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[EDIT] FILE: PrepareBodyMiddleware.php

<?php

namespace GuzzleHttp;

use GuzzleHttp\Promise\PromiseInterface;
use Psr\Http\Message\RequestInterface;

/**
 * Prepares requests that contain a body, adding the Content-Length,
 * Content-Type, and Expect headers.
 *
 * @final
 */
class PrepareBodyMiddleware
{
    /**
     * @var callable(RequestInterface, array): PromiseInterface
     */
    private $nextHandler;

/**
     * @param callable(RequestInterface, array): PromiseInterface $nextHandler Next handler to invoke.
     */
    public function __construct(callable $nextHandler)
    {
        $this->nextHandler = $nextHandler;
    }

public function __invoke(RequestInterface $request, array $options): PromiseInterface
    {
        $fn = $this->nextHandler;

// Don't do anything if the request has no body.
        if ($request->getBody()->getSize() === 0) {
            return $fn($request, $options);
        }

$modify = [];

// Add a default content-type if possible.
        if (!$request->hasHeader('Content-Type')) {
            if ($uri = $request->getBody()->getMetadata('uri')) {
                if (is_string($uri) && $type = Psr7\MimeType::fromFilename($uri)) {
                    $modify['set_headers']['Content-Type'] = $type;
                }
            }
        }

// Add a default content-length or transfer-encoding header.
        if (!$request->hasHeader('Content-Length')
            && !$request->hasHeader('Transfer-Encoding')
        ) {
            $size = $request->getBody()->getSize();
            if ($size !== null) {
                $modify['set_headers']['Content-Length'] = $size;
            } else {
                $modify['set_headers']['Transfer-Encoding'] = 'chunked';
            }
        }

// Add the expect header if needed.
        $this->addExpectHeader($request, $options, $modify);

return $fn(Psr7\Utils::modifyRequest($request, $modify), $options);
    }

/**
     * Add expect header
     */
    private function addExpectHeader(RequestInterface $request, array $options, array &$modify): void
    {
        // Determine if the Expect header should be used
        if ($request->hasHeader('Expect')) {
            return;
        }

$expect = $options['expect'] ?? null;

// Return if disabled or using HTTP/1.0
        if ($expect === false || $request->getProtocolVersion() === '1.0') {
            return;
        }

// The expect header is unconditionally enabled
        if ($expect === true) {
            $modify['set_headers']['Expect'] = '100-Continue';

return;
        }

// By default, send the expect header when the payload is > 1mb
        if ($expect === null) {
            $expect = 1048576;
        }

// Always add if the body cannot be rewound, the size cannot be
        // determined, or the size is greater than the cutoff threshold
        $body = $request->getBody();
        $size = $body->getSize();

if ($size === null || $size >= (int) $expect || !$body->isSeekable()) {
            $modify['set_headers']['Expect'] = '100-Continue';
        }
    }
}

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