Some calls to HTMLConverter.convert() are not robust against rejection of the Promise

[mvanbrab]

Symptoms

• No response is returned
• The occurrence of a fragment like this on the console:

(node:4681) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 623)

Where in the code

Calls in handlers/request-handler.js and handlers/content-negotiation-handler.js, in situations that deal with a previous error. Found on the development branch:

• walder/lib/handlers/request-handler.js

  Lines 44 to 49 in 5205e36

  	} catch (e) {
  	// If something goes wrong with applying the pipe modules, send status code 500
  	console.error('Pipe module error: ' + e.message);
  	const html = await HTMLConverter.convert(htmlInfo['500'], {});
  	this.responseHandler.handle(res, 500, 'text/html')(html);
  	}

• walder/lib/handlers/request-handler.js

  Lines 65 to 71 in 5205e36

  	if (!htmlInfo[String(status)]) {
  	this.logger.warn(`No HTML template is defined for status code ${status}. Sending an empty string instead.`);
  	handleResponse('');
  	} else {
  	const html = await HTMLConverter.convert(htmlInfo[String(status)], {});
  	handleResponse(html);
  	}

  (this one contains a workaround that will work, but changing into proposed solution is more uniform)

• walder/lib/handlers/request-handler.js

  Lines 79 to 84 in 5205e36

  	} catch (err) {
  	this.logger.error(`HTML conversion failed for "${req.path}".`);
  	status = 500;
  	const html = await HTMLConverter.convert(htmlInfo[String(status)], {});
  	this.responseHandler.handle(res, status, 'text/html')(html);
  	}

• walder/lib/handlers/request-handler.js

  Lines 86 to 89 in 5205e36

  	} else {
  	const html = await HTMLConverter.convert(htmlInfo[String(status)], {});
  	this.responseHandler.handle(res, status, 'text/html')(html);
  	}

• walder/lib/handlers/content-negotiation-handler.js

  Lines 81 to 86 in 5205e36

  	catch (error) {
  	// If something goes wrong with the data reformatting, send html with error code 500
  	console.error('Content negotiation (reformatting) error: ' + error.message);
  	const html = await HtmlConverter.convert(htmlInfo['500'], {});
  	this.responseHandler.handle(res, 500, 'text/html')(html);
  	}

How to test (example, not covering every call listed)

• start walder with configuration example/config-errors.yaml
• after walder activation, make the files error*.html in example/views unavailable
• evoke the error conditions, e.g. by visiting
  http://localhost:3000/thisPageSurelyWontExist
  http://localhost:3000/bad_pipeModule
  http://localhost:3000/bad_query

Proposed solution

Do both:

1. Wrap each call in a code snippet like this one (or put this code in a wrapper function)

    let html;
    try {
      html = await HTMLConverter.convert(htmlInfo, data);
    } catch (error) {
      if (logger) {
        logger.warn('Empty HTML returned, because HTML conversion failed: ' + error.message);
      }
      html = '';
    }

2. Modify this code
   

   walder/lib/converters/html-converter.js

   Lines 39 to 46 in 5205e36

   	try {
   	fileContent = await fs.readFile(htmlInfo.file, 'utf8');
   	} catch (err) {
   	const error = new Error(`Reading the file "${htmlInfo.file}" failed.`);
   	error.type = 'IO_READING_FAILED';
   	reject(error);
   	}

   
   into this code (this avoids extra code in the code snipped above)

      try {
        fileContent = await fs.readFile(htmlInfo.file, 'utf8');
      } catch (err) {
        if (htmlInfo && htmlInfo.file) {
          const error = new Error(`Reading the file "${htmlInfo.file ? htmlInfo.file : "(undefined)"}" failed.`);
          error.type = 'IO_READING_FAILED';
          reject(error);
        } else {
          reject(new Error(`Not given: htmlInfo.file.`));
        }
        return;
      }