Use a mutex to avoid race conditions corrupting browsers.json

The specific race here is only hit when no browers.json exists.
In that case, JBL attempts to detect & write the browser config
twice, in parallel. If that happens, and the config doesn't
have exactly the same length in both cases, the write ends up
with spurious data appended, and the config file no longer
parses. We can recover from that, but it breaks in the meantime.
With this change, it should (in theory) never happen.

Author: pimterry

package-lock.json

@@ -29,6 +29,7 @@
     "@oclif/config": "^1.8.8",
     "@oclif/plugin-help": "^2.1.3",
     "@oclif/plugin-update": "^1.3.8",
+    "async-mutex": "^0.1.3",
     "env-paths": "^1.0.0",
     "graphql": "^14.0.2",
     "graphql-yoga": "^1.16.7",

package.json

@@ -1,11 +1,15 @@
 import * as fs from 'fs';
 import * as path from 'path';
-import * as getBrowserLauncher from '@james-proxy/james-browser-launcher';
-import { LaunchOptions, BrowserInstance } from '@james-proxy/james-browser-launcher';
 import { promisify } from 'util';
 
+import { Mutex } from 'async-mutex';
+
+import * as getBrowserLauncherCb from '@james-proxy/james-browser-launcher';
+import { LaunchOptions, BrowserInstance } from '@james-proxy/james-browser-launcher';
+
 const readFile = promisify(fs.readFile);
 const deleteFile = promisify(fs.unlink);
+const getBrowserLauncher = promisify(getBrowserLauncherCb);
 
 const browserConfigPath = (configPath: string) => path.join(configPath, 'browsers.json');
 
@@ -28,8 +32,14 @@ export async function checkBrowserConfig(configPath: string) {
     });
 }
 
-async function getLauncher(configPath: string) {
-    return await promisify(getBrowserLauncher)(browserConfigPath(configPath));
+// It's not safe to call getBrowserLauncher in parallel, as config files can
+// get corrupted. We use a mutex to serialize it.
+const getLauncherMutex = new Mutex();
+
+function getLauncher(configPath: string) {
+    return getLauncherMutex.runExclusive(() =>
+        getBrowserLauncher(browserConfigPath(configPath))
+    );
 }
 
 export const getAvailableBrowsers = async (configPath: string) => {