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Filter Chain
Collaboration diagram for Filter Chain:

Data Structures

union  ap_filter_func
 
struct  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. More...
 
struct  ap_filter_t
 The representation of a filter chain. More...
 

Macros

#define ap_fwrite(f, bb, data, nbyte)   apr_brigade_write(bb, ap_filter_flush, f, data, nbyte)
 
#define ap_fputs(f, bb, str)   apr_brigade_write(bb, ap_filter_flush, f, str, strlen(str))
 
#define ap_fputc(f, bb, c)   apr_brigade_putc(bb, ap_filter_flush, f, c)
 
#define AP_FILTER_PROTO_CHANGE   0x1
 
#define AP_FILTER_PROTO_CHANGE_LENGTH   0x2
 
#define AP_FILTER_PROTO_NO_BYTERANGE   0x4
 
#define AP_FILTER_PROTO_NO_PROXY   0x8
 
#define AP_FILTER_PROTO_NO_CACHE   0x10
 
#define AP_FILTER_PROTO_TRANSFORM   0x20
 

Typedefs

typedef struct ap_filter_t ap_filter_t
 
typedef struct ap_filter_rec_t ap_filter_rec_t
 
typedef struct ap_filter_provider_t ap_filter_provider_t
 

Enumerations

enum  ap_filter_type {
  AP_FTYPE_RESOURCE = 10, AP_FTYPE_CONTENT_SET = 20, AP_FTYPE_PROTOCOL = 30, AP_FTYPE_TRANSCODE = 40,
  AP_FTYPE_CONNECTION = 50, AP_FTYPE_NETWORK = 60
}
 
enum  ap_filter_direction_e { AP_FILTER_INPUT = 1, AP_FILTER_OUTPUT = 2 }
 

Functions

typedef APR_RING_HEAD (ap_filter_ring, ap_filter_t) ap_filter_ring_t
 The representation of a filters' ring. More...
 
apr_status_t ap_get_brigade (ap_filter_t *filter, apr_bucket_brigade *bucket, ap_input_mode_t mode, apr_read_type_e block, apr_off_t readbytes)
 
apr_status_t ap_pass_brigade (ap_filter_t *filter, apr_bucket_brigade *bucket)
 
apr_status_t ap_pass_brigade_fchk (request_rec *r, apr_bucket_brigade *bucket, const char *fmt,...) __attribute__((format(printf
 
apr_status_t ap_filter_rec_tap_register_input_filter (const char *name, ap_in_filter_func filter_func, ap_init_filter_func filter_init, ap_filter_type ftype)
 
ap_filter_rec_tap_register_output_filter (const char *name, ap_out_filter_func filter_func, ap_init_filter_func filter_init, ap_filter_type ftype)
 
ap_filter_rec_tap_register_output_filter_protocol (const char *name, ap_out_filter_func filter_func, ap_init_filter_func filter_init, ap_filter_type ftype, unsigned int proto_flags)
 
ap_filter_tap_add_input_filter (const char *name, void *ctx, request_rec *r, conn_rec *c)
 
ap_filter_tap_add_input_filter_handle (ap_filter_rec_t *f, void *ctx, request_rec *r, conn_rec *c)
 
ap_filter_rec_tap_get_input_filter_handle (const char *name)
 
ap_filter_tap_add_output_filter (const char *name, void *ctx, request_rec *r, conn_rec *c)
 
ap_filter_tap_add_output_filter_handle (ap_filter_rec_t *f, void *ctx, request_rec *r, conn_rec *c)
 
ap_filter_rec_tap_get_output_filter_handle (const char *name)
 
void ap_remove_input_filter (ap_filter_t *f)
 
void ap_remove_output_filter (ap_filter_t *f)
 
apr_status_t ap_remove_input_filter_byhandle (ap_filter_t *next, const char *handle)
 
apr_status_t ap_remove_output_filter_byhandle (ap_filter_t *next, const char *handle)
 
apr_status_t ap_save_brigade (ap_filter_t *f, apr_bucket_brigade **save_to, apr_bucket_brigade **b, apr_pool_t *p)
 
int ap_filter_prepare_brigade (ap_filter_t *f, apr_pool_t **p)
 
apr_status_t ap_filter_setaside_brigade (ap_filter_t *f, apr_bucket_brigade *bb)
 
