Truncate FileContentCache by default to avoid SQL query errors

MySQL has a packet limit of 1MB as a default
(http://dev.mysql.com/doc/refman/5.0/en/packet-too-large.html).
This interferes with the UPDATE queries required
to add file content caches. Since the query can't be terminated
correctly, the whole content will be discarded with a query error.

This change allows to truncate content prior to the UPDATE operation,
and defaults to 500 characters. This leaves some room for multibyte
characters as well as other parts of the SQL query.

README.md

@@ -64,6 +64,10 @@ File:
 ```
 
 By default any extracted content will be cached against the database row.
+In order to stay within common size constraints for SQL queries required in this operation,
+the cache sets a maximum character length after which content gets truncated (default: 500000).
+You can configure this value through `FileTextCache_Database.max_content_length` in your yaml configuration.
+
 
 Alternatively, extracted content can be cached using SS_Cache to prevent excessive database growth.
 In order to swap out the cache backend you can use the following yaml configuration.

_config/config.yml

@@ -6,3 +6,6 @@ Injector:
 
 SolrCellTextExtractor:
 #  base_url: 'http://localhost:8983/solr/update/extract'
+
+FileTextCache_Database:
+  max_content_length: 500000

code/extensions/FileTextCache.php

@@ -19,7 +19,8 @@ interface FileTextCache {
 }
 
 /**
- * Caches the extracted content on the record for the file
+ * Caches the extracted content on the record for the file.
+ * Limits the stored file content by default to avoid hitting query size limits.
  */
 class FileTextCache_Database implements FileTextCache {
 	
@@ -28,7 +29,8 @@ class FileTextCache_Database implements FileTextCache {
 	}
 
 	public function save(File $file, $content) {
-		$file->FileContentCache = $content;
+		$maxLength = Config::inst()->get('FileTextCache_Database', 'max_content_length');
+		$file->FileContentCache = ($maxLength) ? substr($content, 0, $maxLength) : $content;
 		$file->write();
 	}
 

tests/FileTextCacheDatabaseTest.php

@@ -0,0 +1,17 @@
+<?php
+class FileTextCacheDatabaseTest extends SapphireTest {
+	
+	public function testTruncatesByMaxLength() {
+		Config::nest();
+		
+		Config::inst()->update('FileTextCache_Database', 'max_content_length', 5);
+		$cache = new FileTextCache_Database();
+		$file = $this->getMock('File', array('write'));
+		$content = '0123456789';
+		$cache->save($file, $content);
+		$this->assertEquals($cache->load($file), '01234');
+
+		Config::unnest();
+	}
+
+}