Render pipeline separation of concerns (#64)

Follow-up to https://github.com/matrix-org/matrix-public-archive/pull/36

Render pipeline separation of concerns:

 1. Run in `child_process`
 2. Hydrogen render
 
It's now just a generic `child_process` runner that runs the Hydrogen render in it. This eliminates the windy path of the 1-4 steps that was only held together by the file names themselves.

public/js/entry-client.js

@@ -1,2 +1,2 @@
-import mounted from 'matrix-public-archive-shared/4-hydrogen-vm-render-script';
+import mounted from 'matrix-public-archive-shared/hydrogen-vm-render-script';
 console.log('mounted', mounted);

server/child-process-runner/child-fork-script.js

@@ -1,13 +1,11 @@
 'use strict';
 
-// Called by `child_process` `fork` in `render-hydrogen-to-string.js` so we can
-// get the data and exit the process cleanly. We don't want Hydrogen to keep
-// running after we get our initial rendered HTML.
+// Called by `child_process` `fork` in `run-in-child-process.js` so we can
+// get the data and exit the process cleanly.
 
 const assert = require('assert');
 
 const RethrownError = require('../lib/rethrown-error');
-const _renderHydrogenToStringUnsafe = require('./3-render-hydrogen-to-string-unsafe');
 
 // 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.
@@ -42,11 +40,12 @@ async function serializeError(err) {
 }
 
 // We don't exit the process after encountering one of these because maybe it
-// doesn't matter to the main render process in Hydrogen.
+// doesn't matter to the main-line process in the module.
 //
 // If we don't listen for these events, the child will exit with status code 1
 // (error) when they occur.
 process.on('uncaughtException', async (err /*, origin*/) => {
+  console.log('2 uncaughtException', err);
   await serializeError(new RethrownError('uncaughtException in child process', err));
 });
 
@@ -57,17 +56,28 @@ process.on('unhandledRejection', async (reason /*, promise*/) => {
 // Only kick everything off once we receive the options. We pass in the options
 // this way instead of argv because we will run into `Error: spawn E2BIG` and
 // `Error: spawn ENAMETOOLONG` with argv.
-process.on('message', async (renderOptions) => {
+process.on('message', async (runArguments) => {
   try {
-    const resultantHtml = await _renderHydrogenToStringUnsafe(renderOptions);
+    assert(runArguments);
 
-    assert(resultantHtml, `No HTML returned from _renderHydrogenToStringUnsafe.`);
+    // Require the module that we're supposed to run
+    const modulePath = process.argv[2];
+    assert(
+      modulePath,
+      'Expected `modulePath` to be passed into `child-fork-script.js` via argv[2]'
+    );
+    const moduleToRun = require(modulePath);
+
+    // Run the module
+    const result = await moduleToRun(runArguments);
+
+    assert(result, `No result returned from module we ran (${modulePath}).`);
 
     // Send back the data we need to the parent.
     await new Promise((resolve, reject) => {
       process.send(
         {
-          data: resultantHtml,
+          data: result,
         },
         (err) => {
           if (err) {

server/child-process-runner/run-in-child-process.js

@@ -1,22 +1,23 @@
 'use strict';
 
-// We use a child_process because we want to be able to exit the process after
-// we receive the SSR results. We don't want Hydrogen to keep running after we
-// get our initial rendered HTML.
-
-const fork = require('child_process').fork;
+// Generic `child_process` runner that handles running the given module with the
+// given `runArguments` and returning the async result. Handles the complexity
+// error handling, passing large argument objects, and timeouts.
+//
+// Error handling includes main-line errors seen while waiting the async result,
+// as well as keeping track of out of band `uncaughtException` and
+// `unhandledRejection` to give more context if the process exits with code 1
+// (error) or timesout.
 
 const assert = require('assert');
+const fork = require('child_process').fork;
+
 const RethrownError = require('../lib/rethrown-error');
 const { traceFunction } = require('../tracing/trace-utilities');
 
 const config = require('../lib/config');
 const logOutputFromChildProcesses = config.get('logOutputFromChildProcesses');
 
-// The render should be fast. If it's taking more than 5 seconds, something has
-// gone really wrong.
-const RENDER_TIMEOUT = 5000;
-
 if (!logOutputFromChildProcesses) {
   console.warn(
     `Silencing logs from child processes (config.logOutputFromChildProcesses = ${logOutputFromChildProcesses})`
@@ -60,7 +61,7 @@ function assembleErrorAfterChildExitsWithErrors(exitCode, childErrors) {
   return childErrorSummary;
 }
 
-async function renderHydrogenToString(renderOptions) {
+async function runInChildProcess(modulePath, runArguments, { timeout }) {
   let abortTimeoutId;
   try {
     let childErrors = [];
@@ -68,9 +69,10 @@ async function renderHydrogenToString(renderOptions) {
 
     const controller = new AbortController();
     const { signal } = controller;
-    // We use a child_process because we want to be able to exit the process after
-    // we receive the SSR results.
-    const child = fork(require.resolve('./2-render-hydrogen-to-string-fork-script'), [], {
+    // We use a child_process because we want to be able to exit the process
+    // after we receive the results. We use `fork` instead of `exec`/`spawn` so
+    // that we can pass a module instead of running a command.
+    const child = fork(require.resolve('./child-fork-script'), [modulePath], {
       signal,
       // Default to silencing logs from the child process. We already have
       // proper instrumentation of any errors that might occur.
@@ -92,15 +94,17 @@ async function renderHydrogenToString(renderOptions) {
       });
     }
 