apr_status_t ap_filter_reinstate_brigade (ap_filter_t *f, apr_bucket_brigade *bb, apr_bucket **flush_upto)
 
int ap_filter_should_yield (ap_filter_t *f)
 
int ap_filter_output_pending (conn_rec *c)
 
int ap_filter_input_pending (conn_rec *c)
 
apr_status_t ap_filter_flush (apr_bucket_brigade *bb, void *ctx)
 
apr_status_t ap_fflush (ap_filter_t *f, apr_bucket_brigade *bb)
 
apr_status_t ap_fputstrs (ap_filter_t *f, apr_bucket_brigade *bb,...) AP_FN_ATTR_SENTINEL
 
apr_status_t ap_fprintf (ap_filter_t *f, apr_bucket_brigade *bb, const char *fmt,...) __attribute__((format(printf
 
apr_status_t void ap_filter_protocol (ap_filter_t *f, unsigned int proto_flags)
 

Filter callbacks

This function type is used for filter callbacks. It will be passed a pointer to "this" filter, and a "bucket brigade" 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).

If the initialization function argument passed to the registration functions is non-NULL, it will be called iff the filter is in the input or output filter chains and before any data is generated to allow the filter to prepare for processing.

The bucket brigade always belongs to the caller, but the filter is free to use the buckets within it as it sees fit. Normally, the brigade will be returned empty. Buckets may not be retained between successive calls to the filter unless they have been "set aside" with a call apr_bucket_setaside. Typically this will be done with ap_save_brigade(). Buckets removed from the brigade become the responsibility of the filter, which must arrange for them to be deleted, either by doing so directly or by inserting them in a brigade which will subsequently be destroyed.

For the input and output filters, the return value of a filter should be an APR status value. For the init function, the return value should be an HTTP error code or OK if it was successful.

typedef apr_status_t(* ap_out_filter_func )(ap_filter_t *f, apr_bucket_brigade *b)
 
typedef apr_status_t(* ap_in_filter_func )(ap_filter_t *f, apr_bucket_brigade *b, ap_input_mode_t mode, apr_read_type_e block, apr_off_t readbytes)
 
typedef int(* ap_init_filter_func )(ap_filter_t *f)
 
typedef union ap_filter_func ap_filter_func
 

Detailed Description

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).

Macro Definition Documentation

#define AP_FILTER_PROTO_CHANGE   0x1

Filter changes contents (so invalidating checksums/etc)

#define AP_FILTER_PROTO_CHANGE_LENGTH   0x2

Filter changes length of contents (so invalidating content-length/etc)

#define AP_FILTER_PROTO_NO_BYTERANGE   0x4

Filter requires complete input and can't work on byteranges

#define AP_FILTER_PROTO_NO_CACHE   0x10

Filter makes output non-cacheable

#define AP_FILTER_PROTO_NO_PROXY   0x8

Filter should not run in a proxy

#define AP_FILTER_PROTO_TRANSFORM   0x20

Filter is incompatible with "Cache-Control: no-transform"

#define ap_fputc (   f,
  bb,
 
)    apr_brigade_putc(bb, ap_filter_flush, f, c)

Write a character for the current filter, buffering if possible.

Parameters
fthe filter we are writing to
bbThe brigade to buffer into
cThe character to write
#define ap_fputs (   f,
  bb,
  str 
)    apr_brigade_write(bb, ap_filter_flush, f, str, strlen(str))

Write a buffer for the current filter, buffering if possible.

Parameters
fthe filter we are writing to
bbThe brigade to buffer into
strThe string to write
#define ap_fwrite (   f,
  bb,
  data,
  nbyte 
)    apr_brigade_write(bb, ap_filter_flush, f, data, nbyte)

Write a buffer for the current filter, buffering if possible.

Parameters
fthe filter we are writing to
bbThe brigade to buffer into
dataThe data to write
nbyteThe number of bytes in the data

Typedef Documentation

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_t ap_filter_t
typedef apr_status_t(* ap_in_filter_func)(ap_filter_t *f, apr_bucket_brigade *b, ap_input_mode_t mode, apr_read_type_e block, apr_off_t readbytes)
typedef int(* ap_init_filter_func)(ap_filter_t *f)
typedef apr_status_t(* ap_out_filter_func)(ap_filter_t *f, apr_bucket_brigade *b)

Enumeration Type Documentation

These flags indicate whether the given filter is an input filter or an output filter.

