Guide to writing output filters - Apache HTTP Server Version 2.4

Apache HTTP Server Version 2.4

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# Guide to writing output filters

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There are a number of common pitfalls encountered when writing output filters; this page aims to document best practice for authors of new or existing filters.

This document is applicable to both version 2.0 and version 2.2 of the Apache HTTP Server; it specifically targets `RESOURCE`-level or `CONTENT_SET`-level filters though some advice is generic to all types of filter.

- Comments

## Filters and bucket brigades

Each time a filter is invoked, it is passed a bucket brigade, containing a sequence of buckets which represent both data content and metadata. Every bucket has a bucket type; a number of bucket types are defined and used by the `httpd` core modules (and the `apr-util` library which provides the bucket brigade interface), but modules are free to define their own types.

Output filters must be prepared to process buckets of non-standard types; with a few exceptions, a filter need not care about the types of buckets being filtered.

A filter can tell whether a bucket represents either data or metadata using the `APR_BUCKET_IS_METADATA` macro. Generally, all metadata buckets should be passed down the filter chain by an output filter. Filters may transform, delete, and insert data buckets as appropriate.

There are two metadata bucket types which all filters must pay attention to: the `EOS` bucket type, and the `FLUSH` bucket type. An `EOS` bucket indicates that the end of the response has been reached and no further buckets need be processed. A `FLUSH` bucket indicates that the filter should flush any buffered buckets (if applicable) down the filter chain immediately.

`FLUSH` buckets are sent when the content generator (or an upstream filter) knows that there may be a delay before more content can be sent. By passing `FLUSH` buckets down the filter chain immediately, filters ensure that the client is not kept waiting for pending data longer than necessary.

Filters can create `FLUSH` buckets and pass these down the filter chain if desired. Generating `FLUSH` buckets unnecessarily, or too frequently, can harm network utilisation since it may force large numbers of small packets to be sent, rather than a small number of larger packets. The section on Non-blocking bucket reads covers a case where filters are encouraged to generate `FLUSH` buckets.

### Example bucket brigade

`HEAP FLUSH FILE EOS`

This shows a bucket brigade which may be passed to a filter; it contains two metadata buckets (`FLUSH` and `EOS`), and two data buckets (`HEAP` and `FILE`).

## Filter invocation

For any given request, an output filter might be invoked only once and be given a single brigade representing the entire response. It is also possible that the number of times a filter is invoked for a single response is proportional to the size of the content being filtered, with the filter being passed a brigade containing a single bucket each time. Filters must operate correctly in either case.

An output filter which allocates long-lived memory every time it is invoked may consume memory proportional to response size. Output filters which need to allocate memory should do so once per response; see Maintaining state below.

An output filter can distinguish the final invocation for a given response by the presence of an `EOS` bucket in the brigade. Any buckets in the brigade after an EOS should be ignored.

An output filter should never pass an empty brigade down the filter chain. To be defensive, filters should be prepared to accept an empty brigade, and should return success without passing this brigade on down the filter chain. The handling of an empty brigade should have no side effects (such as changing any state private to the filter).

### How to handle an empty brigade

        apr_status_t dummy_filter(ap_filter_t *f, apr_bucket_brigade *bb)

        {
            if (APR_BRIGADE_EMPTY(bb)) {
                return APR_SUCCESS;
            }
            ....

## Brigade structure

A bucket brigade is a doubly-linked list of buckets. The list is terminated (at both ends) by a sentinel which can be distinguished from a normal bucket by comparing it with the pointer returned by `APR_BRIGADE_SENTINEL`. The list sentinel is in fact not a valid bucket structure; any attempt to call normal bucket functions (such as `apr_bucket_read`) on the sentinel will have undefined behaviour (i.e. will crash the process).

There are a variety of functions and macros for traversing and manipulating bucket brigades; see the apr_buckets.h header for complete coverage. Commonly used macros include:

`APR_BRIGADE_FIRST(bb)`
  returns the first bucket in brigade bb
`APR_BRIGADE_LAST(bb)`
  returns the last bucket in brigade bb
`APR_BUCKET_NEXT(e)`
  gives the next bucket after bucket e
`APR_BUCKET_PREV(e)`
  gives the bucket before bucket e

The `apr_bucket_brigade` structure itself is allocated out of a pool, so if a filter creates a new brigade, it must ensure that memory use is correctly bounded. A filter which allocates a new brigade out of the request pool (`r->pool`) on every invocation, for example, will fall foul of the warning above concerning memory use. Such a filter should instead create a brigade on the first invocation per request, and store that brigade in its state structure.

