If  wrong sql,the console print error one times。
eg：
<pre>
spark-sql> show tabless;
show tabless;
14/10/13 21:03:48 INFO ParseDriver: Parsing command: show tabless
............
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver.processCmd(SparkSQLCLIDriver.scala:274)
	at org.apache.hadoop.hive.cli.CliDriver.processLine(CliDriver.java:413)
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver$.main(SparkSQLCLIDriver.scala:209)
	at org.apache.spark.sql.hive.thriftserver.SparkSQLCLIDriver.main(SparkSQLCLIDriver.scala)
Caused by: org.apache.hadoop.hive.ql.parse.ParseException: line 1:5 cannot recognize input near 'show' 'tabless' '<EOF>' in ddl statement

	at org.apache.hadoop.hive.ql.parse.ParseDriver.parse(ParseDriver.java:193)
	at org.apache.hadoop.hive.ql.parse.ParseDriver.parse(ParseDriver.java:161)
	at org.apache.spark.sql.hive.HiveQl$.getAst(HiveQl.scala:218)
	at org.apache.spark.sql.hive.HiveQl$.createPlan(HiveQl.scala:226)
	... 47 more
Time taken: 4.35 seconds
14/10/13 21:03:51 INFO CliDriver: Time taken: 4.35 seconds
</pre>

Author: wangxiaojing <u9jing@gmail.com>

Closes #2790 from wangxiaojing/spark-3940 and squashes the following commits:

e2e5c14 [wangxiaojing] sql Print the error code three times

Some developers want to replace `Optimizer` to fit their projects but can't do so because currently `Optimizer` is an `object`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2825 from ueshin/issues/SPARK-3969 and squashes the following commits:

abbc53c [Takuya UESHIN] Re-rename Optimizer object.
4d2e1bc [Takuya UESHIN] Rename Optimizer object.
9547a23 [Takuya UESHIN] Extract abstract class from Optimizer for developers to be able to replace Optimizer.

Write properties of hive-site.xml to HiveContext when initilize session state in SparkSQLEnv.scala.

The method of SparkSQLEnv.init() in HiveThriftServer2.scala can not write the properties of hive-site.xml to HiveContext. Such as: add configuration property spark.sql.shuffle.partititions in the hive-site.xml.

Author: luogankun <luogankun@gmail.com>

Closes #2800 from luogankun/SPARK-3945 and squashes the following commits:

3679efc [luogankun] [SPARK-3945]Write properties of hive-site.xml to HiveContext when initilize session state In SparkSQLEnv.scala

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2820 from ueshin/issues/SPARK-3966 and squashes the following commits:

ca4a745 [Takuya UESHIN] Fix nullabilities of Cast related to DateType.

Author: Michael Armbrust <michael@databricks.com>

Closes #2658 from marmbrus/nestedAggs and squashes the following commits:

862b763 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into nestedAggs
3234521 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into nestedAggs
8b06fdc [Michael Armbrust] possible fix for grouping on nested fields

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

Closes #2767 from liancheng/multi-join and squashes the following commits:

9dc0d18 [Cheng Lian] Adds multiple join support for SQLContext

DecisionTree splits on continuous features by choosing an array of values from a subsample of the data.
Currently, it does not check for identical values in the subsample, so it could end up having multiple copies of the same split. In this PR, we choose splits for a continuous feature in 3 steps:

1. Sort sample values for this feature
2. Get number of occurrence of each distinct value
3. Iterate the value count array computed in step 2 to choose splits.

After find splits, `numSplits` and `numBins` in metadata will be updated.

CC: mengxr manishamde jkbradley, please help me review this, thanks.

