[SPARK-4037][SQL] Removes the SessionState instance created in HiveThriftServer2

`HiveThriftServer2` creates a global singleton `SessionState` instance and overrides `HiveContext` to inject the `SessionState` object. This messes up `SessionState` initialization and causes problems.

This PR replaces the global `SessionState` with `HiveContext.sessionState` to avoid the initialization conflict. Also `HiveContext` reuses existing started `SessionState` if any (this is required by `SparkSQLCLIDriver`, which uses specialized `CliSessionState`).

Author: Cheng Lian <lian@databricks.com>

Closes #2887 from liancheng/spark-4037 and squashes the following commits:

8446675 [Cheng Lian] Removes redundant Driver initialization
a28fef5 [Cheng Lian] Avoid starting HiveContext.sessionState multiple times
49b1c5b [Cheng Lian] Reuses existing started SessionState if any
3cd6fab [Cheng Lian] Fixes SPARK-4037

1 files changed, 30 insertions, 14 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
index fad4091d48..ff8fa44194 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala
@@ -224,21 +224,29 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
   }
 
   /**
-   * SQLConf and HiveConf contracts: when the hive session is first initialized, params in
-   * HiveConf will get picked up by the SQLConf.  Additionally, any properties set by
-   * set() or a SET command inside sql() will be set in the SQLConf *as well as*
-   * in the HiveConf.
+   * SQLConf and HiveConf contracts:
+   *
+   * 1. reuse existing started SessionState if any
+   * 2. when the Hive session is first initialized, params in HiveConf will get picked up by the
+   *    SQLConf.  Additionally, any properties set by set() or a SET command inside sql() will be
+   *    set in the SQLConf *as well as* in the HiveConf.
    */
-  @transient lazy val hiveconf = new HiveConf(classOf[SessionState])
-  @transient protected[hive] lazy val sessionState = {
-    val ss = new SessionState(hiveconf)
-    setConf(hiveconf.getAllProperties)  // Have SQLConf pick up the initial set of HiveConf.
-    SessionState.start(ss)
-    ss.err = new PrintStream(outputBuffer, true, "UTF-8")
-    ss.out = new PrintStream(outputBuffer, true, "UTF-8")
-
-    ss
-  }
+  @transient protected[hive] lazy val (hiveconf, sessionState) =
+    Option(SessionState.get())
+      .orElse {
+        val newState = new SessionState(new HiveConf(classOf[SessionState]))
+        // Only starts newly created `SessionState` instance.  Any existing `SessionState` instance
+        // returned by `SessionState.get()` must be the most recently started one.
+        SessionState.start(newState)
+        Some(newState)
+      }
+      .map { state =>
+        setConf(state.getConf.getAllProperties)
+        if (state.out == null) state.out = new PrintStream(outputBuffer, true, "UTF-8")
+        if (state.err == null) state.err = new PrintStream(outputBuffer, true, "UTF-8")
+        (state.getConf, state)
+      }
+      .get
 
   override def setConf(key: String, value: String): Unit = {
     super.setConf(key, value)
@@ -288,6 +296,14 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
       val cmd_1: String = cmd_trimmed.substring(tokens(0).length()).trim()
       val proc: CommandProcessor = HiveShim.getCommandProcessor(Array(tokens(0)), hiveconf)
 
+      // Makes sure the session represented by the `sessionState` field is activated. This implies
+      // Spark SQL Hive support uses a single `SessionState` for all Hive operations and breaks
+      // session isolation under multi-user scenarios (i.e. HiveThriftServer2).
+      // TODO Fix session isolation
+      if (SessionState.get() != sessionState) {
+        SessionState.start(sessionState)
+      }
+
       proc match {
         case driver: Driver =>
           val results = HiveShim.createDriverResultsArray