Catches 'PHP Fatal error Allowed memory size of xxxxxxx bytes exhausted' and 'RedisException' debug.log messages and flushes object cache.

auto-flush-object-cache.php

<?php
/**
 * Auto-flush object cache.
 *
 * @package Fragen\Auto_Flush_Cache
 *
 * Plugin Name:       Auto-Flush Object Cache
 * Plugin URI:        https://gist.github.com/afragen/d46d3cc2c7e07d99f921560dfbb70246
 * Description:       Auto-flush the object cache when error log records PHP Fatal error for allowed memory size.
 * Version:           0.7.1
 * Author:            Andy Fragen
 * License:           MIT
 * Requires at least: 5.2
 * Requires PHP:      7.1
 * Gist Plugin URI:   https://gist.github.com/afragen/d46d3cc2c7e07d99f921560dfbb70246
 */

namespace Fragen;

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

// Allow plugins to define location of error log.
add_action(
	'plugins_loaded',
	function() {
		$error_log_path = ini_get( 'error_log' );
		if ( ! empty( $error_log_path ) ) {
			( new Auto_Flush_Cache() )->run( $error_log_path );
		}
	}
);

/**
 * Class Auto_Flush_Cache
 */
class Auto_Flush_Cache {

	/**
	 * Let's get going.
	 *
	 * @param string $error_log_path File path to error log.
	 *
	 * @return void
	 */
	public function run( $error_log_path ) {
		$tail_error_log = $this->tail_custom( $error_log_path, 50 );
		$regex_errors   = [ '/Allowed memory size of (\d+) bytes exhausted/', '/RedisException:/' ];

		foreach ( $regex_errors as $regex ) {
			if ( preg_match( $regex, $tail_error_log ) ) {
				wp_cache_flush();
				file_put_contents( $error_log_path, '' );
				error_log( 'Object cache has been flushed and error log purged.' );
			}
		}
	}

	/**
	 * Slightly modified version of http://www.geekality.net/2011/05/28/php-tail-tackling-large-files/
	 *
	 * @author Torleif Berger, Lorenzo Stanco
	 * @link http://stackoverflow.com/a/15025877/995958
	 * @license http://creativecommons.org/licenses/by/3.0/
	 *
	 * @param string $filepath File path.
	 * @param int    $lines    Number of lines to tail.
	 * @param bool   $adaptive Dynamic buffer length.
	 */
	private function tail_custom( $filepath, $lines = 1, $adaptive = true ) {

		// Open file.
		$f = @fopen( $filepath, 'rb' );
		if ( false === $f ) {
			return false;
		}

		// Sets buffer size, according to the number of lines to retrieve.
		// This gives a performance boost when reading a few lines from the file.
		if ( ! $adaptive ) {
			$buffer = 4096;
		} else {
			$buffer = ( $lines < 2 ? 64 : ( $lines < 10 ? 512 : 4096 ) );
		}

		// Jump to last character.
		fseek( $f, -1, SEEK_END );

		// Read it and adjust line number if necessary.
		// (Otherwise the result would be wrong if file doesn't end with a blank line).
		if ( fread( $f, 1 ) !== "\n" ) {
			--$lines;
		}

		// Start reading.
		$output = '';
		$chunk  = '';

		// While we would like more.
		while ( ftell( $f ) > 0 && $lines >= 0 ) {
			// Figure out how far back we should jump.
			$seek = min( ftell( $f ), $buffer );

			// Do the jump (backwards, relative to where we are).
			fseek( $f, -$seek, SEEK_CUR );

			// Read a chunk and prepend it to our output.
			$output = ( $chunk = fread( $f, $seek ) ) . $output;

			// Jump back to where we started reading.
			fseek( $f, -mb_strlen( $chunk, '8bit' ), SEEK_CUR );

			// Decrease our line counter.
			$lines -= substr_count( $chunk, "\n" );
		}

		// While we have too many lines.
		// (Because of buffer size we might have read too many).
		while ( $lines++ < 0 ) {
			// Find first newline and remove all text before that.
			$output = substr( $output, strpos( $output, "\n" ) + 1 );
		}

		// Close file and return.
		fclose( $f );
		return trim( $output );
	}
}