rfc:function_autoloading

PHP RFC: Function Autoloading

Introduction

PHP presently offers the ability to import class-like structures (classes, interfaces and traits) via a callback (or series of them) that can be registered. This lets a developer “catch” the fact that a class isn't present, and gives them a chance to load it. This is used to great effect in the PHP community.

Presently, other types of symbols tables are not autoloadable. This RFC proposes a new unified autoloading mechanism to unify autoloading efforts across all four symbol tables (class, function, stream wrapper and constant).

Proposal

This RFC proposes to add a suite of functions and constants in the php namespace to achieve total control over autoloading.

Constants

This proposal registers the following constants:

  • php\AUTOLOAD_CLASS ⇒ 1 - Represents Class autoloading
  • php\AUTOLOAD_FUNCTION ⇒ 2 - Represents Function autoloading
  • php\AUTOLOAD_CONSTANT ⇒ 4 - Represents Constant autoloading
  • php\AUTOLOAD_STREAM = 8 - Represents Stream autoloading

Userland Functions

This proposal registers / modifies the following functions:

bool php\autoload_register(callable $callback, int $type, bool $prepend = false)

This function behaves similar to the current spl_autoload_register function. You can pass in a bitmask of types to register for this autoloader.

bool php\autoload_unregister(callable $callback, int $type = 0)

This function behaves similar to the current spl_autoload_unregister function, and unregisters a callback that was previously registered. Note that if you registered the same callback for multiple types, this will unregister all of them unless the $type argument is specified.

array php\autoload_list(int $type)

This function will return a list of all registered autoloaders for a specific type.

function_exists()

A new optional boolean argument is added to function_exists() to match the behavior of class_exists() when it comes to autoloading functions.

defined()

A new optional boolean argument is added to defined() to match the behavior of class_exists() when it comes to autoloading constants.

Internal Typedefs

This proposal adds the following internal structures:

zend_autoload_func

This structure stores the autoloader function.

typedef struct zend_autoload_func {
    zend_fcall_info fci;
    zend_fcall_info_cache fcc;
    zval *callable;
    int pass_type;
} _zend_autoload_func;

Internal Functions

This proposal adds the following ZEND_API functions/macros:

void* zend_autoload_call(zend_string *name, zend_string *lname, int type)

This function will call an autoloader of the specified type. It requires two versions of the name, one which is the called case, and one which is the normalized lookup case. In the case of case-sensitive constants, they should be identical. In the case of insensitive constants, functions and classes, lname should be a lowercase version of the name.

int zend_autoload_register(zend_autoload_func* func, int type, int flags)

This will register an autoload function with the specified flags (prepend is currently the only supported flag).

int zend_autoload_unregister(zend_autoload_func* func, int type)

This will unregister an autoloader function that's passed.

int zend_lookup_function(const char *name, int name_length, zend_function <nowiki>**</nowiki>fbc)

Lookup a function by name, using autoloading

int zend_lookup_function_ex(const char *name, int name_length, const zend_literal *key, int use_autoload, zend_function <nowiki>**</nowiki>fbc)

Lookup a function by name, optionally calling the autoloader.

ZEND_LOOKUP_FUNCTION_BY_NAME(name, fbc)

This macro will find a function in the symbol table, or attempt to autoload it if not defined. The name must be a zend_string.

ZEND_LOOKUP_FUNCTION_BY_KEY(name, key, fbc)

This macro will find a function in the symbol table, or attempt to autoload it if not defined. This separates the called function in name from the looked up function in key. Both name and key must be zend_string.

ZEND_LOOKUP_FUNCTION_BY_NS_KEY(name, key, fbc)

This macro will find a function in the symbol table, or attempt to autoload it if not defined. This separates the called function in name from the looked up function in key. Both name and key must be zend_string.

Behavior

Registering autoloaders with the new API will allow callbacks to be fired on class, function and/or constant missing errors.

Single Type Behavior

By passing a single constant to the register function, the callback will only be called for types that match (the $type parameter is still set, but will never vary).

single_type.php
<?php
php\autoload_register(function($name, $type) {
    var_dump($name, $type);
    eval("function $name(){}");
    // We don't need a switch, since we only register for functions.
}, php\AUTOLOAD_FUNCTION);
foo(); // string(3) "foo" int(2)
new foo(); // FATAL_ERROR as no autoloader is registered
?>

Multiple Type Behavior

By passing a bitwise-or'd constant to the register function, the callback will only be called for types that match).

multiple_type.php
<?php
php\autoload_register(function($name, $type) {
    var_dump($name, $type);
    switch ($type) {
       case php\AUTOLOAD_FUNCTION:
           eval("function $name(){}");
           break;
       case php\AUTOLOAD_CONSTANT:
           define($name, $name);
           break;
    }
}, php\AUTOLOAD_FUNCTION | php\AUTOLOAD_CONSTANT);
foo(); // string(3) "foo" int(2)
FOO; // string(3) "FOO" int(4)
new foo(); // FATAL_ERROR as no autoloader is registered
?>

Registering The Same Callback Multiple Times For Different Types

multiple_registration.php
<?php
$callback = function($name, $type) {
    var_dump($name);
    if ($name === 'foo') {
        eval("function $name(){}");
    } else {
        define($name, $name);
    }
};
php\autoload_register($callback, php\AUTOLOAD_FUNCTION);
php\autoload_register($callback, php\AUTOLOAD_CONSTANT);
foo(); // string(3) "foo" int(2)
FOO; // string(3) "FOO", "FOO"
?>

Userland Backwards Compatibility

SPL

This RFC proposes to strip the current spl_autoload_register functionality, and make spl_autoload_* simple proxies for registering core autoloaders. They will function exactly as they do now, but under the hood they will be using the new interface.

