Symlink 404 cache files atomically to avoid potential race condition.

See wpsharks/comet-cache#242

quick-cache-pro/includes/advanced-cache.tpl.php

@@ -1284,8 +1284,10 @@ public function output_buffer_callback_handler($buffer, $phase)
 
 			# This is where a new 404 request might be detected for the first time; and where the 404 error file already exists in this case.
 
+			$cache_file_symlink_tmp = $this->cache_file.'.'.uniqid('', TRUE).'.tmp'; // Symlink creation is atomic; e.g. tmp file w/ rename.
+
 			if($this->is_404 && is_file($this->cache_file_404))
-				if(!symlink($this->cache_file_404, $this->cache_file))
+				if(!(symlink($this->cache_file_404, $cache_file_symlink_tmp) && rename($cache_file_symlink_tmp, $this->cache_file_404)))
 					throw new \exception(sprintf(__('Unable to create symlink: `%1$s` » `%2$s`. Possible permissions issue (or race condition), please check your cache directory: `%3$s`.', $this->text_domain), $this->cache_file, $this->cache_file_404, QUICK_CACHE_DIR));
 				else return (boolean)$this->maybe_set_debug_info($this::NC_DEBUG_1ST_TIME_404_SYMLINK);
 
@@ -1301,15 +1303,22 @@ public function output_buffer_callback_handler($buffer, $phase)
 				                                                ($this->is_404) ? '404 [error document]' : $this->salt_location, (($this->user_token) ? '; '.sprintf(__('user token: %1$s', $this->text_domain), $this->user_token) : ''), $total_time, date('M jS, Y @ g:i a T'))).' -->';
 				$cache .= "\n".'<!-- '.htmlspecialchars(sprintf(__('This Quick Cache file will auto-expire (and be rebuilt) on: %1$s (based on your configured expiration time).', $this->text_domain), date('M jS, Y @ g:i a T', strtotime('+'.QUICK_CACHE_MAX_AGE)))).' -->';
 			}
+
 			$cache_file_tmp = $this->cache_file.'.'.uniqid('', TRUE).'.tmp'; // Cache creation is atomic; e.g. tmp file w/ rename.
+
 			/*
 			 * This is NOT a 404, or it is 404 and the 404 cache file doesn't yet exist (so we need to create it).
 			 */
 			if($this->is_404) // This is a 404; let's create 404 cache file and symlink to it.
 			{
 				if(file_put_contents($cache_file_tmp, serialize($this->headers_list()).'<!--headers-->'.$cache) && rename($cache_file_tmp, $this->cache_file_404))
-					if(symlink($this->cache_file_404, $this->cache_file)) // If this fails an exception will be thrown down below.
+				{
+					$cache_file_symlink_tmp = $this->cache_file.'.'.uniqid('', TRUE).'.tmp'; // Symlink creation is atomic; e.g. tmp file w/ rename.
+					if(!(symlink($this->cache_file_404, $cache_file_symlink_tmp) && rename($cache_file_symlink_tmp, $this->cache_file)))
+						throw new \exception(sprintf(__('Unable to create symlink: `%1$s` » `%2$s`. Possible permissions issue (or race condition), please check your cache directory: `%3$s`.', $this->text_domain), $this->cache_file, $this->cache_file_404, QUICK_CACHE_DIR));
+					else
 						return $cache; // Return the newly built cache; with possible debug information also.
+				}
 
 			} // NOT a 404; let's write a new cache file.
 			else if(file_put_contents($cache_file_tmp, serialize($this->headers_list()).'<!--headers-->'.$cache) && rename($cache_file_tmp, $this->cache_file))