Author: Qiping Li <liqiping1991@gmail.com>
Author: chouqin <liqiping1991@gmail.com>
Author: liqi <liqiping1991@gmail.com>
Author: qiping.lqp <qiping.lqp@alibaba-inc.com>

Closes #2780 from chouqin/dt-findsplits and squashes the following commits:

18d0301 [Qiping Li] check explicitly findsplits return distinct splits
8dc28ab [chouqin] remove blank lines
ffc920f [chouqin] adjust code based on comments and add more test cases
9857039 [chouqin] Merge branch 'master' of https://github.com/apache/spark into dt-findsplits
d353596 [qiping.lqp] fix pyspark doc test
9e64699 [Qiping Li] fix random forest unit test
3c72913 [Qiping Li] fix random forest unit test
092efcb [Qiping Li] fix bug
f69f47f [Qiping Li] fix bug
ab303a4 [Qiping Li] fix bug
af6dc97 [Qiping Li] fix bug
2a8267a [Qiping Li] fix bug
c339a61 [Qiping Li] fix bug
369f812 [Qiping Li] fix style
8f46af6 [Qiping Li] add comments and unit test
9e7138e [Qiping Li] Merge branch 'dt-findsplits' of https://github.com/chouqin/spark into dt-findsplits
1b25a35 [Qiping Li] Merge branch 'master' of https://github.com/apache/spark into dt-findsplits
0cd744a [liqi] fix bug
3652823 [Qiping Li] fix bug
af7cb79 [Qiping Li] Choose splits for continuous features in DecisionTree more adaptively

Before, if the master node is killed and restarted, the worker nodes
would not attempt to reconnect to the Master. Therefore, when the Master
node was restarted, the worker nodes needed to be restarted as well.

Now, when the Master node is disconnected, the worker nodes will
continuously ping the master node in attempts to reconnect to it. Once
the master node restarts, it will detect one of the registration
requests from its former workers. The result is that the cluster
re-enters a healthy state.

In addition, when the master does not receive a heartbeat from the
worker, the worker was removed; however, when the worker sent a
heartbeat to the master, the master used to ignore the heartbeat. Now,
a master that receives a heartbeat from a worker that had been
disconnected will request the worker to re-attempt the registration
process, at which point the worker will send a RegisterWorker request
and be re-connected accordingly.

Re-connection attempts per worker are submitted every N seconds, where N
is configured by the property spark.worker.reconnect.interval - this has
a default of 60 seconds right now.

Author: mcheah <mcheah@palantir.com>

Closes #2828 from mccheah/reconnect-dead-workers and squashes the following commits:

83f8bc9 [mcheah] [SPARK-3736] More informative log message, and fixing some indentation.
fe0e02f [mcheah] [SPARK-3736] Moving reconnection logic to registerWithMaster().
94ddeca [mcheah] [SPARK-3736] Changing a log warning to a log info.
a698e35 [mcheah] [SPARK-3736] Addressing PR comment to make some defs private.
b9a3077 [mcheah] [SPARK-3736] Addressing PR comments related to reconnection.
2ad5ed5 [mcheah] [SPARK-3736] Cancel attempts to reconnect if the master changes.
b5b34af [mcheah] [SPARK-3736] Workers reconnect when disassociated from the master.

Package names of 2 test suites are different from their directory names.
- `GeneratedEvaluationSuite`
- `GeneratedMutableEvaluationSuite`

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2835 from ueshin/issues/SPARK-3986 and squashes the following commits:

fa2cc05 [Takuya UESHIN] Fix package names to fit their directory names.

The problem caused by #1966
CC YanTangZhai andrewor14

Author: GuoQiang Li <witgo@qq.com>

Closes #2858 from witgo/SPARK-4010 and squashes the following commits:

9866fbf [GuoQiang Li] Spark UI returns 500 in yarn-client mode

Kernel 2.6.32 bug will lead to unexpected behavior of transferTo in copyStream, and this will corrupt the shuffle output file in sort-based shuffle, which will somehow introduce PARSING_ERROR(2), deserialization error or offset out of range. Here fix this by adding append flag, also add some position checking code. Details can be seen in [SPARK-3948](https://issues.apache.org/jira/browse/SPARK-3948).

Author: jerryshao <saisai.shao@intel.com>

Closes #2824 from jerryshao/SPARK-3948 and squashes the following commits:

be0533a [jerryshao] Address the comments
a82b184 [jerryshao] add configuration to control the NIO way of copying stream
e17ada2 [jerryshao] Fix kernel 2.6.32 bug led unexpected behavior of transferTo

This PR adds a Java API for AsyncRDDActions and promotes the API from `Experimental` to stable.

