[KYUUBI #1472] allow the engine not to self-terminate.

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### _Why are the changes needed?_
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Allow the engine not to self-terminate.  For details: #1472.

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [X] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #1475 from wForget/KYUUBI-1472.

Closes #1472

97412e5 [Wang Zhen] Merge remote-tracking branch 'remotes/origin/master'
8ff0cf7 [Wang Zhen] [KYUUBI-1472] allow the engine not to self-terminate. Use zero or negative to represent no limit.
7cfa03c [Wang Zhen] [KYUUBI-1472] allow the engine not to self-terminate.

Authored-by: Wang Zhen <wangzhen07@qiyi.com>
Signed-off-by: ulysses-you <ulyssesyou@apache.org>

Author: wForget

docs/deployment/settings.md

@@ -298,7 +298,7 @@ kyuubi\.session\.check<br>\.interval|<div style='width: 65pt;word-wrap: break-wo
 kyuubi\.session\.conf<br>\.ignore\.list|<div style='width: 65pt;word-wrap: break-word;white-space: normal'></div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>A comma separated list of ignored keys. If the client connection contains any of them, the key and the corresponding value will be removed silently during engine bootstrap and connection setup. Note that this rule is for server-side protection defined via administrators to prevent some essential configs from tampering but will not forbid users to set dynamic configurations via SET syntax.</div>|<div style='width: 30pt'>seq</div>|<div style='width: 20pt'>1.2.0</div>
 kyuubi\.session\.conf<br>\.restrict\.list|<div style='width: 65pt;word-wrap: break-word;white-space: normal'></div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>A comma separated list of restricted keys. If the client connection contains any of them, the connection will be rejected explicitly during engine bootstrap and connection setup. Note that this rule is for server-side protection defined via administrators to prevent some essential configs from tampering but will not forbid users to set dynamic configurations via SET syntax.</div>|<div style='width: 30pt'>seq</div>|<div style='width: 20pt'>1.2.0</div>
 kyuubi\.session\.engine<br>\.check\.interval|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>PT5M</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>The check interval for engine timeout</div>|<div style='width: 30pt'>duration</div>|<div style='width: 20pt'>1.0.0</div>
-kyuubi\.session\.engine<br>\.idle\.timeout|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>PT30M</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>engine timeout, the engine will self-terminate when it's not accessed for this duration</div>|<div style='width: 30pt'>duration</div>|<div style='width: 20pt'>1.0.0</div>
+kyuubi\.session\.engine<br>\.idle\.timeout|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>PT30M</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>engine timeout, the engine will self-terminate when it's not accessed for this duration. 0 or negative means not to self-terminate.</div>|<div style='width: 30pt'>duration</div>|<div style='width: 20pt'>1.0.0</div>
 kyuubi\.session\.engine<br>\.initialize\.timeout|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>PT3M</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>Timeout for starting the background engine, e.g. SparkSQLEngine.</div>|<div style='width: 30pt'>duration</div>|<div style='width: 20pt'>1.0.0</div>
 kyuubi\.session\.engine<br>\.launch\.async|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>false</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>When opening kyuubi session, whether to launch backend engine asynchronously. When true, the Kyuubi server will set up the connection with the client without delay as the backend engine will be created asynchronously.</div>|<div style='width: 30pt'>boolean</div>|<div style='width: 20pt'>1.4.0</div>
 kyuubi\.session\.engine<br>\.log\.timeout|<div style='width: 65pt;word-wrap: break-word;white-space: normal'>PT24H</div>|<div style='width: 170pt;word-wrap: break-word;white-space: normal'>If we use Spark as the engine then the session submit log is the console output of spark-submit. We will retain the session submit log until over the config value.</div>|<div style='width: 30pt'>duration</div>|<div style='width: 20pt'>1.1.0</div>

kyuubi-common/src/main/scala/org/apache/kyuubi/config/KyuubiConf.scala

@@ -623,7 +623,8 @@ object KyuubiConf {
     .createWithDefault(Duration.ofMinutes(5).toMillis)
 
   val ENGINE_IDLE_TIMEOUT: ConfigEntry[Long] = buildConf("session.engine.idle.timeout")
-    .doc("engine timeout, the engine will self-terminate when it's not accessed for this duration")
+    .doc("engine timeout, the engine will self-terminate when it's not accessed for this " +
+      "duration. 0 or negative means not to self-terminate.")
     .version("1.0.0")
     .timeConf
     .createWithDefault(Duration.ofMinutes(30L).toMillis)

kyuubi-common/src/main/scala/org/apache/kyuubi/session/SessionManager.scala

@@ -270,15 +270,17 @@ abstract class SessionManager(name: String) extends CompositeService(name) {
     _latestLogoutTime = System.currentTimeMillis()
     val interval = conf.get(ENGINE_CHECK_INTERVAL)
     val idleTimeout = conf.get(ENGINE_IDLE_TIMEOUT)
-    val checkTask = new Runnable {
-      override def run(): Unit = {
-        if (!shutdown &&
-          System.currentTimeMillis() - latestLogoutTime > idleTimeout && getOpenSessionCount <= 0) {
-          info(s"Idled for more than $idleTimeout ms, terminating")
-          stop()
+    if (idleTimeout > 0) {
+      val checkTask = new Runnable {
+        override def run(): Unit = {
+          if (!shutdown && System.currentTimeMillis() - latestLogoutTime > idleTimeout &&
+            getOpenSessionCount <= 0) {
+            info(s"Idled for more than $idleTimeout ms, terminating")
+            stop()
+          }
         }
       }
+      timeoutChecker.scheduleWithFixedDelay(checkTask, interval, interval, TimeUnit.MILLISECONDS)
     }
-    timeoutChecker.scheduleWithFixedDelay(checkTask, interval, interval, TimeUnit.MILLISECONDS)
   }
 }