This means that calls to spl_autoload_functions() will include any autoloader (which indicates support for php\AUTOLOAD_CLASS) registered through php\autoload_register(). However, all autoloaders registered via spl_autoload_register will set the pass_type flag to 0, meaning that only a single argument will be passed to the callback. This is for compatiblity.

__autoload()

The legacy __autoload() function still works (only for classes) if no autoloader has been registered. If any autoloader is registered (class, function or constant), the legacy system will disable itself (this is how it works currently).

C API Backwards Compatibility

SPL

The autoload related SPL globals have been removed, due to the implementation being centralized.

Zend

A pair of new functions have been added:

  • ZEND_API int zend_lookup_function(const char *name, int name_length, zend_function **fbc TSRMLS_DC)
  • ZEND_API int zend_lookup_function_ex(const char *name, int name_length, const zend_literal *key, int use_autoload, zend_function **fbc TSRMLS_DC)

These will do a normal lookup for a function, and then fall back to an autoloader.

A pair of new “helper macros” have also been added:

  • ZEND_LOOKUP_FUNCTION_BY_NAME(name, name_length, fbc)
  • ZEND_LOOKUP_FUNCTION_BY_LITERAL(name, name_length, literal, fbc)

These two will do a legacy style hash-table lookup before triggering the autoloading function call (to zend_lookup_function()). The reason for this is performance.

Opcodes which lookup functions, are using this new macro. This way, there should be no performance regression at all (thanks to short-circuit operators) for defined functions.

General questions & answers

Why Rewrite A Complete Autoloader?

Initially, I implemented this as spl_function_autoload_register. Quickly, it became clear that there was a lot of duplication, and the original system was a bit stringy.

This implementation greatly simplifies the (internal) handling of autoloading in general.

Why not support multiple "types" for a single autoloader?

Existing autoloaders may support a second parameter. If we attempted to provide the type as a second argument to an existing autoloader, it could clash. This happens with the existing spl_autoload() autoloader.

What Filename Conventions Does This Support?

None, and all. This proposal presently implements no type of file loading handler.

The only thing that is implemented is the ability to register a callback to attempt to load a function or constant (or class) when it is missing. How the callback maps structures to files is outside of the scope of this RFC.

Doesn't This Complicate The Engine?

Nope! The reason is that the current autoloading mechanism for classes is extremely fragile as is.

For example, the implementation hinges on a global variable which sets the php-level callback to call on autoload. This requires setting up a zend_fcall_info struct, and a zend_fcall_info_cache struct, as well as dispatching a function internally to autoload. The Current Implementation.

The implementation of spl_autoload_call and spl_autoload_register are also extremely complicated. The current SPL implementation.

This refactor cleans both of these pieces up significantly.

Backward Incompatible Changes

Userland

There should be no user-land BC changes.

PECL

EG(autoload_func)

PECL extensions which rely on the EG(autoload_func) global variable will break (due to refactor).

A quick scan of LXR shows that only the optimizer extension would change.

autoload_func_info

PECL extensions which reply on the SPL type autoload_func_info will break (due to refactor).

A quick scan of LXR shows that no extensions use this.

SPL_G(autoload_functions)

PECL extensions which rely on the SPL globals will break (due to refactor).

A quick scan of LXR shows that no extensions use this.

Proposed PHP Version(s)

PHP 7.1.x

SAPIs Impacted

None.

Impact to Existing Extensions

See Backward Incompatible Changes

php.ini Defaults

None.

Open Issues

None yet.

Discussion Points

Autoloading Constants

Summary

When refactoring the autoloader to support functions, adding support for constants isn't significantly difficult, yet can have an advantage.

Stance

This RFC takes the stance that it is worth while autoloading constants for consistency.

Deprecation of __autoload()

Summary

The legacy function __autoload() can be deprecated.

Stance

This is out of scope for this RFC.

Deprecation of spl_autoload_register()

Summary

Since there is a new implementation which supports class autoloading, spl_autoload_register() is redundant. Therefore, it can be deprecated.

Stance

This RFC takes the stance that deprecation should not happen right away, if at all. It is therefore out of scope for this RFC.

Patches and Tests

A patch will be created shortly

References

Rejected Features

- None.

Vote

Changelog

  • 2013-08-29 0.1 Initial Creation
  • 2013-08-30 0.2 Add performance section and basic benchmarks
  • 2015-09-04 0.3 Re-proposed
rfc/function_autoloading.txt · Last modified: 2015/09/03 07:10 by ircmaxell