Author: Josh Rosen <joshrosen@apache.org>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #2760 from JoshRosen/async-rdd-actions-in-java and squashes the following commits:

0d45fbc [Josh Rosen] Whitespace fix.
ad3ae53 [Josh Rosen] Merge remote-tracking branch 'origin/master' into async-rdd-actions-in-java
c0153a5 [Josh Rosen] Remove unused variable.
e8e2867 [Josh Rosen] Updates based on Marcelo's review feedback
7a1417f [Josh Rosen] Removed unnecessary java.util import.
6f8f6ac [Josh Rosen] Fix import ordering.
ff28e49 [Josh Rosen] Add MiMa excludes and fix a scalastyle error.
346e46e [Josh Rosen] [SPARK-3902] Stabilize AsyncRDDActions; add Java API.

This patch attempts to fix SPARK-2546 in `branch-1.0` and `branch-1.1`.  The underlying problem is that thread-safety issues in Hadoop Configuration objects may cause Spark tasks to get stuck in infinite loops.  The approach taken here is to clone a new copy of the JobConf for each task rather than sharing a single copy between tasks.  Note that there are still Configuration thread-safety issues that may affect the driver, but these seem much less likely to occur in practice and will be more complex to fix (see discussion on the SPARK-2546 ticket).

This cloning is guarded by a new configuration option (`spark.hadoop.cloneConf`) and is disabled by default in order to avoid unexpected performance regressions for workloads that are unaffected by the Configuration thread-safety issues.

Author: Josh Rosen <joshrosen@apache.org>

Closes #2684 from JoshRosen/jobconf-fix-backport and squashes the following commits:

f14f259 [Josh Rosen] Add configuration option to control cloning of Hadoop JobConf.
b562451 [Josh Rosen] Remove unused jobConfCacheKey field.
dd25697 [Josh Rosen] [SPARK-2546] [1.0 / 1.1 backport] Clone JobConf for each task.

(cherry picked from commit 2cd40db2)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>

Conflicts:
	core/src/main/scala/org/apache/spark/rdd/HadoopRDD.scala

Having Python examples in Streaming Programming Guide.

Also add RecoverableNetworkWordCount example.

Author: Davies Liu <davies.liu@gmail.com>
Author: Davies Liu <davies@databricks.com>

Closes #2808 from davies/pyguide and squashes the following commits:

8d4bec4 [Davies Liu] update readme
26a7e37 [Davies Liu] fix format
3821c4d [Davies Liu] address comments, add missing file
7e4bb8a [Davies Liu] add Python examples in Streaming Programming Guide

Make JavaPairRDD.collectAsMap result Serializable since Java Maps generally are

Author: Sean Owen <sowen@cloudera.com>

Closes #2805 from srowen/SPARK-3926 and squashes the following commits:

ecb78ee [Sean Owen] Fix conflict between java.io.Serializable and use of Scala's Serializable
f4717f9 [Sean Owen] Oops, fix compile problem
ae1b36f [Sean Owen] Expand to cover Maps returned from other Java API methods as well
51c26c2 [Sean Owen] Make JavaPairRDD.collectAsMap result Serializable since Java Maps generally are

SPARK-3934: When run with a mix of unordered categorical and continuous features, on multiclass classification, RandomForest fails. The bug is in the sanity checks in getFeatureOffset and getLeftRightFeatureOffsets, which use the wrong indices for checking whether features are unordered.
Fix: Remove the sanity checks since they are not really needed, and since they would require DTStatsAggregator to keep track of an extra set of indices (for the feature subset).

Added test to RandomForestSuite which failed with old version but now works.

SPARK-3918: Added baggedInput.unpersist at end of training.

Also:
* I removed DTStatsAggregator.isUnordered since it is no longer used.
* DecisionTreeMetadata: Added logWarning when maxBins is automatically reduced.
* Updated DecisionTreeRunner to explicitly fix the test data to have the same number of features as the training data.  This is a temporary fix which should eventually be replaced by pre-indexing both datasets.
* RandomForestModel: Updated toString to print total number of nodes in forest.
* Changed Predict class to be public DeveloperApi.  This was necessary to allow users to create their own trees by hand (for testing).

CC: mengxr  manishamde chouqin codedeft  Just notifying you of these small bug fixes.