Enumerator
AP_FILTER_INPUT 

Input filters

AP_FILTER_OUTPUT 

Output filters

Filters have different types/classifications. These are used to group and sort the filters to properly sequence their operation.

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().

Enumerator
AP_FTYPE_RESOURCE 

These filters are used to alter the content that is passed through them. Examples are SSI or PHP.

AP_FTYPE_CONTENT_SET 

These filters are used to alter the content as a whole, but after all AP_FTYPE_RESOURCE filters are executed. These filters should not change the content-type. An example is deflate.

AP_FTYPE_PROTOCOL 

These filters are used to handle the protocol between server and client. Examples are HTTP and POP.

AP_FTYPE_TRANSCODE 

These filters implement transport encodings (e.g., chunking).

AP_FTYPE_CONNECTION 

These filters will alter the content, but in ways that are more strongly associated with the connection. Examples are splitting an HTTP connection into multiple requests and buffering HTTP responses across multiple requests.

It is important to note that these types of filters are not allowed in a sub-request. A sub-request's output can certainly be filtered by AP_FTYPE_RESOURCE filters, but all of the "final processing" is determined by the main request.

AP_FTYPE_NETWORK 

These filters don't alter the content. They are responsible for sending/receiving data to/from the client.

Function Documentation

ap_filter_t* ap_add_input_filter ( const char *  name,
void ctx,
request_rec r,
conn_rec c 
)

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.

Parameters
nameThe name of the filter to add
ctxContext data to provide to the filter
rThe request to add this filter for (or NULL if it isn't associated with a request)
cThe connection to add the fillter for
ap_filter_t* ap_add_input_filter_handle ( ap_filter_rec_t f,
void ctx,
request_rec r,
conn_rec c 
)

Variant of ap_add_input_filter() that accepts a registered filter handle (as returned by ap_register_input_filter()) rather than a filter name

Parameters
fThe filter handle to add
ctxContext data to provide to the filter
rThe request to add this filter for (or NULL if it isn't associated with a request)
cThe connection to add the fillter for
ap_filter_t* ap_add_output_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.

Parameters
nameThe name of the filter to add
ctxContext data to set in the filter
rThe request to add this filter for (or NULL if it isn't associated with a request)
cThe connection to add this filter for
Note
If adding a connection-level output filter (i.e. where the type is >= AP_FTYPE_CONNECTION) during processing of a request, the request object r must be passed in to ensure the filter chains are modified correctly. f->r will still be initialized as NULL in the new filter.
ap_filter_t* ap_add_output_filter_handle ( ap_filter_rec_t f,
void ctx,
request_rec r,
conn_rec c 
)

Variant of ap_add_output_filter() that accepts a registered filter handle (as returned by ap_register_output_filter()) rather than a filter name

Parameters
fThe filter handle to add
ctxContext data to set in the filter
rThe request to add this filter for (or NULL if it isn't associated with a request)
cThe connection to add the filter for
Note
If adding a connection-level output filter (i.e. where the type is >= AP_FTYPE_CONNECTION) during processing of a request, the request object r must be passed in to ensure the filter chains are modified correctly. f->r will still be initialized as NULL in the new filter.
apr_status_t ap_fflush ( ap_filter_t f,
apr_bucket_brigade bb 
)

Flush the current brigade down the filter stack.

Parameters
fThe filter we are passing to
bbThe brigade to flush
apr_status_t ap_filter_flush ( apr_bucket_brigade bb,
void ctx 
)

Flush function for apr_brigade_* calls. This calls ap_pass_brigade to flush the brigade if the brigade buffer overflows.

Parameters
bbThe brigade to flush
ctxThe filter to pass the brigade to
Note
this function has nothing to do with FLUSH buckets. It is simply a way to flush content out of a brigade and down a filter stack.
int ap_filter_input_pending ( conn_rec c)

This function determines whether there is pending data in the input filters. Pending data is data that has been read from the underlying socket but not yet returned to the application.

Parameters
cThe connection.
Returns
If no pending data remains, this function returns DECLINED. If some pending data remains, this function returns OK.
int ap_filter_output_pending ( conn_rec c)

This function determines whether there is unwritten data in the output filters, and if so, attempts to make a single write to each filter with unwritten data.

