Avoid ConcurrentModificationException when getting machine map in AppInfo (#205)

- Avoid ConcurrentModificationException when `getMachines()` by deep clone the machine map
- Enhance the code to avoid potential ConcurrentModificationException
- Add test cases

sentinel-dashboard/pom.xml

@@ -99,6 +99,11 @@
             <groupId>com.alibaba</groupId>
             <artifactId>fastjson</artifactId>
         </dependency>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>

sentinel-dashboard/src/main/java/com/taobao/csp/sentinel/dashboard/discovery/AppInfo.java

@@ -15,15 +15,16 @@
  */
 package com.taobao.csp.sentinel.dashboard.discovery;
 
+import java.util.HashSet;
 import java.util.Optional;
 import java.util.Set;
-import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class AppInfo {
 
     private String app = "";
 
-    private Set<MachineInfo> machines = new TreeSet<MachineInfo>();
+    private Set<MachineInfo> machines = ConcurrentHashMap.newKeySet();
 
     public AppInfo() {
     }
@@ -40,8 +41,13 @@ public class AppInfo {
         this.app = app;
     }
 
-    public synchronized Set<MachineInfo> getMachines() {
-        return machines;
+    /**
+     * Get the current machines.
+     *
+     * @return a new copy of the current machines.
+     */
+    public Set<MachineInfo> getMachines() {
+        return new HashSet<>(machines);
     }
 
     @Override
@@ -49,7 +55,7 @@ public class AppInfo {
         return "AppInfo{" + "app='" + app + ", machines=" + machines + '}';
     }
 
-    public synchronized boolean addMachine(MachineInfo machineInfo) {
+    public boolean addMachine(MachineInfo machineInfo) {
         machines.remove(machineInfo);
         return machines.add(machineInfo);
     }

sentinel-dashboard/src/main/java/com/taobao/csp/sentinel/dashboard/discovery/MachineInfo.java

@@ -16,6 +16,7 @@
 package com.taobao.csp.sentinel.dashboard.discovery;
 
 import java.util.Date;
+import java.util.Objects;
 
 import com.alibaba.csp.sentinel.util.StringUtil;
 
@@ -117,29 +118,16 @@ public class MachineInfo implements Comparable<MachineInfo> {
 
     @Override
     public boolean equals(Object o) {
-        if (this == o) {
-            return true;
-        }
-        if (!(o instanceof MachineInfo)) {
-            return false;
-        }
-
+        if (this == o) { return true; }
+        if (!(o instanceof MachineInfo)) { return false; }
         MachineInfo that = (MachineInfo)o;
-
-        if (app != null ? !app.equals(that.app) : that.app != null) {
-            return false;
-        }
-        if (hostname != null ? !hostname.equals(that.hostname) : that.hostname != null) {
-            return false;
-        }
-        return ip != null ? ip.equals(that.ip) : that.ip == null;
+        return Objects.equals(app, that.app) &&
+            Objects.equals(ip, that.ip) &&
+            Objects.equals(port, that.port);
     }
 
     @Override
     public int hashCode() {
-        int result = app != null ? app.hashCode() : 0;
-        result = 31 * result + (hostname != null ? hostname.hashCode() : 0);
-        result = 31 * result + (ip != null ? ip.hashCode() : 0);
-        return result;
+        return Objects.hash(app, ip, port);
     }
 }

sentinel-dashboard/src/test/java/com/taobao/csp/sentinel/dashboard/discovery/AppInfoTest.java

@@ -0,0 +1,52 @@
+package com.taobao.csp.sentinel.dashboard.discovery;
+
+import java.util.ConcurrentModificationException;
+import java.util.Set;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class AppInfoTest {
+    @Test
+    public void testConcurrentGetMachines() throws Exception {
+        AppInfo appInfo = new AppInfo("testApp");
+        appInfo.addMachine(genMachineInfo("hostName1", "10.18.129.91"));
+        appInfo.addMachine(genMachineInfo("hostName2", "10.18.129.92"));
+        Set<MachineInfo> machines = appInfo.getMachines();
+        new Thread(() -> {
+            try {
+                for (MachineInfo m : machines) {
+                    System.out.println(m);
+                    try {
+                        Thread.sleep(200);
+                    } catch (InterruptedException e) {
+                    }
+                }
+            } catch (ConcurrentModificationException e) {
+                e.printStackTrace();
+                assertTrue(false);
+            }
+
+        }).start();
+        Thread.sleep(100);
+        try {
+            appInfo.addMachine(genMachineInfo("hostName3", "10.18.129.93"));
+        } catch (ConcurrentModificationException e) {
+            e.printStackTrace();
+            assertTrue(false);
+        }
+        Thread.sleep(1000);
+    }
+
+    private MachineInfo genMachineInfo(String hostName, String ip) {
+        MachineInfo machine = new MachineInfo();
+        machine.setApp("testApp");
+        machine.setHostname(hostName);
+        machine.setIp(ip);
+        machine.setPort(8719);
+        machine.setVersion(String.valueOf(System.currentTimeMillis()));
+        return machine;
+    }
+
+}