fix(#713): handle unexpected hangups in multiprocess appender

Handle the `'error'` event on client sockets of worker logger.

lib/appenders/multiprocess.js

@@ -64,8 +64,21 @@ function logServer(config, actualAppender, levels) {
       }
     }
 
+    function handleError(error) {
+      const loggingEvent = {
+        startTime: new Date(),
+        categoryName: 'log4js',
+        level: levels.ERROR,
+        data: ['A worker log process hung up unexpectedly', error],
+        remoteAddress: clientSocket.remoteAddress,
+        remotePort: clientSocket.remotePort
+      };
+      actualAppender(loggingEvent);
+    }
+
     clientSocket.on('data', chunkReceived);
     clientSocket.on('end', chunkReceived);
+    clientSocket.on('error', handleError);
   });
 
   server.listen(config.loggerPort || 5000, config.loggerHost || 'localhost', function () {

test/tap/multiprocess-shutdown-test.js

@@ -3,6 +3,7 @@
 const test = require('tap').test;
 const log4js = require('../../lib/log4js');
 const net = require('net');
+const childProcess = require('child_process');
 const sandbox = require('sandboxed-module');
 
 test('multiprocess appender shutdown (master)', { timeout: 2000 }, (t) => {
@@ -88,3 +89,48 @@ test('multiprocess appender shutdown (worker)', (t) => {
     t.end();
   }, 500);
 });
+
+test('multiprocess appender crash (worker)', (t) => {
+  const loggerPort = 12346;
+  const messages = [];
+  const fakeConsole = {
+    log: function (msg) {
+      messages.push(msg);
+    }
+  };
+  const log4jsWithFakeConsole = sandbox.require(
+    '../../lib/log4js',
+    {
+      globals: {
+        console: fakeConsole
+      }
+    }
+  );
+  log4jsWithFakeConsole.configure({
+    appenders: {
+      console: { type: 'console', layout: { type: 'messagePassThrough' } },
+      multi: {
+        type: 'multiprocess',
+        mode: 'master',
+        loggerPort: loggerPort,
+        appender: 'console'
+      }
+    },
+    categories: { default: { appenders: ['multi'], level: 'debug' } }
+  });
+
+  setTimeout(() => {
+    const worker = childProcess.fork(
+      require.resolve('./multiprocess-worker'),
+      ['start-multiprocess-worker', loggerPort]
+    );
+
+    setTimeout(() => {
+      worker.kill();
+      setTimeout(() => {
+        t.equal(messages[0], 'Logging from worker');
+        log4jsWithFakeConsole.shutdown(() => t.end());
+      }, 250);
+    }, 250);
+  }, 250);
+});

test/tap/multiprocess-test.js

@@ -236,6 +236,14 @@ test('Multiprocess Appender', (batch) => {
       assert.end();
     });
 
+    t.test('should log the error on "error" event', (assert) => {
+      net.cbs.error(new Error('Expected error'));
+      const logEvents = recording.replay();
+      assert.plan(2);
+      assert.equal(logEvents.length, 1);
+      assert.equal('A worker log process hung up unexpectedly', logEvents[0].data[0]);
+    });
+
     t.test('when a client connects', (assert) => {
       const logString = `${JSON.stringify({
         level: { level: 10000, levelStr: 'DEBUG' },

test/tap/multiprocess-worker.js

@@ -0,0 +1,11 @@
+if (process.argv.indexOf('start-multiprocess-worker') >= 0) {
+  const log4js = require('../../lib/log4js');
+  const port = parseInt(process.argv[process.argv.length - 1], 10);
+  log4js.configure({
+    appenders: {
+      multi: { type: 'multiprocess', mode: 'worker', loggerPort: port }
+    },
+    categories: { default: { appenders: ['multi'], level: 'debug' } }
+  });
+  log4js.getLogger('worker').info('Logging from worker');
+}