-    // Pass the renderOptions to the child by sending instead of via argv because we
-    // will run into `Error: spawn E2BIG` and `Error: spawn ENAMETOOLONG` with
-    // argv.
-    child.send(renderOptions);
+    // Pass the runArguments to the child by sending instead of via argv because
+    // we will run into `Error: spawn E2BIG` and `Error: spawn ENAMETOOLONG`
+    // with argv.
+    child.send(runArguments);
 
     // Stops the child process if it takes too long
-    abortTimeoutId = setTimeout(() => {
-      controller.abort();
-    }, RENDER_TIMEOUT);
+    if (timeout) {
+      abortTimeoutId = setTimeout(() => {
+        controller.abort();
+      }, timeout);
+    }
 
     const returnedData = await new Promise((resolve, reject) => {
       let data = '';
@@ -112,8 +116,8 @@ async function renderHydrogenToString(renderOptions) {
           childError.name = result.name;
           childError.message = result.message;
           childError.stack = result.stack;
-          // When an error happens while rendering Hydrogen, we only expect one
-          // error to come through here from the main line to render Hydrogen.
+          // When an error happens while running the module, we only expect one
+          // error to come through here from the main-line to run the module.
           // But it's possible to get multiple errors from async out of context
           // places since we also listen to `uncaughtException` and
           // `unhandledRejection`.
@@ -146,7 +150,7 @@ async function renderHydrogenToString(renderOptions) {
           );
           reject(
             new RethrownError(
-              `Timed out while rendering Hydrogen to string so we aborted the child process after ${RENDER_TIMEOUT}ms. Any child errors? (${childErrors.length})`,
+              `Timed out while running ${modulePath} so we aborted the child process after ${timeout}ms. Any child errors? (${childErrors.length})`,
               childErrorSummary
             )
           );
@@ -159,19 +163,12 @@ async function renderHydrogenToString(renderOptions) {
     if (!returnedData) {
       const childErrorSummary = assembleErrorAfterChildExitsWithErrors(childExitCode, childErrors);
       throw new RethrownError(
-        `No HTML sent from child process to render Hydrogen. Any child errors? (${childErrors.length})`,
+        `No \`returnedData\` sent from child process while running the module (${modulePath}). Any child errors? (${childErrors.length})`,
         childErrorSummary
       );
     }
 
     return returnedData;
-  } catch (err) {
-    throw new RethrownError(
-      `Failed to render Hydrogen to string. In order to reproduce, feed in these arguments into \`renderHydrogenToString(...)\`:\n    renderToString arguments: ${JSON.stringify(
-        renderOptions
-      )}`,
-      err
-    );
   } finally {
     // We don't have to add a undefined/null check here because `clearTimeout`
     // works with any value you give it and doesn't throw an error.
@@ -179,4 +176,4 @@ async function renderHydrogenToString(renderOptions) {
   }
 }
 
-module.exports = traceFunction(renderHydrogenToString);
+module.exports = traceFunction(runInChildProcess);

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

@@ -18,8 +18,6 @@ const { readFile } = require('fs').promises;
 const crypto = require('crypto');
 const { parseHTML } = require('linkedom');
 
-const config = require('../lib/config');
-
 // Setup the DOM context with any necessary shims/polyfills and ensure the VM
 // context global has everything that a normal document does so Hydrogen can
 // render.
@@ -62,25 +60,16 @@ function createDomAndSetupVmContext() {
   };
 }
 
-async function _renderHydrogenToStringUnsafe({ fromTimestamp, roomData, events, stateEventMap }) {
-  assert(fromTimestamp);
-  assert(roomData);
-  assert(events);
-  assert(stateEventMap);
+async function _renderHydrogenToStringUnsafe(renderOptions) {
+  assert(renderOptions);
+  assert(renderOptions.vmRenderScriptFilePath);
+  assert(renderOptions.vmRenderContext);
 
   const { dom, vmContext } = createDomAndSetupVmContext();
 
   // Define this for the SSR context
   dom.window.matrixPublicArchiveContext = {
-    fromTimestamp,
-    roomData,
-    events,
-    stateEventMap,
-    config: {
-      basePort: config.get('basePort'),
-      basePath: config.get('basePath'),
-      matrixServerUrl: config.get('matrixServerUrl'),
-    },
+    ...renderOptions.vmRenderContext,
   };
   // Serialize it for when we run this again client-side
   dom.document.body.insertAdjacentHTML(
@@ -92,18 +81,16 @@ async function _renderHydrogenToStringUnsafe({ fromTimestamp, roomData, events,
       `
   );
 
