Apache HTTP Server Version 2.4

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# Converting Modules from Apache 1.3 to Apache 2.0

This is a first attempt at writing the lessons I learned when trying to convert the `mod_mmap_static` module to Apache 2.0. It's by no means definitive and probably won't even be correct in some ways, but it's a start.

## The easier changes ...

### Cleanup Routines

These now need to be of type `apr_status_t` and return a value of that type. Normally the return value will be `APR_SUCCESS` unless there is some need to signal an error in the cleanup. Be aware that even though you signal an error not all code yet checks and acts upon the error.

### Initialisation Routines

These should now be renamed to better signify where they sit in the overall process. So the name gets a small change from `mmap_init` to `mmap_post_config`. The arguments passed have undergone a radical change and now look like

- `apr_pool_t *p`
- `apr_pool_t *plog`
- `apr_pool_t *ptemp`
- `server_rec *s`

### Data Types

A lot of the data types have been moved into the APR. This means that some have had a name change, such as the one shown above. The following is a brief list of some of the changes that you are likely to have to make.

- `pool` becomes `apr_pool_t`
- `table` becomes `apr_table_t`

## The messier changes...

### Register Hooks

The new architecture uses a series of hooks to provide for calling your functions. These you'll need to add to your module by way of a new function, `static void register_hooks(void)`. The function is really reasonably straightforward once you understand what needs to be done. Each function that needs calling at some stage in the processing of a request needs to be registered, handlers do not. There are a number of phases where functions can be added, and for each you can specify with a high degree of control the relative order that the function will be called in.

This is the code that was added to `mod_mmap_static`:

    static void register_hooks(void)
    {
        static const char * const aszPre[]={ "http_core.c",NULL };
        ap_hook_post_config(mmap_post_config,NULL,NULL,HOOK_MIDDLE);
        ap_hook_translate_name(mmap_static_xlat,aszPre,NULL,HOOK_LAST);
    };

This registers 2 functions that need to be called, one in the `post_config` stage (virtually every module will need this one) and one for the `translate_name` phase. note that while there are different function names the format of each is identical. So what is the format?

`ap_hook_phase_name(function_name, predecessors, successors, position);`

There are 3 hook positions defined...

- `HOOK_FIRST`
- `HOOK_MIDDLE`
- `HOOK_LAST`

To define the position you use the position and then modify it with the predecessors and successors. Each of the modifiers can be a list of functions that should be called, either before the function is run (predecessors) or after the function has run (successors).

In the `mod_mmap_static` case I didn't care about the `post_config` stage, but the `mmap_static_xlat` must be called after the core module had done its name translation, hence the use of the aszPre to define a modifier to the position `HOOK_LAST`.

### Module Definition

There are now a lot fewer stages to worry about when creating your module definition. The old definition looked like

    module MODULE_VAR_EXPORT module_name_module =
    {
        STANDARD_MODULE_STUFF,
        /* initializer */
        /* dir config creater */
        /* dir merger --- default is to override */
        /* server config */
        /* merge server config */
        /* command handlers */
        /* handlers */
        /* filename translation */
        /* check_user_id */
        /* check auth */
        /* check access */
        /* type_checker */
        /* fixups */
        /* logger */
        /* header parser */
        /* child_init */
        /* child_exit */
        /* post read-request */
    };

The new structure is a great deal simpler...

    module MODULE_VAR_EXPORT module_name_module =
    {
        STANDARD20_MODULE_STUFF,
        /* create per-directory config structures */
        /* merge per-directory config structures  */
        /* create per-server config structures    */
        /* merge per-server config structures     */
        /* command handlers */
        /* handlers */
        /* register hooks */
    };

Some of these read directly across, some don't. I'll try to summarise what should be done below.

The stages that read directly across :

`/* dir config creater */`
  `/* create per-directory config structures */`
`/* server config */`
  `/* create per-server config structures */`
`/* dir merger */`
  `/* merge per-directory config structures */`
`/* merge server config */`
  `/* merge per-server config structures */`
`/* command table */`
  `/* command apr_table_t */`
`/* handlers */`
  `/* handlers */`

The remainder of the old functions should be registered as hooks. There are the following hook stages defined so far...

`ap_hook_pre_config`
  do any setup required prior to processing configuration directives
`ap_hook_check_config`
  review configuration directive interdependencies
`ap_hook_test_config`
  executes only with `-t` option
`ap_hook_open_logs`
  open any specified logs
`ap_hook_post_config`
  this is where the old `_init` routines get registered
`ap_hook_http_method`
  retrieve the http method from a request. (legacy)
`ap_hook_auth_checker`
  check if the resource requires authorization
`ap_hook_access_checker`
  check for module-specific restrictions
`ap_hook_check_user_id`
  check the user-id and password
`ap_hook_default_port`
  retrieve the default port for the server
`ap_hook_pre_connection`
  do any setup required just before processing, but after accepting
`ap_hook_process_connection`
  run the correct protocol
`ap_hook_child_init`
  call as soon as the child is started
`ap_hook_create_request`
  ??
`ap_hook_fixups`
  last chance to modify things before generating content
`ap_hook_handler`
  generate the content
`ap_hook_header_parser`
  lets modules look at the headers, not used by most modules, because they use `post_read_request` for this
`ap_hook_insert_filter`
  to insert filters into the filter chain
`ap_hook_log_transaction`
  log information about the request
`ap_hook_optional_fn_retrieve`
  retrieve any functions registered as optional
`ap_hook_post_read_request`
  called after reading the request, before any other phase
`ap_hook_quick_handler`
  called before any request processing, used by cache modules.
`ap_hook_translate_name`
  translate the URI into a filename
`ap_hook_type_checker`
  determine and/or set the doc type