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git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1651 48356398-32a2-884e-a903-53898d9a118a
280 lines
12 KiB
Plaintext
280 lines
12 KiB
Plaintext
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INCLUDES, AUTOLOAD, BYTECODE CACHES and OPTIMIZATION
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The Problem
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-----------
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HTML Purifier contains a number of extra components that are not used all
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of the time, only if the user explicitly specifies that we should use
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them.
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Some of these optional components are optionally included (Filter,
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Language, Lexer, Printer), while others are included all the time
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(Injector, URIFilter, HTMLModule, URIScheme). We will stipulate that these
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are all developer specified: it is conceivable that certain Tokens are not
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used, but this is user-dependent and should not be trusted.
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We should come up with a consistent way to handle these things and ensure
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that we get the maximum performance when there is bytecode caches and
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when there are not. Unfortunately, these two goals seem contrary to each
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other.
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A peripheral issue is the performance of ConfigSchema, which has been
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shown take a large, constant amount of initialization time, and is
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intricately linked to the issue of includes due to its pervasive use
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in our plugin architecture.
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Pros and Cons
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-------------
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We will assume that user-based extensions will be included by them.
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Conditional includes:
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Pros:
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- User management is simplified; only a single directive needs to be set
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- Only necessary code is included
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Cons:
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- Doesn't play nicely with opcode caches
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- Adds complexity to standalone version
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- Optional configuration directives are not exposed without a little
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extra coaxing (not implemented yet)
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Include it all:
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Pros:
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- User management is still simple
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- Plays nicely with opcode caches and standalone version
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- All configuration directives are present
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Cons:
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- Lots of (how much?) extra code is included
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- Classes that inherit from external libraries will cause compile
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errors
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Build an include stub (Let's do this!):
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Pros:
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- Only necessary code is included
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- Plays nicely with opcode caches and standalone version
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- require (without once) can be used, see above
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- Could further extend as a compilation to one file
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Cons:
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- Not implemented yet
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- Requires user intervention and use of a command line script
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- Standalone script must be chained to this
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- More complex and compiled-language-like
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- Requires a whole new class of system-wide configuration directives,
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as configuration objects can be reused
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- Determining what needs to be included can be complex (see above)
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- No way of autodetecting dynamically instantiated classes
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- Might be slow
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Include stubs
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-------------
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This solution may be "just right" for users who are heavily oriented
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towards performance. However, there are a number of picky implementation
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details to work out beforehand.
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The number one concern is how to make the HTML Purifier files "work
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out of the box", while still being able to easily get them into a form
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that works with this setup. As the codebase stands right now, it would
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be necessary to strip out all of the require_once calls. The only way
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we could get rid of the require_once calls is to use __autoload or
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use the stub for all cases (which might not be a bad idea).
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Aside
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-----
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An important thing to remember, however, is that these require_once's
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are valuable data about what classes a file needs. Unfortunately, there's
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no distinction between whether or not the file is needed all the time,
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or whether or not it is one of our "optional" files. Thus, it is
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effectively useless.
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Deprecated
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----------
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One of the things I'd like to do is have the code search for any classes
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that are explicitly mentioned in the code. If a class isn't mentioned, I
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get to assume that it is "optional," i.e. included via introspection.
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The choice is either to use PHP's tokenizer or use regexps; regexps would
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be faster but a tokenizer would be more correct. If this ends up being
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unfeasible, adding dependency comments isn't a bad idea. (This could
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even be done automatically by search/replacing require_once, although
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we'd have to manually inspect the results for the optional requires.)
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NOTE: This ends up not being necessary, as we're going to make the user
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figure out all the extra classes they need, and only include the core
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which is predetermined.
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Using the autoload framework with include stubs works nicely with
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introspective classes: instead of having to have require_once inside
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the function, we can let autoload do the work; we simply need to
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new $class or accept the object straight from the caller. Handling filters
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becomes a simple matter of ticking off configuration directives, and
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if ConfigSchema spits out errors, adding the necessary includes. We could
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also use the autoload framework as a fallback, in case the user forgets
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to make the include, but doesn't really care about performance.
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Insight
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-------
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All of this talk is merely a natural extension of what our current
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standalone functionality does. However, instead of having our code
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perform the includes, or attempting to inline everything that possibly
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could be used, we boot the issue to the user, making them include
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everything or setup the fallback autoload handler.
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Configuration Schema
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--------------------
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A common deficiency for all of the conditional include setups (including
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the dynamically built include PHP stub) is that if one of this
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conditionally included files includes a configuration directive, it
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is not accessible to configdoc. A stopgap solution for this problem is
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to have it piggy-back off of the data in the merge-library.php script
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to figure out what extra files it needs to include, but if the file also
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inherits classes that don't exist, we're in big trouble.
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I think it's high time we centralized the configuration documentation.
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However, the type checking has been a great boon for the library, and
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I'd like to keep that. The compromise is to use some other source, and
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then parse it into the ConfigSchema internal format (sans all of those
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nasty documentation strings which we really don't need at runtime) and
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serialize that for future use.
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The next question is that of format. XML is very verbose, and the prospect
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of setting defaults in it gives me willies. However, this may be necessary.