Parameters
cThe connection.
Returns
If no unwritten data remains, this function returns DECLINED. If some unwritten data remains, this function returns OK. If any attempt to write data failed, this functions returns a positive integer.
int ap_filter_prepare_brigade ( ap_filter_t f,
apr_pool_t **  p 
)

Prepare the filter to allow brigades to be set aside. This can be used within an input filter to allocate space to set aside data in the input filters, or can be used within an output filter by being called via ap_filter_setaside_brigade().

Parameters
fThe current filter
pThe pool that was used to create the brigade. In a request filter this will be the request pool, in a connection filter this will be the connection pool.
Returns
OK if a brigade was created, DECLINED otherwise.
apr_status_t void ap_filter_protocol ( ap_filter_t f,
unsigned int  proto_flags 
)

set protocol requirements for an output content filter (only works with AP_FTYPE_RESOURCE and AP_FTYPE_CONTENT_SET)

Parameters
fthe filter in question
proto_flagsLogical OR of AP_FILTER_PROTO_* bits
apr_status_t ap_filter_reinstate_brigade ( ap_filter_t f,
apr_bucket_brigade bb,
apr_bucket **  flush_upto 
)

Reinstate a brigade setaside earlier, and calculate the amount of data we should write based on the presence of flush buckets, size limits on in memory buckets, and the number of outstanding requests in the pipeline. This is a safety mechanism to protect against a module that might try generate data too quickly for downstream to handle without yielding as it should.

If the brigade passed in is empty, we reinstate the brigade and return immediately on the assumption that any buckets needing to be flushed were flushed before being passed to ap_filter_setaside_brigade().

Parameters
fThe current filter
bbThe bucket brigade to restore to.
flush_uptoWork out the bucket we need to flush up to, based on the presence of a flush bucket, size limits on in-memory buckets, size limits on the number of requests outstanding in the pipeline.
Returns
APR_SUCCESS.
apr_status_t ap_filter_setaside_brigade ( ap_filter_t f,
apr_bucket_brigade bb 
)

Prepare a bucket brigade to be setaside, creating a dedicated pool if necessary within the filter to handle the lifetime of the setaside brigade.

Parameters
fThe current filter
bbThe bucket brigade to set aside. This brigade is always empty on return
int ap_filter_should_yield ( ap_filter_t f)

This function calculates whether there are any as yet unsent buffered brigades in downstream filters, and returns non zero if so.

A filter should use this to determine whether the passing of data downstream might block, and so defer the passing of brigades downstream with ap_filter_setaside_brigade().

This function can be called safely from a handler.

apr_status_t ap_fprintf ( ap_filter_t f,
apr_bucket_brigade bb,
const char *  fmt,
  ... 
)

Output data to the filter in printf format

Parameters
fthe filter we are writing to
bbThe brigade to buffer into
fmtThe format string
...The arguments to use to fill out the format string
apr_status_t ap_fputstrs ( ap_filter_t f,
apr_bucket_brigade bb,
  ... 
)

Write an unspecified number of strings to the current filter

Parameters
fthe filter we are writing to
bbThe brigade to buffer into
...The strings to write
apr_status_t ap_get_brigade ( ap_filter_t filter,
apr_bucket_brigade bucket,
ap_input_mode_t  mode,
apr_read_type_e  block,
apr_off_t  readbytes 
)

Get the current bucket brigade from the next filter on the filter stack. The filter returns an apr_status_t value. If the bottom-most filter doesn't read from the network, then AP_NOBODY_READ is returned. The bucket brigade will be empty when there is nothing left to get.

Parameters
filterThe next filter in the chain
bucketThe current bucket brigade. The original brigade passed to ap_get_brigade() must be empty.
modeThe way in which the data should be read
blockHow the operations should be performed APR_BLOCK_READ, APR_NONBLOCK_READ
readbytesHow many bytes to read from the next filter.
ap_filter_rec_t* ap_get_input_filter_handle ( const char *  name)

Returns the filter handle for use with ap_add_input_filter_handle.

Parameters
nameThe filter name to look up
ap_filter_rec_t* ap_get_output_filter_handle ( const char *  name)

Returns the filter handle for use with ap_add_output_filter_handle.

Parameters
nameThe filter name to look up
apr_status_t ap_pass_brigade ( ap_filter_t filter,
apr_bucket_brigade bucket 
)

Pass the current bucket brigade down to the next filter on the filter stack. The filter returns an apr_status_t value. If the bottom-most filter doesn't write to the network, then AP_NOBODY_WROTE is returned.