-  const hydrogenRenderScriptCode = await readFile(
-    path.resolve(__dirname, '../../shared/4-hydrogen-vm-render-script.js'),
-    'utf8'
-  );
+  const vmRenderScriptFilePath = renderOptions.vmRenderScriptFilePath;
+  const hydrogenRenderScriptCode = await readFile(vmRenderScriptFilePath, 'utf8');
   const hydrogenRenderScript = new vm.Script(hydrogenRenderScriptCode, {
-    filename: '4-hydrogen-vm-render-script.js',
+    filename: path.basename(vmRenderScriptFilePath),
   });
   // Note: The VM does not exit after the result is returned here and is why
   // this should be run in a `child_process` that we can exit.
   const vmResult = hydrogenRenderScript.runInContext(vmContext);
   // Wait for everything to render
-  // (waiting on the promise returned from `4-hydrogen-vm-render-script.js`)
+  // (waiting on the promise returned from the VM render script)
   await vmResult;
 
   const documentString = dom.document.body.toString();

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

@@ -0,0 +1,54 @@
+'use strict';
+
+// Server-side render Hydrogen to a string.
+//
+// We use a `child_process` because we want to be able to exit the process after
+// we receive the SSR results. We don't want Hydrogen to keep running after we
+// get our initial rendered HTML.
+
+const assert = require('assert');
+const RethrownError = require('../lib/rethrown-error');
+const runInChildProcess = require('../child-process-runner/run-in-child-process');
+
+// The render should be fast. If it's taking more than 5 seconds, something has
+// gone really wrong.
+const RENDER_TIMEOUT = 5000;
+
+async function renderHydrogenToString(renderOptions) {
+  assert(renderOptions.vmRenderScriptFilePath);
+  assert(renderOptions.vmRenderContext);
+
+  try {
+    // In development, if you're running into a hard to track down error with
+    // the render hydrogen stack and fighting against the multiple layers of
+    // complexity with `child_process `and `vm`; you can get away with removing
+    // the `child_process` part of it by using
+    // `render-hydrogen-to-string-unsafe` directly.
+    // ```js
+    // const _renderHydrogenToStringUnsafe = require('../hydrogen-render/render-hydrogen-to-string-unsafe');
+    // const hydrogenHtmlOutput = await _renderHydrogenToStringUnsafe(renderOptions);
+    // ```
+    //
+    // We use a child_process because we want to be able to exit the process after
+    // we receive the SSR results. We don't want Hydrogen to keep running after we
+    // get our initial rendered HTML.
+    const hydrogenHtmlOutput = await runInChildProcess(
+      require.resolve('./render-hydrogen-to-string-unsafe'),
+      renderOptions,
+      {
+        timeout: RENDER_TIMEOUT,
+      }
+    );
+
+    return hydrogenHtmlOutput;
+  } catch (err) {
+    throw new RethrownError(
+      `Failed to render Hydrogen to string. In order to reproduce, feed in these arguments into \`renderHydrogenToString(...)\`:\n    renderHydrogenToString arguments: ${JSON.stringify(
+        renderOptions
+      )}`,
+      err
+    );
+  }
+}
+
+module.exports = renderHydrogenToString;

server/routes/install-routes.js

@@ -13,7 +13,7 @@ const timeoutMiddleware = require('./timeout-middleware');
 const fetchRoomData = require('../fetch-room-data');
 const fetchEventsInRange = require('../fetch-events-in-range');
 const ensureRoomJoined = require('../ensure-room-joined');
-const renderHydrogenToString = require('../hydrogen-render/1-render-hydrogen-to-string');
+const renderHydrogenToString = require('../hydrogen-render/render-hydrogen-to-string');
 const sanitizeHtml = require('../lib/sanitize-html');
 const safeJson = require('../lib/safe-json');
 
@@ -182,21 +182,21 @@ function installRoutes(app) {
         throw new Error('TODO: Redirect user to smaller hour range');
       }
 
-      // In development, if you're running into a hard to track down error with
-      // the render hydrogen stack and fighting against the multiple layers of
-      // complexity with `child_process `and `vm`; you can get away with removing
-      // the `child_process` part of it by using
-      // `3-render-hydrogen-to-string-unsafe` directly.
-      // ```js
-      // const _renderHydrogenToStringUnsafe = require('../hydrogen-render/3-render-hydrogen-to-string-unsafe');
-      // const hydrogenHtmlOutput = await _renderHydrogenToStringUnsafe({ /* renderData */ });
-      // ```
-      //
       const hydrogenHtmlOutput = await renderHydrogenToString({
-        fromTimestamp,
-        roomData,
-        events,
-        stateEventMap,
+        vmRenderScriptFilePath: path.resolve(
+          __dirname,
+          '../../shared/hydrogen-vm-render-script.js'
+        ),
+        vmRenderContext: {
+          fromTimestamp,
+          roomData,
+          events,
+          stateEventMap,
+          config: {
+            basePath: config.get('basePath'),
+            matrixServerUrl: config.get('matrixServerUrl'),
+          },
+        },
       });
 
       const serializableSpans = getSerializableSpans();

server/start-dev.js

@@ -19,6 +19,7 @@ build(
   })
 );
 
+// Pass through some args
 const args = [];
 if (process.argv.includes('--tracing')) {
   args.push('--tracing');