It is generally never advisable to use `apr_brigade_destroy` to "destroy" a brigade unless you know for certain that the brigade will never be used again, even then, it should be used rarely. The memory used by the brigade structure will not be released by calling this function (since it comes from a pool), but the associated pool cleanup is unregistered. Using `apr_brigade_destroy` can in fact cause memory leaks; if a "destroyed" brigade contains buckets when its containing pool is destroyed, those buckets will not be immediately destroyed.

In general, filters should use `apr_brigade_cleanup` in preference to `apr_brigade_destroy`.

## Processing buckets

When dealing with non-metadata buckets, it is important to understand that the "`apr_bucket *`" object is an abstract representation of data:

1. The amount of data represented by the bucket may or may not have a determinate length; for a bucket which represents data of indeterminate length, the `->length` field is set to the value `(apr_size_t)-1`. For example, buckets of the `PIPE` bucket type have an indeterminate length; they represent the output from a pipe.
2. The data represented by a bucket may or may not be mapped into memory. The `FILE` bucket type, for example, represents data stored in a file on disk.

Filters read the data from a bucket using the `apr_bucket_read` function. When this function is invoked, the bucket may morph into a different bucket type, and may also insert a new bucket into the bucket brigade. This must happen for buckets which represent data not mapped into memory.

To give an example; consider a bucket brigade containing a single `FILE` bucket representing an entire file, 24 kilobytes in size:

`FILE(0K-24K)`

When this bucket is read, it will read a block of data from the file, morph into a `HEAP` bucket to represent that data, and return the data to the caller. It also inserts a new `FILE` bucket representing the remainder of the file; after the `apr_bucket_read` call, the brigade looks like:

`HEAP(8K) FILE(8K-24K)`

## Filtering brigades

The basic function of any output filter will be to iterate through the passed-in brigade and transform (or simply examine) the content in some manner. The implementation of the iteration loop is critical to producing a well-behaved output filter.

Taking an example which loops through the entire brigade as follows:

### Bad output filter -- do not imitate!

    apr_bucket *e = APR_BRIGADE_FIRST(bb);
    const char *data;
    apr_size_t length;

    while (e != APR_BRIGADE_SENTINEL(bb)) {
        apr_bucket_read(e, &data, &length, APR_BLOCK_READ);
        e = APR_BUCKET_NEXT(e);
    }

    return ap_pass_brigade(bb);

The above implementation would consume memory proportional to content size. If passed a `FILE` bucket, for example, the entire file contents would be read into memory as each `apr_bucket_read` call morphed a `FILE` bucket into a `HEAP` bucket.

In contrast, the implementation below will consume a fixed amount of memory to filter any brigade; a temporary brigade is needed and must be allocated only once per response, see the Maintaining state section.

### Better output filter

    apr_bucket *e;
    const char *data;
    apr_size_t length;

    while ((e = APR_BRIGADE_FIRST(bb)) != APR_BRIGADE_SENTINEL(bb)) {
       rv = apr_bucket_read(e, &data, &length, APR_BLOCK_READ);
       if (rv) ...;
       /* Remove bucket e from bb. */
       APR_BUCKET_REMOVE(e);
       /* Insert it into  temporary brigade. */
       APR_BRIGADE_INSERT_HEAD(tmpbb, e);
       /* Pass brigade downstream. */
       rv = ap_pass_brigade(f->next, tmpbb);
       if (rv) ...;
       apr_brigade_cleanup(tmpbb);
    }

## Maintaining state

A filter which needs to maintain state over multiple invocations per response can use the `->ctx` field of its `ap_filter_t` structure. It is typical to store a temporary brigade in such a structure, to avoid having to allocate a new brigade per invocation as described in the Brigade structure section.

### Example code to maintain filter state

    struct dummy_state {
       apr_bucket_brigade *tmpbb;
       int filter_state;
       ...
    };

    apr_status_t dummy_filter(ap_filter_t *f, apr_bucket_brigade *bb)
    {

        struct dummy_state *state;

        state = f->ctx;
        if (state == NULL) {

           /* First invocation for this response: initialise state structure.
            */
           f->ctx = state = apr_palloc(f->r->pool, sizeof *state);

           state->tmpbb = apr_brigade_create(f->r->pool, f->c->bucket_alloc);
           state->filter_state = ...;

        }
        ...