Parameters
filterThe next filter in the chain
bucketThe current bucket brigade
Remarks
Ownership of the brigade is retained by the caller. On return, the contents of the brigade are UNDEFINED, and the caller must either call apr_brigade_cleanup or apr_brigade_destroy on the brigade.
apr_status_t ap_pass_brigade_fchk ( request_rec r,
apr_bucket_brigade bucket,
const char *  fmt,
  ... 
)

Pass the current bucket brigade down to the next filter on the filter stack checking for filter errors. The filter returns an apr_status_t value. Returns OK if the brigade is successfully passed AP_FILTER_ERROR on a filter error HTTP_INTERNAL_SERVER_ERROR on all other errors

Parameters
rThe request rec
bucketThe current bucket brigade
fmtThe format string. If NULL defaults to "ap_pass_brigade returned"
...The arguments to use to fill out the format string
Remarks
Ownership of the brigade is retained by the caller. On return, the contents of the brigade are UNDEFINED, and the caller must either call apr_brigade_cleanup or apr_brigade_destroy on the brigade.
apr_status_t ap_filter_rec_t* ap_register_input_filter ( const char *  name,
ap_in_filter_func  filter_func,
ap_init_filter_func  filter_init,
ap_filter_type  ftype 
)

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.

Parameters
nameThe name to attach to the filter function
filter_funcThe filter function to name
filter_initThe function to call before the filter handlers are invoked
ftypeThe type of filter function, either AP_FTYPE_CONTENT_SET or AP_FTYPE_CONNECTION
See Also
add_input_filter()
ap_filter_rec_t* ap_register_output_filter ( const char *  name,
ap_out_filter_func  filter_func,
ap_init_filter_func  filter_init,
ap_filter_type  ftype 
)
ap_filter_rec_t* ap_register_output_filter_protocol ( const char *  name,
ap_out_filter_func  filter_func,
ap_init_filter_func  filter_init,
ap_filter_type  ftype,
unsigned int  proto_flags 
)

This function is used to register an output filter with the system. After this registration is performed, then a filter may be added directly to the filter chain by using ap_add_output_filter() and simply specifying the name, or as a provider under mod_filter.

Parameters
nameThe name to attach to the filter function
filter_funcThe filter function to name
filter_initThe function to call before the filter handlers are invoked
ftypeThe type of filter function, either AP_FTYPE_CONTENT_SET or AP_FTYPE_CONNECTION
proto_flagsProtocol flags: logical OR of AP_FILTER_PROTO_* bits
Returns
the filter rec
See Also
ap_add_output_filter()
void ap_remove_input_filter ( ap_filter_t f)

Remove an input filter from either the request or connection stack it is associated with.

Parameters
fThe filter to remove
apr_status_t ap_remove_input_filter_byhandle ( ap_filter_t next,
const char *  handle 
)

Remove an input filter from either the request or connection stack it is associated with.

Parameters
nextThe filter stack to search
handleThe filter handle (name) to remove
Returns
APR_SUCCESS on removal or error
void ap_remove_output_filter ( ap_filter_t f)

Remove an output filter from either the request or connection stack it is associated with.

Parameters
fThe filter to remove
apr_status_t ap_remove_output_filter_byhandle ( ap_filter_t next,
const char *  handle 
)

Remove an output filter from either the request or connection stack it is associated with.

Parameters
nextThe filter stack to search
handleThe filter handle (name) to remove
Returns
APR_SUCCESS on removal or error
apr_status_t ap_save_brigade ( ap_filter_t f,
apr_bucket_brigade **  save_to,
apr_bucket_brigade **  b,
apr_pool_t p 
)

Prepare a bucket brigade to be setaside. If a different brigade was set-aside earlier, then the two brigades are concatenated together.

If *save_to is NULL, the brigade will be created, and a cleanup registered to clear the brigade address when the pool is destroyed.

Parameters
fThe current filter
save_toThe brigade that was previously set-aside. Regardless, the new bucket brigade is returned in this location.
bThe bucket brigade to save aside. This brigade is always empty on return
pEnsure that all data in the brigade lives as long as this pool
typedef APR_RING_HEAD ( ap_filter_ring  ,
ap_filter_t   
)

The representation of a filters' ring.