the filter function
 * @param filter_func The filter function to name
 * @param The type of filter function, either AP_FTYPE_CONTENT or AP_FTYPE_CONNECTION
 * @deffunc void ap_register_input_filter(const char *name, ap_in_filter_func filter_func, ap_filter_type ftype)
 */
API_EXPORT(void) ap_register_input_filter(const char *name,
                                          ap_in_filter_func filter_func,
                                          ap_filter_type ftype);
/*
 * ap_register_output_filter():
 *
 * This function is used to register an output filter with the system. 
 * After this registration is performed, then a filter may be added 
 * into the filter chain by using ap_add_output_filter() and simply 
 * specifying the name.
 *
 * The filter's callback and type should be passed.
 */
/**
 * Register an output filter for later use.  This allows modules to name their 
 * filter functions for later addition to a specific request
 * @param name The name to attach to the filter function
 * @param filter_func The filter function to name
 * @param The type of filter function, either AP_FTYPE_CONTENT or AP_FTYPE_CONNECTION
 * @deffunc void ap_register_output_filter(const char *name, ap_out_filter_func filter_func, ap_filter_type ftype)
 */
API_EXPORT(void) ap_register_output_filter(const char *name,
                                           ap_out_filter_func filter_func,
                                           ap_filter_type ftype);

/*
 * ap_add_filter():
 *
 * Adds a named filter into the filter chain on the specified request record.
 * The filter will be installed with the specified context pointer.
 *
 * Filters added in this way will always be placed at the end of the filters
 * that have the same type (thus, the filters have the same order as the
 * calls to ap_add_filter). If the current filter chain contains filters
 * from another request, then this filter will be added before those other
 * filters.
 * 
 * To re-iterate that last comment.  This function is building a FIFO
 * list of filters.  Take note of that when adding your filter to the chain.
 */
/**
 * Add a filter to the current connection.  Filters are added in a FIFO manner.
 * The first filter added will be the first filter called.
 * @param name The name of the filter to add
 * @param r The request to add this filter for (or NULL if it isn't associated with a request)
 * @param c The connection to add the fillter for
 * @deffunc void ap_add_input_filter(const char *name, void *ctx, request_rec *r, conn_rec *c)
 */
API_EXPORT(void) ap_add_input_filter(const char *name, void *ctx, request_rec *r, conn_rec *c);

/**
 * Add a filter to the current request.  Filters are added in a FIFO manner.
 * The first filter added will be the first filter called.
 * @param name The name of the filter to add
 * @param ctx Context data to set in the filter
 * @param r The request to add this filter for (or NULL if it isn't associated with a request)
 * @param c The connection to add this filter for
 * @deffunc void ap_add_output_filter(const char *name, void *ctx, request_rec *r, conn_rec *c)
 */
API_EXPORT(void) ap_add_output_filter(const char *name, void *ctx, 
                                      request_rec *r, conn_rec *c);

/* The next two filters are for abstraction purposes only.  They could be
 * done away with, but that would require that we break modules if we ever
 * want to change our filter registration method.  The basic idea, is that
 * all filters have a place to store data, the ctx pointer.  These functions
 * fill out that pointer with a bucket brigade, and retrieve that data on
 * the next call.  The nice thing about these functions, is that they
 * automatically concatenate the bucket brigades together for you.  This means
 * that if you have already stored a brigade in the filters ctx pointer, then
 * when you add more it will be tacked onto the end of that brigade.  When
 * you retrieve data, if you pass in a bucket brigade to the get function,
 * it will append the current brigade onto the one that you are retrieving.
 */

/**
 * prepare a bucket brigade to be setaside.  If a different brigade was 
 * set-aside earlier, then the two brigades are concatenated together.
 * @param f The current filter
 * @param save_to The brigade that was previously set-aside.  Regardless, the
 *             new bucket brigade is returned in this location.
 * @param b The bucket brigade to save aside.  This brigade is always empty
 *          on return
 * @deffunc void ap_save_brigade(ap_filter_t *f, ap_bucket_brigade **save_to, ap_bucket_brigade **b)
 */
API_EXPORT(void) ap_save_brigade(ap_filter_t *f, ap_bucket_brigade **save_to,
                                        ap_bucket_brigade **b);    

#ifdef __cplusplus
}
#endif

#endif	/* !AP_FILTER_H */