[SPARK-2350] Don't NPE while launching drivers

Prior to this change, we could throw a NPE if we launch a driver while another one is waiting, because removing from an iterator while iterating over it is not safe.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1289 from aarondav/master-fail and squashes the following commits:

1cf1cf4 [Aaron Davidson] SPARK-2350: Don't NPE while launching drivers
[SPARK-2307][Reprise] Correctly report RDD blocks on SparkUI

**Problem.** The existing code in `ExecutorPage.scala` requires a linear scan through all the blocks to filter out the uncached ones. Every refresh could be expensive if there are many blocks and many executors.

**Solution.** The proper semantics should be the following: `StorageStatusListener` should contain only block statuses that are cached. This means as soon as a block is unpersisted by any mean, its status should be removed. This is reflected in the changes made in `StorageStatusListener.scala`.

Further, the `StorageTab` must stop relying on the `StorageStatusListener` changing a dropped block's status to `StorageLevel.NONE` (which no longer happens). This is reflected in the changes made in `StorageTab.scala` and `StorageUtils.scala`.

----------

If you have been following this chain of PRs like pwendell, you will quickly notice that this reverts the changes in #1249, which reverts the changes in #1080. In other words, we are adding back the changes from #1080, and fixing SPARK-2307 on top of those changes. Please ask questions if you are confused.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1255 from andrewor14/storage-ui-fix-reprise and squashes the following commits:

45416fa [Andrew Or] Merge branch 'master' of github.com:apache/spark into storage-ui-fix-reprise
a82ea25 [Andrew Or] Add tests for StorageStatusListener
8773b01 [Andrew Or] Update comment / minor changes
3afde3f [Andrew Or] Correctly report the number of blocks on SparkUI
SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark

JIRA: https://issues.apache.org/jira/browse/SPARK-2282

This issue is caused by a buildup of sockets in the TIME_WAIT stage of TCP, which is a stage that lasts for some period of time after the communication closes.

This solution simply allows us to reuse sockets that are in TIME_WAIT, to avoid issues with the buildup of the rapid creation of these sockets.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1220 from aarondav/SPARK-2282 and squashes the following commits:

2e5cab3 [Aaron Davidson] SPARK-2282: Reuse PySpark Accumulator sockets to avoid crashing Spark
[SPARK-2059][SQL] Don't throw TreeNodeException in `execution.ExplainCommand`

This is a fix for the problem revealed by PR #1265.

Currently `HiveComparisonSuite` ignores output of `ExplainCommand` since Catalyst query plan is quite different from Hive query plan. But exceptions throw from `CheckResolution` still breaks test cases. This PR catches any `TreeNodeException` and reports it as part of the query explanation.

After merging this PR, PR #1265 can also be merged safely.

For a normal query:

```
scala> hql("explain select key from src").foreach(println)
...
[Physical execution plan:]
[HiveTableScan [key#9], (MetastoreRelation default, src, None), None]
```

For a wrong query with unresolved attribute(s):

```
scala> hql("explain select kay from src").foreach(println)
...
[Error occurred during query planning: ]
[Unresolved attributes: 'kay, tree:]
[Project ['kay]]
[ LowerCaseSchema ]
[  MetastoreRelation default, src, None]
```

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1294 from liancheng/safe-explain and squashes the following commits:

4318911 [Cheng Lian] Don't throw TreeNodeException in `execution.ExplainCommand`
[SPARK-1199][REPL] Remove VALId and use the original import style for defined classes.

This is an alternate solution to #1176.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #1179 from ScrapCodes/SPARK-1199/repl-fix-second-approach and squashes the following commits:

820b34b [Prashant Sharma] Here we generate two kinds of import wrappers based on whether it is a class or not.
Update SQLConf.scala

use concurrent.ConcurrentHashMap instead of util.Collections.synchronizedMap

Author: baishuo(白硕) <vc_java@hotmail.com>

Closes #1272 from baishuo/master and squashes the following commits:

51ec55d [baishuo(白硕)] Update SQLConf.scala
63da043 [baishuo(白硕)] Update SQLConf.scala
36b6dbd [baishuo(白硕)] Update SQLConf.scala
864faa0 [baishuo(白硕)] Update SQLConf.scala
593096b [baishuo(白硕)] Update SQLConf.scala
7304d9b [baishuo(白硕)] Update SQLConf.scala
843581c [baishuo(白硕)] Update SQLConf.scala
1d3e4a2 [baishuo(白硕)] Update SQLConf.scala
0740f28 [baishuo(白硕)] Update SQLConf.scala
[SPARK-2059][SQL] Add analysis checks

This replaces #1263 with a test case.