Author: Joseph K. Bradley <joseph.kurata.bradley@gmail.com>

Closes #2785 from jkbradley/dtrunner-update and squashes the following commits:

9132321 [Joseph K. Bradley] merged with master, fixed imports
9dbd000 [Joseph K. Bradley] Merge remote-tracking branch 'upstream/master' into dtrunner-update
e116473 [Joseph K. Bradley] Changed Predict class to be public DeveloperApi.
f502e65 [Joseph K. Bradley] bug fix for SPARK-3934
7f3d60f [Joseph K. Bradley] Merge remote-tracking branch 'upstream/master' into dtrunner-update
ba567ab [Joseph K. Bradley] Changed DTRunner to load test data using same number of features as in training data.
4e88c1f [Joseph K. Bradley] changed RF toString to print total number of nodes

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #2834 from adrian-wang/sqlpypath and squashes the following commits:

da7aa95 [Daoyuan Wang] fix file path using path.join

In the current implementation it was possible for the reference to change after analysis.

Author: Michael Armbrust <michael@databricks.com>

Closes #2717 from marmbrus/pythonUdfResults and squashes the following commits:

da14879 [Michael Armbrust] Fix test
6343bcb [Michael Armbrust] add test
9533286 [Michael Armbrust] Correctly preserve the result attribute of python UDFs though transformations

This avoids issues when HDFS is configured in a way that would not
allow the hardcoded default replication of "3".

Note: getDefaultReplication(Path) was added in 0.23.3, and the oldest
one available on Maven Central is 0.23.7, so I chose to not add code
to access that method via reflection.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #2831 from vanzin/SPARK-3979 and squashes the following commits:

b0e3a97 [Marcelo Vanzin] [SPARK-3979] [yarn] Use fs's default replication.

There is a unused variable(count) in saveAsHadoopDataset in PairRDDFunctions.scala. The initial idea of this variable seems to count the number of records, so I am adding a log statement to log the number of records that has been written to the writer.

Author: likun <jacky.likun@huawei.com>
Author: jackylk <jacky.likun@huawei.com>

Closes #2791 from jackylk/SPARK-3935 and squashes the following commits:

a874047 [jackylk] removing the unused variable in PairRddFunctions.scala
3bf43c7 [likun] log the number of records has been written

Its hard to debug which broadcast variables refer to what in a big codebase. Printing call site information helps in debugging.

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #2829 from shivaram/spark-broadcast-print and squashes the following commits:

cd6dbdf [Shivaram Venkataraman] Print call site information for broadcasts

JobProgressPage could not show Fair Scheduler Pools section sometimes.
SparkContext starts webui and then postEnvironmentUpdate. Sometimes JobProgressPage is accessed between webui starting and postEnvironmentUpdate, then the lazy val isFairScheduler will be false. The Fair Scheduler Pools section will not display any more.

Author: yantangzhai <tyz0303@163.com>
Author: YanTangZhai <hakeemzhai@tencent.com>

Closes #1966 from YanTangZhai/SPARK-3067 and squashes the following commits:

d4323f8 [yantangzhai] update [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
8a00106 [YanTangZhai] Merge pull request #6 from apache/master
b6391cc [yantangzhai] revert [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
d2226cd [yantangzhai] [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
cbcba66 [YanTangZhai] Merge pull request #3 from apache/master
aac7f7b [yantangzhai] [SPARK-3067] JobProgressPage could not show Fair Scheduler Pools section sometimes
cdef539 [YanTangZhai] Merge pull request #1 from apache/master

Sorry. I found that I forgot to add `afterExecute` for `handleConnectExecutor` in #2593.

Author: zsxwing <zsxwing@gmail.com>

Closes #2794 from zsxwing/SPARK-3741 and squashes the following commits:

a0bc4dd [zsxwing] Add afterExecute for handleConnectExecutor

Introduced in https://github.com/pwendell/spark/commit/f7e79bc42c1635686c3af01eef147dae92de2529, I'm not sure why we need two spark.executor.memory here.

