Make sure we finish sending the HTML payload before we exit the process (#38)

I encountered a page which responded successfully but all of the Hydrogen HTML was missing. It just had the boilerplate around it.

What I am guessing happened is that since `process.send` is async, with a sufficiently large
payload and race condition, `process.exit(0)` was being called before it finished sending.

Related:
 - https://stackoverflow.com/questions/34627546/process-send-is-sync-async-on-nix-windows
 - 56d9584a0e
 - https://github.com/nodejs/node/issues/6767

server/hydrogen-render/1-render-hydrogen-to-string.js

@@ -6,6 +6,7 @@
 
 const fork = require('child_process').fork;
 
+const assert = require('assert');
 const RethrownError = require('../lib/rethrown-error');
 
 // The render should be fast. If it's taking more than 5 seconds, something has
@@ -60,12 +61,16 @@ async function renderHydrogenToString(options) {
         } else {
           let extraErrorsMessage = '';
           if (childErrors.length > 1) {
-            extraErrorsMessage = ` (somehow we saw ${childErrors.length} errors but we really always expect 1 error)`;
+            extraErrorsMessage = ` (somehow we saw ${
+              childErrors.length
+            } errors but we really always expect 1 error)\n${childErrors
+              .map((childError, index) => ` ${index}. ${childError.message} ${childError.stack}`)
+              .join('\n')}`;
           }
 
           const error = new RethrownError(
             `Child process failed with exit code ${exitCode}${extraErrorsMessage}`,
-            childErrors[0]
+            childErrors[0] || new Error('No child errors')
           );
           reject(error);
         }
@@ -84,6 +89,11 @@ async function renderHydrogenToString(options) {
       });
     });
 
+    assert(
+      data,
+      `No HTML sent from child process to render Hydrogen. Any child errors? (${childErrors.length})`
+    );
+
     return data;
   } catch (err) {
     throw new RethrownError(

server/hydrogen-render/2-render-hydrogen-to-string-fork-script.js

@@ -4,6 +4,8 @@
 // get the data and exit the process cleanly. We don't want Hydrogen to keep
 // running after we get our initial rendered HTML.
 
+const assert = require('assert');
+
 const _renderHydrogenToStringUnsafe = require('./3-render-hydrogen-to-string-unsafe');
 
 // Only kick everything off once we receive the options. We pass in the options
@@ -13,12 +15,31 @@ process.on('message', async (options) => {
   try {
     const resultantHtml = await _renderHydrogenToStringUnsafe(options);
 
-    // Send back the data we need
+    assert(resultantHtml, `No HTML returned from _renderHydrogenToStringUnsafe.`);
-    process.send({
+
+    // Send back the data we need to the parent.
+    await new Promise((resolve, reject) => {
+      process.send(
+        {
           data: resultantHtml,
-    });
+        },
-    // End the process gracefully. We got all the data we need.
+        (err) => {
+          if (err) {
+            return reject(err);
+          }
+
+          // Exit once we know the data was sent out. We can't gurantee the
+          // message was received but this should work pretty well.
+          //
+          // Related:
+          //  - https://stackoverflow.com/questions/34627546/process-send-is-sync-async-on-nix-windows
+          //  - https://github.com/nodejs/node/commit/56d9584a0ead78874ca9d4de2e55b41c4056e502
+          //  - https://github.com/nodejs/node/issues/6767
           process.exit(0);
+          resolve();
+        }
+      );
+    });
   } catch (err) {
     // Serialize the error and send it back up to the parent process so we can
     // interact with it and know what happened when the process exits.

server/hydrogen-render/3-render-hydrogen-to-string-unsafe.js

@@ -94,6 +94,7 @@ async function _renderHydrogenToStringUnsafe({ fromTimestamp, roomData, events,
   await vmResult;
 
   const documentString = dom.document.body.toString();
+  assert(documentString, 'Document body should not be empty after we rendered Hydrogen');
   return documentString;
 }