fix: fix problem with observe responses without observe option (#344)

* fix: fix problem with initial observe request

* feat: add observe example using californium server

* test: add observe server response without option

* test: fix eager deregister test

examples/observe_eclipse.js

@@ -0,0 +1,29 @@
+const coap = require('../') // or coap
+
+const statusRequest = coap.request({
+    method: 'GET',
+    host: 'californium.eclipseprojects.io',
+    pathname: 'obs',
+    observe: true,
+    confirmable: true
+})
+
+let responseCounter = 0
+
+statusRequest.on('response', res => {
+    res.on('error', err => {
+        console.error('Error by receiving: ' + err)
+        this.emit('error', 'Error by receiving: ' + err)
+        res.close()
+    })
+
+    res.on('data', chunk => {
+        console.log(`Server time: ${chunk.toString()}`)
+        responseCounter++
+        if (responseCounter >= 5) {
+            console.log('Successfully received five responses. Closing the ObserveStream.')
+            res.close()
+        }
+    })
+})
+statusRequest.end()

lib/observe_read_stream.ts

@@ -33,20 +33,24 @@ export default class ObserveReadStream extends IncomingMessage {
             packetToMessage(this, packet)
         }
 
+        let observe: number
+
         if (typeof this.headers.Observe !== 'number') {
-            return
+            observe = 0
+        } else {
+            observe = this.headers.Observe
         }
 
         // First notification
         if (this._lastId === undefined) {
-            this._lastId = this.headers.Observe - 1
+            this._lastId = observe - 1
         }
 
-        const dseq = (this.headers.Observe - this._lastId) & 0xffffff
+        const dseq = (observe - this._lastId) & 0xffffff
         const dtime = Date.now() - this._lastTime
 
         if (this._disableFiltering || (dseq > 0 && dseq < (1 << 23)) || dtime > 128 * 1000) {
-            this._lastId = this.headers.Observe
+            this._lastId = observe
             this._lastTime = Date.now()
             this.push(packet.payload)
         }

test/request.ts

@@ -1257,6 +1257,14 @@ describe('request', function () {
                         return
                     }
 
+                    ssend(rsinfo, {
+                        messageId: packet.messageId,
+                        token: packet.token,
+                        payload: Buffer.from('42'),
+                        ack: true,
+                        code: '2.05'
+                    })
+
                     ssend(rsinfo, {
                         messageId: packet.messageId,
                         token: packet.token,
@@ -1382,7 +1390,7 @@ describe('request', function () {
                         return
                     }
 
-                    server.on('message', (msg, rsinfo) => {
+                    server.once('message', (msg, rsinfo) => {
                         const packet = parse(msg)
                         if (packet.ack && (packet.code === '0.00')) {
                             return