Author: Reynold Xin <rxin@apache.org>
Author: Michael Armbrust <michael@databricks.com>

Closes #1265 from rxin/sql-analysis-error and squashes the following commits:

a639e01 [Reynold Xin] Added a test case for unresolved attribute analysis.
7371e1b [Reynold Xin] Merge pull request #1263 from marmbrus/analysisChecks
448c088 [Michael Armbrust] Add analysis checks
[SPARK-2234][SQL]Spark SQL basicOperators  add Except operator

Hi all,
I want to submit a Except operator in basicOperators.scala
In SQL case.SQL support two table do except operator.
select * from table1
except
select * from table2
This operator support the substract function .Return an table with the elements from `this` that are not in `other`.This operator should limit the input SparkPlan Seq only has two member.The check will later support
JIRA:https://issues.apache.org/jira/browse/SPARK-2234

Author: Yanjie Gao <gaoyanjie55@163.com>
Author: YanjieGao <396154235@qq.com>
Author: root <root@node4.(none)>
Author: gaoyanjie <gaoyanjie55@163.com>

Closes #1151 from YanjieGao/patch-6 and squashes the following commits:

f19f899 [YanjieGao] add a new blank line in basicoperators.scala
2ff7d73 [YanjieGao] resolve the identation in SqlParser and SparkStrategies
fdb5227 [YanjieGao] Merge remote branch 'upstream/master' into patch-6
9940d19 [YanjieGao] make comment less than 100c
09c7413 [YanjieGao] pr 1151 SqlParser add cache ,basic Operator rename Except and modify comment
b4b5867 [root] Merge remote branch 'upstream/master' into patch-6
b4c3869 [Yanjie Gao] change SparkStrategies Sparkcontext to SqlContext
7e0ec29 [Yanjie Gao] delete multi test
7e7c83f [Yanjie Gao] delete conflict except
b01beb8 [YanjieGao] resolve conflict sparkstrategies and basicOperators
4dc8166 [YanjieGao] resolve conflict
fa68a98 [Yanjie Gao] Update joins.scala
8e6bb00 [Yanjie Gao] delete conflict except
dd9ba5e [Yanjie Gao] Update joins.scala
a0d4e73 [Yanjie Gao] delete skew join
60f5ddd [Yanjie Gao] update less than 100c
0e72233 [Yanjie Gao] update SQLQuerySuite on master branch
7f916b5 [Yanjie Gao] update execution/basicOperators on master branch
a28dece [Yanjie Gao] Update logical/basicOperators on master branch
a639935 [Yanjie Gao] Update SparkStrategies.scala
3bf7def [Yanjie Gao] update SqlParser on master branch
26f833f [Yanjie Gao] update SparkStrategies.scala on master branch
8dd063f [Yanjie Gao] Update logical/basicOperators on master branch
9847dcf [Yanjie Gao] update SqlParser on masterbranch
d6a4604 [Yanjie Gao] Update joins.scala
424c507 [Yanjie Gao] Update joins.scala
7680742 [Yanjie Gao] Update SqlParser.scala
a7193d8 [gaoyanjie] [SPARK-2234][SQL]Spark SQL basicOperators add Except operator #1151
5c8a224 [Yanjie Gao] update the line less than 100c
ee066b3 [Yanjie Gao] Update basicOperators.scala
32a80ab [Yanjie Gao] remove except in HiveQl
cf232eb [Yanjie Gao] update 1comment 2space3 left.out
f1ea3f3 [Yanjie Gao] remove comment
7ea9b91 [Yanjie Gao] remove annotation
7f3d613 [Yanjie Gao] update .map(_.copy())
670a1bb [Yanjie Gao] Update HiveQl.scala
3fe7746 [Yanjie Gao] Update SQLQuerySuite.scala
a36eb0a [Yanjie Gao] Update basicOperators.scala
7859e56 [Yanjie Gao] Update SparkStrategies.scala
052346d [Yanjie Gao] Subtract is conflict with Subtract(e1,e2)
aab3785 [Yanjie Gao] Update SQLQuerySuite.scala
4bf80b1 [Yanjie Gao] update subtract to except
4bdd520 [Yanjie Gao] Update SqlParser.scala
2d4bfbd [Yanjie Gao] Update SQLQuerySuite.scala
0808921 [Yanjie Gao] SQLQuerySuite
a8a1948 [Yanjie Gao] SparkStrategies
1fe96c0 [Yanjie Gao] HiveQl.scala update
3305e40 [Yanjie Gao] SqlParser
7a98c37 [Yanjie Gao] Update basicOperators.scala
cf5b9d0 [Yanjie Gao] Update basicOperators.scala
8945835 [Yanjie Gao] object SkewJoin extends Strategy
2b98962 [Yanjie Gao] Update SqlParser.scala
dd32980 [Yanjie Gao] update1
68815b2 [Yanjie Gao] Reformat the code style
4eb43ec [Yanjie Gao] Update basicOperators.scala
aa06072 [Yanjie Gao] Reformat the code sytle
resolve conflict and add annotation on basicOperator and remove HiveQl