Author: WangTaoTheTonic <barneystinson@aliyun.com>
Author: WangTao <barneystinson@aliyun.com>

Closes #2745 from WangTaoTheTonic/redundantconfig and squashes the following commits:

e7564dc [WangTao] too long line
fdbdb1f [WangTaoTheTonic] trivial workaround
d06b6e5 [WangTaoTheTonic] remove redundant spark.executor.memory in doc

In BlockManagermasterActor, _remainingMem would increase memSize for twice when updateBlockInfo if new storageLevel is invalid and old storageLevel is "useMemory". Also, _remainingMem should increase with original memory size instead of new memSize.

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #2792 from liyezhang556520/spark-3941-remainMem and squashes the following commits:

3d487cc [Zhang, Liye] make the code concise
0380a32 [Zhang, Liye] [SPARK-3941][CORE] _remainingmem should not increase twice when updateBlockInfo

Author: Kun Li <jacky.likun@gmail.com>

Closes #2809 from jackylk/patch-1 and squashes the following commits:

46c926b [Kun Li] typo in HiveFromSpark

Something about the 2.3.4 upgrade seems to have made the issue manifest where all the services disconnect from each other after exactly 1000 seconds (which is the heartbeat interval). [This post](https://groups.google.com/forum/#!topic/akka-user/X3xzpTCbEFs) suggests that heartbeat pause should be greater than heartbeat interval, and increasing the pause from 600s to 6000s seems to have rectified the issue. My current cluster has now exceeded 1400s of uptime without failure!

I do not know why this fixed it, because the threshold we have set for the failure detector is the exponent of a timeout, and 300 is extremely large. Perhaps the default failure detector changed in 2.3.4 and now ignores threshold.

Author: Aaron Davidson <aaron@databricks.com>

Closes #2784 from aarondav/fix-timeout and squashes the following commits:

bd1151a [Aaron Davidson] Increase pause, don't decrease interval
9cb0372 [Aaron Davidson] [SPARK-3923] Decrease Akka heartbeat interval below heartbeat pause

This is a small number of clean-up changes on top of #2782. Closes #2782.

Author: Prashant Sharma <prashant.s@imaginea.com>
Author: Patrick Wendell <pwendell@gmail.com>

Closes #2803 from pwendell/pr-2782 and squashes the following commits:

56d5b7a [Patrick Wendell] Minor clean-up
44089ec [Patrick Wendell] Clean-up the TaskContext API.
ed551ce [Prashant Sharma] Fixed a typo
df261d0 [Prashant Sharma] Josh's suggestion
facf3b1 [Prashant Sharma] Fixed the mima issue.
7ecc2fe [Prashant Sharma] CR, Moved implementations to TaskContextImpl
bbd9e05 [Prashant Sharma] adding missed out files to git.
ef633f5 [Prashant Sharma] SPARK-3874, Provide stable TaskContext API

The removed `Future` was used to end the test case as soon as the Spark SQL CLI process exits. When the process exits prematurely, this mechanism prevents the test case to wait until timeout. But it also creates a race condition: when `foundAllExpectedAnswers.tryFailure` is called, there are chances that the last expected output line of the CLI process hasn't been caught by the main logics of the test code, thus fails the test case.

Removing this `Future` doesn't affect correctness.

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

Closes #2823 from liancheng/clean-clisuite and squashes the following commits:

489a97c [Cheng Lian] Fixes the race condition that may cause test failure

Customized pickler should be registered before unpickling, but in executor, there is no way to register the picklers before run the tasks.

So, we need to register the picklers in the tasks itself, duplicate the javaToPython() and pythonToJava() in MLlib, call SerDe.initialize() before pickling or unpickling.

Author: Davies Liu <davies.liu@gmail.com>

Closes #2830 from davies/fix_pickle and squashes the following commits:

0c85fb9 [Davies Liu] revert the privacy change
6b94e15 [Davies Liu] use JavaConverters instead of JavaConversions
0f02050 [Davies Liu] hotfix: Customized pickler does not work in cluster

Author: Shiti <ssaxena.ece@gmail.com>

Closes #2810 from Shiti/master and squashes the following commits:

051d82f [Shiti] setting the default value of uri scheme to "file"  where matching "file" or None yields the same result

Author: prudhvi <prudhvi953@gmail.com>

Closes #2799 from prudhvije/ScalaStyle/space-after-comment-start and squashes the following commits:

fc263a1 [prudhvi] [Core] Using scalastyle to check the space after comment start

This is another implementation about #1256
cc andrewor14 vanzin

Author: GuoQiang Li <witgo@qq.com>

Closes #2379 from witgo/SPARK-2098-new and squashes the following commits:

4ef1cbd [GuoQiang Li] review commit
49ef70e [GuoQiang Li] Refactor getDefaultPropertiesFile
c45d20c [GuoQiang Li] All Spark processes should support spark-defaults.conf, config file

HT to Diana, just proposing an implementation of her suggestion, which I rather agreed with. Is there a second/third for the motion?