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Splitting up the file into manageable chunks may alleviate this trouble,
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and we may be even want to create our own format optimized for specifying
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configuration. It might look like (based off the PHPT format, which is
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nicely compact yet unambiguous and human-readable):
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Core.HiddenElements
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TYPE: lookup
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DEFAULT: array('script', 'style') // auto-converted during processing
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--ALIASES--
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Core.InvisibleElements, Core.StupidElements
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--DESCRIPTION--
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<p>
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Blah blah
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</p>
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The first line is the directive name, the lines after that prior to the
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first --HEADER-- block are single-line values, and then after that
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the multiline values are there. No value is restricted to a particular
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format: DEFAULT could very well be multiline if that would be easier.
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This would make it insanely easy, also, to add arbitrary extra parameters,
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like:
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VERSION: 3.0.0
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ALLOWED: 'none', 'light', 'medium', 'heavy' // this is wrapped in array()
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EXTERNAL: CSSTidy // this would be documented somewhere else with a URL
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The final loss would be that you wouldn't know what file the directive
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was used in; with some clever regexps it should be possible to
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figure out where $config->get($ns, $d); occurs. Reflective calls to
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the configuration object is mitigated by the fact that getBatch is
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used, so we can simply talk about that in the namespace definition page.
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This might be slow, but it would only happen when we are creating
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the documentation for consumption, and is sugar.
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We can put this in a schema/ directory, outside of HTML Purifier. The serialized
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data gets treated like entities.ser.
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The final thing that needs to be handled is user defined configurations.
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They can be added at runtime using ConfigSchema::registerDirectory()
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which globs the directory and grabs all of the directives to be incorporated
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in. Then, the result is saved. We may want to take advantage of the
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DefinitionCache framework, although it is not altogether certain what
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configuration directives would be used to generate our key (meta-directives!)
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Further thoughts
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----------------
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Our master configuration schema will only need to be updated once
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every new version, so it's easily versionable. User specified
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schema files are far more volatile, but it's far too expensive
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to check the filemtimes of all the files, so a DefinitionRev style
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mechanism works better. However, we can uniquely identify the
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schema based on the directories they loaded, so there's no need
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for a DefinitionId until we give them full programmatic control.
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These variables should be directly incorporated into ConfigSchema,
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and ConfigSchema should handle serialization. Some refactoring will be
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necessary for the DefinitionCache classes, as they are built with
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Config in mind. If the user changes something, the cache file gets
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rebuilt. If the version changes, the cache file gets rebuilt. Since
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our unit tests flush the caches before we start, and the operation is
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pretty fast, this will not negatively impact unit testing.
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One last thing: certain configuration directives require that files
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get added. They may even be specified dynamically. It is not a good idea
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for the HTMLPurifier_Config object to be used directly for such matters.
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Instead, the userland code should explicitly perform the includes. We may
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put in something like:
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REQUIRES: HTMLPurifier_Filter_ExtractStyleBlocks
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To indicate that if that class doesn't exist, and the user is attempting
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to use the directive, we should fatally error out. The stub includes the core files,
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and the user includes everything else. Any reflective things like new
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$class would be required to tie in with the configuration.
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It would work very well with rarely used configuration options, but it
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wouldn't be so good for "core" parts that can be disabled. In such cases
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the core include file would need to be modified, and the only way
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to properly do this is use the configuration object. Once again, our
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ability to create cache keys saves the day again: we can create arbitrary
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stub files for arbitrary configurations and include those. They could
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even be the single file affairs. The only thing we'd need to include,
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then, would be HTMLPurifier_Config! Then, the configuration object would
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load the library.
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An aside...
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-----------
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One questions, however, the wisdom of letting PHP files write other PHP
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files. It seems like a recipe for disaster, or at least lots of headaches
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in highly secured setups, where PHP does not have the ability to write
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to its root. In such cases, we could use sticky bits or tell the user
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to manually generate the file.
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The other troublesome bit is actually doing the calculations necessary.
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For certain cases, it's simple (such as URIScheme), but for AttrDef
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and HTMLModule the dependency trees are very complex in relation to
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%HTML.Allowed and friends. I think that this idea should be shelved
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and looked at a later, less insane date.
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An interesting dilemma presents itself when a configuration form is offered
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to the user. Normally, the configuration object is not accessible without
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editing PHP code; this facility changes thing. The sensible thing to do
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is stipulate that all classes required by the directives you allow must
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be included.
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Unit testing
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------------
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Setting up the parsing and translation into our existing format would not
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be difficult to do. It might represent a good time for us to rethink our
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tests for these facilities; as creative as they are, they are often hacky
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and require public visibility for things that ought to be protected.
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This is especially applicable for our DefinitionCache tests.
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Migration
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---------
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Because we are not *adding* anything essentially new, it should be trivial
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to write a script to take our existing data and dump it into the new format.
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Well, not trivial, but fairly easy to accomplish. Primary implementation
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difficulties would probably involve formatting the file nicely.
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Backwards-compatibility
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-----------------------
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I expect that the ConfigSchema methods should stick around for a little bit,
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but display E_USER_NOTICE warnings that they are deprecated. This will
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require documentation!
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New stuff
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---------
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VERSION: Version number directive was introduced
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DEPRECATED-VERSION: If the directive was deprecated, when was it deprecated?
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DEPRECATED-USE: If the directive was deprecated, what should the user use now?
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REQUIRES: What classes does this configuration directive require, but are
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not part of the HTML Purifier core?
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