DeadContext handling (#39)

* Added processing of accidental context losses

* Deleted extra middleware

* Added custom errors for more flexible development.
Now we response with not only 422 but also with 500

* Major logic of middleware:
set status and send response

helpers/exceptions.js

@@ -0,0 +1,15 @@
+exports.PageNotFoundError = class PageNotFoundError extends Error {
+    constructor(message="Page not found", ...args) {
+        super(message, ...args);
+        this.message = message;
+        this.name = "PageNotFoundError";
+    }
+}
+
+exports.ContextNotFoundError = class ContextNotFoundError extends Error {
+    constructor(message="Context not found", ...args) {
+        super(message, ...args);
+        this.message = message;
+        this.name = "ContextNotFoundError";
+    }
+}

helpers/middlewares.js

@@ -1,3 +1,5 @@
+const exceptions = require("./exceptions");
+
 exports.exceptionMiddleware = async function exceptionMiddleware(err, req, res, next) {
     if (res.headersSent) {
         return next(err);
@@ -7,14 +9,25 @@ exports.exceptionMiddleware = async function exceptionMiddleware(err, req, res,
     const pageId = err.pageId || req.query.pageId;
     const errorMessage = err.message || 'Unknown error';
 
-    res.status(500);
     if (contextId) {
         res.header('scrapy-puppeteer-service-context-id', contextId);
     }
+
+    if (err.contextId) {  // there was a context, but something went wrong
+        res.status(500);
+    } else {  // No context. Possibly, our service was restarted
+        if (err instanceof exceptions.PageNotFoundError || err instanceof exceptions.ContextNotFoundError) {
+            res.status(422);
+        } else {
+            res.status(500);
+        }
+    }
+
     res.send({
         contextId,
         pageId,
         error: errorMessage
     });
+
     next(err);
 }

helpers/utils.js

@@ -1,4 +1,6 @@
+const exceptions = require("./exceptions");
 const { proxyRequest } = require('puppeteer-proxy');
+
 const PROXY_URL_KEY = 'puppeteer-service-proxy-url'
 
 async function findContextInBrowser(browser, contextId) {
@@ -7,7 +9,7 @@ async function findContextInBrowser(browser, contextId) {
             return context;
         }
     }
-    throw new Error("Context not found");
+    throw new exceptions.ContextNotFoundError();
 }
 
 async function findPageInContext(context, pageId) {
@@ -16,7 +18,7 @@ async function findPageInContext(context, pageId) {
             return page;
         }
     }
-    throw new Error("Page not found");
+    throw new exceptions.PageNotFoundError();
 }
 
 exports.closeContexts = async function closeContexts(browser, contextIds) {