From bc882e2adf12eef92af449da4f7c0f6e117771e3 Mon Sep 17 00:00:00 2001
From: Yuchen Shi <yuchenshi@google.com>
Date: Thu, 5 Nov 2020 08:38:14 -0800
Subject: [PATCH] Work around Java emulators + WSL connectivity issues. (#2780)

---
 CHANGELOG.md                          |  1 +
 src/emulator/controller.ts            | 12 +++++++++++-
 src/emulator/downloadableEmulators.ts | 14 ++++++++++++++
 src/utils.ts                          |  8 ++++++++
 4 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e69de29bb2d..0eef5daa6ed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -0,0 +1 @@
+- Work around Java emulators + WSL connectivity issues.
diff --git a/src/emulator/controller.ts b/src/emulator/controller.ts
index c24c55857f5..fd80bbdc867 100644
--- a/src/emulator/controller.ts
+++ b/src/emulator/controller.ts
@@ -41,10 +41,20 @@ import { FLAG_EXPORT_ON_EXIT_NAME } from "./commandUtils";
 import { fileExistsSync } from "../fsutils";
 
 async function getAndCheckAddress(emulator: Emulators, options: any): Promise<Address> {
-  const host = Constants.normalizeHost(
+  let host = Constants.normalizeHost(
     options.config.get(Constants.getHostKey(emulator), Constants.getDefaultHost(emulator))
   );
 
+  if (host === "localhost" && utils.isRunningInWSL()) {
+    // HACK(https://github.com/firebase/firebase-tools-ui/issues/332): Use IPv4
+    // 127.0.0.1 instead of localhost. This, combined with the hack in
+    // downloadableEmulators.ts, forces the emulator to listen on IPv4 ONLY.
+    // The CLI (including the hub) will also consistently report 127.0.0.1,
+    // causing clients to connect via IPv4 only (which mitigates the problem of
+    // some clients resolving localhost to IPv6 and get connection refused).
+    host = "127.0.0.1";
+  }
+
   const portVal = options.config.get(Constants.getPortKey(emulator), undefined);
   let port;
   let findAvailablePort = false;
diff --git a/src/emulator/downloadableEmulators.ts b/src/emulator/downloadableEmulators.ts
index 27b0afc67f6..ffb0b791ce6 100644
--- a/src/emulator/downloadableEmulators.ts
+++ b/src/emulator/downloadableEmulators.ts
@@ -168,6 +168,20 @@ function _getCommand(
 
   const cmdLineArgs = baseCmd.args.slice();
 
+  if (
+    baseCmd.binary === "java" &&
+    utils.isRunningInWSL() &&
+    (!args.host || !args.host.includes(":"))
+  ) {
+    // HACK(https://github.com/firebase/firebase-tools-ui/issues/332): Force
+    // Java to use IPv4 sockets in WSL (unless IPv6 address explicitly used).
+    // Otherwise, Java will open a tcp6 socket (even if IPv4 address is used),
+    // which handles both 4/6 on Linux but NOT IPv4 from the host to WSL.
+    // This is a hack because it breaks all IPv6 connections as a side effect.
+    // See: https://docs.oracle.com/javase/8/docs/api/java/net/doc-files/net-properties.html
+    cmdLineArgs.unshift("-Djava.net.preferIPv4Stack=true"); // first argument
+  }
+
   const logger = EmulatorLogger.forEmulator(emulator);
   Object.keys(args).forEach((key) => {
     if (!baseCmd.optionalArgs.includes(key)) {
diff --git a/src/utils.ts b/src/utils.ts
index b448f32957c..259b460dd8b 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -478,3 +478,11 @@ export function datetimeString(d: Date): string {
 export function isCloudEnvironment() {
   return !!process.env.CODESPACES;
 }
+
+/**
+ * Indicates whether or not this process is likely to be running in WSL.
+ * @return true if we're likely in WSL, false otherwise
+ */
+export function isRunningInWSL(): boolean {
+  return !!process.env.WSL_DISTRO_NAME;
+}