Refer to "self-contained" rather than "standalone" apps to avoid confusion with standalone deployment mode. And fix placement of reference to this in MLlib docs.

Author: Sean Owen <sowen@cloudera.com>

Closes #2787 from srowen/SPARK-1307 and squashes the following commits:

b5b82e2 [Sean Owen] Refer to "self-contained" rather than "standalone" apps to avoid confusion with standalone deployment mode. And fix placement of reference to this in MLlib docs.

Modified not to pollute environment variables.
Just moved the main logic into `XXX2.cmd` from `XXX.cmd`, and call `XXX2.cmd` with cmd command in `XXX.cmd`.
`pyspark.cmd` and `spark-class.cmd` are already using the same way, but `spark-shell.cmd`, `spark-submit.cmd` and `/python/docs/make.bat` are not.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2797 from tsudukim/feature/SPARK-3943 and squashes the following commits:

b397a7d [Masayoshi TSUZUKI] [SPARK-3943] Some scripts bin\*.cmd pollutes environment variables in Windows

When _JAVA_OPTIONS environment variable is set, a command "java -version" outputs a message like "Picked up _JAVA_OPTIONS: -Dfile.encoding=UTF-8".
./bin/spark-class knows java version from the first line of "java -version" output, so it mistakes java version with _JAVA_OPTIONS set.

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2725 from cocoatomo/issues/3869-mistake-java-version and squashes the following commits:

f894ebd [cocoatomo] [SPARK-3869] ./bin/spark-class miss Java version with _JAVA_OPTIONS set

Avoid overflow in computing n*(n+1)/2 as much as possible; throw explicit error when Gramian computation will fail due to negative array size; warn about large result when computing Gramian too

Author: Sean Owen <sowen@cloudera.com>

Closes #2801 from srowen/SPARK-3803 and squashes the following commits:

b4e6d92 [Sean Owen] Avoid overflow in computing n*(n+1)/2 as much as possible; throw explicit error when Gramian computation will fail due to negative array size; warn about large result when computing Gramian too

Author: shitis <ssaxena.ece@gmail.com>

Closes #2795 from Shiti/master and squashes the following commits:

46897d7 [shitis] Using Option Wrapper to convert String with value null to None

Modified to ignore not the docs/ directory, but only the docs/_build/ which is the output directory of sphinx build.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2796 from tsudukim/feature/SPARK-3946 and squashes the following commits:

2bea6a9 [Masayoshi TSUZUKI] [SPARK-3946] gitignore in /python includes wrong directory

SPARK-3178  setting SPARK_WORKER_MEMORY to a value without a label (m or g) sets the worker memory limit to zero

Validate the memory is greater than zero when set from the SPARK_WORKER_MEMORY environment variable or command line without a g or m label.  Added unit tests. If memory is 0 an IllegalStateException is thrown. Updated unit tests to mock environment variables by subclassing SparkConf (tip provided by Josh Rosen).   Updated WorkerArguments to use SparkConf.getenv instead of System.getenv for reading the SPARK_WORKER_MEMORY environment variable.

Author: Bill Bejeck <bbejeck@gmail.com>

Closes #2309 from bbejeck/spark-memory-worker and squashes the following commits:

51cf915 [Bill Bejeck] SPARK-3178 - Validate the memory is greater than zero when set from the SPARK_WORKER_MEMORY environment variable or command line without a g or m label.  Added unit tests. If memory is 0 an IllegalStateException is thrown. Updated unit tests to mock environment variables by subclassing SparkConf (tip provided by Josh Rosen).   Updated WorkerArguments to use SparkConf.getenv instead of System.getenv for reading the SPARK_WORKER_MEMORY environment variable.