## Buffering buckets

If a filter decides to store buckets beyond the duration of a single filter function invocation (for example storing them in its `->ctx` state structure), those buckets must be set aside. This is necessary because some bucket types provide buckets which represent temporary resources (such as stack memory) which will fall out of scope as soon as the filter chain completes processing the brigade.

To setaside a bucket, the `apr_bucket_setaside` function can be called. Not all bucket types can be setaside, but if successful, the bucket will have morphed to ensure it has a lifetime at least as long as the pool given as an argument to the `apr_bucket_setaside` function.

Alternatively, the `ap_save_brigade` function can be used, which will move all the buckets into a separate brigade containing buckets with a lifetime as long as the given pool argument. This function must be used with care, taking into account the following points:

1. On return, `ap_save_brigade` guarantees that all the buckets in the returned brigade will represent data mapped into memory. If given an input brigade containing, for example, a `PIPE` bucket, `ap_save_brigade` will consume an arbitrary amount of memory to store the entire output of the pipe.
2. When `ap_save_brigade` reads from buckets which cannot be setaside, it will always perform blocking reads, removing the opportunity to use Non-blocking bucket reads.
3. If `ap_save_brigade` is used without passing a non-NULL "`saveto`" (destination) brigade parameter, the function will create a new brigade, which may cause memory use to be proportional to content size as described in the Brigade structure section.

Filters must ensure that any buffered data is processed and passed down the filter chain during the last invocation for a given response (a brigade containing an EOS bucket). Otherwise such data will be lost.

## Non-blocking bucket reads

The `apr_bucket_read` function takes an `apr_read_type_e` argument which determines whether a blocking or non-blocking read will be performed from the data source. A good filter will first attempt to read from every data bucket using a non-blocking read; if that fails with `APR_EAGAIN`, then send a `FLUSH` bucket down the filter chain, and retry using a blocking read.

This mode of operation ensures that any filters further down the filter chain will flush any buffered buckets if a slow content source is being used.

A CGI script is an example of a slow content source which is implemented as a bucket type. `mod_cgi` will send `PIPE` buckets which represent the output from a CGI script; reading from such a bucket will block when waiting for the CGI script to produce more output.

### Example code using non-blocking bucket reads

    apr_bucket *e;
    apr_read_type_e mode = APR_NONBLOCK_READ;

    while ((e = APR_BRIGADE_FIRST(bb)) != APR_BRIGADE_SENTINEL(bb)) {
        apr_status_t rv;

        rv = apr_bucket_read(e, &data, &length, mode);
        if (rv == APR_EAGAIN && mode == APR_NONBLOCK_READ) {

            /* Pass down a brigade containing a flush bucket: */
            APR_BRIGADE_INSERT_TAIL(tmpbb, apr_bucket_flush_create(...));
            rv = ap_pass_brigade(f->next, tmpbb);
            apr_brigade_cleanup(tmpbb);
            if (rv != APR_SUCCESS) return rv;

            /* Retry, using a blocking read. */
            mode = APR_BLOCK_READ;
            continue;
        } else if (rv != APR_SUCCESS) {
            /* handle errors */
        }

        /* Next time, try a non-blocking read first. */
        mode = APR_NONBLOCK_READ;
        ...
    }

## Ten rules for output filters

In summary, here is a set of rules for all output filters to follow:

1. Output filters should not pass empty brigades down the filter chain, but should be tolerant of being passed empty brigades.
2. Output filters must pass all metadata buckets down the filter chain; `FLUSH` buckets should be respected by passing any pending or buffered buckets down the filter chain.
3. Output filters should ignore any buckets following an `EOS` bucket.
4. Output filters must process a fixed amount of data at a time, to ensure that memory consumption is not proportional to the size of the content being filtered.
5. Output filters should be agnostic with respect to bucket types, and must be able to process buckets of unfamiliar type.
6. After calling `ap_pass_brigade` to pass a brigade down the filter chain, output filters should call `apr_brigade_cleanup` to ensure the brigade is empty before reusing that brigade structure; output filters should never use `apr_brigade_destroy` to "destroy" brigades.
7. Output filters must setaside any buckets which are preserved beyond the duration of the filter function.
8. Output filters must not ignore the return value of `ap_pass_brigade`, and must return appropriate errors back up the filter chain.
9. Output filters must only create a fixed number of bucket brigades for each response, rather than one per invocation.
10. Output filters should first attempt non-blocking reads from each data bucket, and send a `FLUSH` bucket down the filter chain if the read blocks, before retrying with a blocking read.

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