PHP: rfc:xmlreader_writer_streams

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====== PHP RFC: Add stream open functions to XML{Reader,Writer} ======
  * Version: 0.10.0
  * Date: 2024-04-21
  * Author: Niels Dossche <nielsdos@php.net>
  * Status: Under Discussion
  * First Published at: https://wiki.php.net/rfc/xmlreader_writer_streams

===== Introduction =====

The <php>XMLReader</php> and <php>XMLWriter</php> classes deal with XML in a stream-oriented manner. The former implements an XML "pull parser". This means that instead of keeping the data in memory or building a document tree, the document is streamed and the developer can instruct <php>XMLReader</php> to parse chunks at the current cursor and either process or skip the data. The advantage is that developers can process and filter large documents while requiring few resources. It is most often used as a lower-level building block for more complex handling of large XML documents. Similarly, <php>XMLWriter</php> writes an XML document to a stream or memory by using functions like <php>startElement</php> and <php>writeElement</php>.

There is however a strange limitation to these classes: they cannot operate on an already-open stream! This is bizarre as the APIs (both internally and user-facing) are stream-oriented. Streams that are already open are common when working, for example, with HTTP requests, data passed from a framework, or just XML data embedded in an existing stream. The lack of an API that works with already-opened streams causes developers to rely on workarounds, e.g. reading the stream entirely to memory and then using the <php>XMLReader</php> APIs, or writing an XML file using <php>XMLWriter</php> and then having to pass that into an already-open stream. That's just wasteful and needlessly difficult. This RFC aims to fix that problem and aims to fix some other inconsistency as well.

===== Proposal =====

I propose to add 2 new functions, one to <php>XMLReader</php> and one to <php>XMLWriter</php>, to create an instance from a stream. Here is how they would look like:

<PHP>
class XMLReader {
    /** @param resource $stream */
    public static function fromStream($stream, ?string $encoding = null, int $flags = 0, ?string $documentUri = null): static {}
}

class XMLWriter {
    /** @param resource $stream */
    public static function toStream($stream): static {}
}
</PHP>

The signatures are heavily inspired by the existing function <php>public static XMLReader::open(string $uri, ?string $encoding = null, int $flags = 0): bool|XMLReader</php> that operate on files.
However, a major difference is that <php>XMLReader::fromStream()</php> is static-only, whereas the other open functions of <php>XMLReader</php> can either be statically or non-statically called and change their return-value behaviour depending on that. The disadvantage of the existing static methods is that they can only return an instance of <php>XMLReader</php>, therefore when <php>XMLReader</php> is inherited by a user subclass we run into the problem that it doesn't return an object of the right type. We solve this by choosing <php>static</php> as return-type, and letting the method internally call the constructor of the static type (with no arguments). As we seem to move away from overloaded functions, I decided to only make a static method variation available.

The <php>$documentUri</php> parameter is used mostly for when libxml outputs error messages, such that you can put an origin name in there.

The signature for <php>XMLWriter::toStream()</php> should be self-explanatory. It is also modeled like the other open functions, but it is considerably simpler. You'll also notice the lack of an encoding argument, and that's because this is already handled by the <php>XMLWriter::startDocument()</php> function.

While implementing this, I found some strange behaviour regarding the <php>?string $encoding</php> parameter of the existing functions <php>XMLReader::open()</php> and <php>XMLReader::XML()</php>. The first oddity is that they emit a warning instead of throwing a ValueError when the encoding contains NULL bytes. This is inconsistent with how other functions handle it. I propose to promote this warning to a ValueError instead. The second oddity is that invalid encoding names are ignored entirely. This means that it won't emit a warning or anything, but just silently not set the encoding. This can hide bugs. I propose to also throw a ValueError in this case stating "Argument #X ($encoding) must be a valid character encoding".

An earlier version of this RFC proposed adding <php>openStream()</php> methods to both classes, but the naming was not ideal and the behaviour of being an instance method was not liked. Therefore, this was changed to static-only methods that return an instance of the respective class.

===== Backward Incompatible Changes =====

There are three minor BC breaks.

The first one is the fact that we're adding the <php>fromStream()</php> or <php>toStream</php> method. If a user extends the <php>XMLReader</php> or <php>XMLWriter</php> class, and their class implements a method with the same name but an incompatible signature, a compile error will occur. I analyzed the top 2500 Composer packages, and none used the toStream or fromStream names in subclasses of the XML classes.. This means that the top 2500 packages don't suffer a BC break because of this. That doesn't mean there will be none, but it gives a good indication.

The second BC break is caused by throwing a <php>ValueError</php> on invalid encodings instead of silently ignoring invalid encodings. If we don't signal the invalid encoding in any way to the user, this can subtly hide bugs. For example, this could hide typos or silently pass invalid user input to the respective functions. Forcing developers to handle this error explicitly will result in more robust code in the end.

The third BC break is the promotion of the NULL-byte warning to a <php>ValueError</php>. This makes the <php>XMLReader</php> and <php>XMLWriter</php> class more consistent with other extensions that throw instead of issuing a warning. The migration for developers should be quite simple: instead of silencing the warning and/or checking the return value of the function, they should use a try-catch construct to handle the error.

===== Example usages =====

==== Minimal XMLReader example ====

<PHP>
// Could be any stream, but this is for simplicity sake
$h = fopen("php://memory", "w+");
fwrite($h, "<root><!--my comment--><child/></root>");
fseek($h, 0);

$reader = XMLReader::fromStream($h);

while ($reader->read()) {
    switch ($reader->nodeType) {
        case XMLReader::ELEMENT:
            echo "Element: ", $reader->name, "\n";
            break;
        case XMLReader::COMMENT:
            echo "Comment: ", $reader->value, "\n";
            break;
    }
}
</PHP>

==== Minimal XMLWriter example ====

<PHP>
// Could be any stream, but this is for simplicity sake
$h = fopen("php://output", "w");

$writer = XMLWriter::toStream($h);

$writer->startElement("root");
$writer->writeAttribute("align", "left");
$writer->writeComment("hello");
$writer->endElement();
$amount = $writer->flush();
echo "\nAmount of bytes written: ";
var_dump($amount);
</PHP>

===== Proposed PHP Version(s) =====

Next PHP 8.x, this is PHP 8.4 at the time of writing.

===== RFC Impact =====

==== To Existing Extensions ====

Only ext/xmlreader and ext/xmlwriter are affected.

===== Open Issues =====

None yet.

===== Unaffected PHP Functionality =====

Everything else, why do we have this section?

===== Future Scope =====

None yet.

===== Proposed Voting Choices =====

One primary vote requiring 2/3rd majority to accept the RFC as a whole.

===== Patches and Tests =====

Implementation PR: https://github.com/php/php-src/pull/14030

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature
  - a link to the language specification section (if any)

===== References =====

  - https://bugs.php.net/bug.php?id=63506
  - https://bugs.php.net/bug.php?id=46146

===== Rejected Features =====

None yet.

===== Changelog =====

  - 0.10.0: Static again
  - 0.9.2: Add example usages of the new APIs.
  - 0.9.1: Made XMLReader::openStream() non-static instead such that it works with overridden classes.
  - 0.9: Initial version under discussion
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