Detect existing Oasis server running on interface

Problem: Sometimes you try to run Oasis twice and the second time you
run the program it blows up in your face about the port being taken.
Most other software just focuses or reopens the window instead of
throwing errors at you.

Solution: Implement `/.well-known/oasis` as a sanity check to see
whether the thing listening on the HTTP port. If it replies "oasis" then
we can open the tab (unless `config.open === false`) and avoid throwing
the error.

src/index.js

@@ -7,12 +7,21 @@
 const fs = require("fs");
 const envPaths = require("env-paths");
 const path = require("path");
+const nodeHttp = require("http");
+const debug = require("debug")("oasis");
 
 const defaultConfigFile = path.join(
   envPaths("oasis", { suffix: "" }).config,
   "/default.json"
 );
 
+const log = (...args) => {
+  const isDebugEnabled = debug.enabled;
+  debug.enabled = true;
+  debug(...args);
+  debug.enabled = isDebugEnabled;
+};
+
 const defaultConfig = {};
 var haveConfig;
 
@@ -36,6 +45,10 @@ const config = cli(defaultConfig, defaultConfigFile);
 delete config._;
 delete config.$0;
 
+const { host } = config;
+const { port } = config;
+const url = `http://${host}:${port}`;
+
 console.log();
 if (haveConfig) {
   console.log(`Configuration read defaults from ${defaultConfigFile}`);
@@ -57,13 +70,31 @@ if (config.debug) {
   process.env.DEBUG = "oasis,oasis:*";
 }
 
+const oasisCheckPath = "/.well-known/oasis";
+
 process.on("uncaughtException", function(err) {
   // This isn't `err.code` because TypeScript doesn't like that.
   if (err["code"] === "EADDRINUSE") {
-    const url = `http://${config.host}:${config.port}`;
-
-    throw new Error(
-      `Another server is already running at ${url}.
+    nodeHttp.get(url + oasisCheckPath, res => {
+      let rawData = "";
+      res.on("data", chunk => {
+        rawData += chunk;
+      });
+      res.on("end", () => {
+        log(rawData);
+        if (rawData === "oasis") {
+          log(`Oasis is already running on host ${host} and port ${port}`);
+          if (config.open === true) {
+            log("Opening link to existing instance of Oasis");
+            open(url);
+          } else {
+            log(
+              "Not opening your browser because opening is disabled by your config"
+            );
+          }
+          process.exit(0);
+        } else {
+          throw new Error(`Another server is already running at ${url}.
 It might be another copy of Oasis or another program on your computer.
 You can run Oasis on a different port number with this option:
 
@@ -74,8 +105,10 @@ Alternatively, you can set the default port in ${defaultConfigFile} with:
     {
       "port": ${config.port + 1}
     }
-`
-    );
+`);
+        }
+      });
+    });
   } else {
     throw err;
   }
@@ -91,7 +124,6 @@ process.argv = [];
 
 const http = require("./http");
 
-const debug = require("debug")("oasis");
 const koaBody = require("koa-body");
 const { nav, ul, li, a } = require("hyperaxe");
 const open = require("open");
@@ -184,6 +216,9 @@ router
   .get("/robots.txt", ctx => {
     ctx.body = "User-agent: *\nDisallow: /";
   })
+  .get(oasisCheckPath, ctx => {
+    ctx.body = "oasis";
+  })
   .get("/public/popular/:period", async ctx => {
     const { period } = ctx.params;
     const publicPopular = async ({ period }) => {
@@ -690,9 +725,6 @@ router
     ctx.redirect("/settings");
   });
 
-const { host } = config;
-const { port } = config;
-
 const routes = router.routes();
 
 const middleware = [
@@ -714,13 +746,8 @@ const middleware = [
 
 http({ host, port, middleware });
 
-const uri = `http://${host}:${port}/`;
-
-const isDebugEnabled = debug.enabled;
-debug.enabled = true;
-debug(`Listening on ${uri}`);
-debug.enabled = isDebugEnabled;
+log(`Listening on ${url}`);
 
 if (config.open === true) {
-  open(uri);
+  open(url);
 }