Apache HTTP Server Version 2.4

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# How filters work in Apache 2.0

### Warning

This is a cut 'n paste job from an email (<022501c1c529$f63a9550$7f00000a@KOJ>) and only reformatted for better readability. It's not up to date but may be a good start for further research.

## Filter Types

There are three basic filter types (each of these is actually broken down into two categories, but that comes later).

`CONNECTION`
  Filters of this type are valid for the lifetime of this connection. (`AP_FTYPE_CONNECTION`, `AP_FTYPE_NETWORK`)
`PROTOCOL`
  Filters of this type are valid for the lifetime of this request from the point of view of the client, this means that the request is valid from the time that the request is sent until the time that the response is received. (`AP_FTYPE_PROTOCOL`, `AP_FTYPE_TRANSCODE`)
`RESOURCE`
  Filters of this type are valid for the time that this content is used to satisfy a request. For simple requests, this is identical to `PROTOCOL`, but internal redirects and sub-requests can change the content without ending the request. (`AP_FTYPE_RESOURCE`, `AP_FTYPE_CONTENT_SET`)

It is important to make the distinction between a protocol and a resource filter. A resource filter is tied to a specific resource, it may also be tied to header information, but the main binding is to a resource. If you are writing a filter and you want to know if it is resource or protocol, the correct question to ask is: "Can this filter be removed if the request is redirected to a different resource?" If the answer is yes, then it is a resource filter. If it is no, then it is most likely a protocol or connection filter. I won't go into connection filters, because they seem to be well understood. With this definition, a few examples might help:

Byterange
  We have coded it to be inserted for all requests, and it is removed if not used. Because this filter is active at the beginning of all requests, it can not be removed if it is redirected, so this is a protocol filter.
http_header
  This filter actually writes the headers to the network. This is obviously a required filter (except in the asis case which is special and will be dealt with below) and so it is a protocol filter.
Deflate
  The administrator configures this filter based on which file has been requested. If we do an internal redirect from an autoindex page to an index.html page, the deflate filter may be added or removed based on config, so this is a resource filter.

The further breakdown of each category into two more filter types is strictly for ordering. We could remove it, and only allow for one filter type, but the order would tend to be wrong, and we would need to hack things to make it work. Currently, the `RESOURCE` filters only have one filter type, but that should change.

## How are filters inserted?

This is actually rather simple in theory, but the code is complex. First of all, it is important that everybody realize that there are three filter lists for each request, but they are all concatenated together:

- `r->output_filters` (corresponds to RESOURCE)
- `r->proto_output_filters` (corresponds to PROTOCOL)
- `r->connection->output_filters` (corresponds to CONNECTION)

The problem previously, was that we used a singly linked list to create the filter stack, and we started from the "correct" location. This means that if I had a `RESOURCE` filter on the stack, and I added a `CONNECTION` filter, the `CONNECTION` filter would be ignored. This should make sense, because we would insert the connection filter at the top of the `c->output_filters` list, but the end of `r->output_filters` pointed to the filter that used to be at the front of `c->output_filters`. This is obviously wrong. The new insertion code uses a doubly linked list. This has the advantage that we never lose a filter that has been inserted. Unfortunately, it comes with a separate set of headaches.

The problem is that we have two different cases were we use subrequests. The first is to insert more data into a response. The second is to replace the existing response with an internal redirect. These are two different cases and need to be treated as such.

In the first case, we are creating the subrequest from within a handler or filter. This means that the next filter should be passed to `make_sub_request` function, and the last resource filter in the sub-request will point to the next filter in the main request. This makes sense, because the sub-request's data needs to flow through the same set of filters as the main request. A graphical representation might help:

    Default_handler --> includes_filter --> byterange --> ...

If the includes filter creates a sub request, then we don't want the data from that sub-request to go through the includes filter, because it might not be SSI data. So, the subrequest adds the following:

    Default_handler --> includes_filter -/-> byterange --> ...
                                        /
    Default_handler --> sub_request_core

What happens if the subrequest is SSI data? Well, that's easy, the `includes_filter` is a resource filter, so it will be added to the sub request in between the `Default_handler` and the `sub_request_core` filter.

The second case for sub-requests is when one sub-request is going to become the real request. This happens whenever a sub-request is created outside of a handler or filter, and NULL is passed as the next filter to the `make_sub_request` function.

In this case, the resource filters no longer make sense for the new request, because the resource has changed. So, instead of starting from scratch, we simply point the front of the resource filters for the sub-request to the front of the protocol filters for the old request. This means that we won't lose any of the protocol filters, neither will we try to send this data through a filter that shouldn't see it.

The problem is that we are using a doubly-linked list for our filter stacks now. But, you should notice that it is possible for two lists to intersect in this model. So, you do you handle the previous pointer? This is a very difficult question to answer, because there is no "right" answer, either method is equally valid. I looked at why we use the previous pointer. The only reason for it is to allow for easier addition of new servers. With that being said, the solution I chose was to make the previous pointer always stay on the original request.

This causes some more complex logic, but it works for all cases. My concern in having it move to the sub-request, is that for the more common case (where a sub-request is used to add data to a response), the main filter chain would be wrong. That didn't seem like a good idea to me.

## Asis

The final topic. :-) Mod_Asis is a bit of a hack, but the handler needs to remove all filters except for connection filters, and send the data. If you are using `mod_asis`, all other bets are off.

## Explanations

The absolutely last point is that the reason this code was so hard to get right, was because we had hacked so much to force it to work. I wrote most of the hacks originally, so I am very much to blame. However, now that the code is right, I have started to remove some hacks. Most people should have seen that the `reset_filters` and `add_required_filters` functions are gone. Those inserted protocol level filters for error conditions, in fact, both functions did the same thing, one after the other, it was really strange. Because we don't lose protocol filters for error cases any more, those hacks went away. The `HTTP_HEADER`, `Content-length`, and `Byterange` filters are all added in the `insert_filters` phase, because if they were added earlier, we had some interesting interactions. Now, those could all be moved to be inserted with the `HTTP_IN`, `CORE`, and `CORE_IN` filters. That would make the code easier to follow.