#### `EventReceiverFirehoseFactory`
Fixed several concurrency bugs in `EventReceiverFirehoseFactory`:
- Race condition over putting an entry into `producerSequences` in `checkProducerSequence()`.
- `Stopwatch` used to measure time across threads, but it's a non-thread-safe class.
- Use `System.nanoTime()` instead of `System.currentTimeMillis()` because the latter are [not suitable](https://stackoverflow.com/a/351571/648955) for measuring time intervals.
- `close()` was not synchronized by could be called from multiple threads concurrently.
Removed unnecessary `readLock` (protecting `hasMore()` and `nextRow()` which are always called from a single thread). Removed unnecessary `volatile` modifiers.
Documented threading model and concurrent control flow of `EventReceiverFirehose` instances.
**Important:** please read the updated Javadoc for `EventReceiverFirehose.addAll()`. It allows events from different requests (batches) to be interleaved in the buffer. Is this OK?
#### `TimedShutoffFirehoseFactory`
- Fixed a race condition that was possible because `close()` that was not properly synchronized.
Documented threading model and concurrent control flow of `TimedShutoffFirehose` instances.
#### `Firehose`
Refined concurrency contract of `Firehose` based on `EventReceiverFirehose` implementation. Importantly, now it states that `close()` doesn't affect `hasMore()` and `nextRow()` and could be called concurrently with them. In other words, specified that `close()` is for "row supply" side rather than "row consume" side. However, I didn't check that other `Firehose` implementatations adhere to this contract.
<hr>
This issue is the result of reviewing `EventReceiverFirehose` and `TimedShutoffFirehose` using [this checklist](https://medium.com/@leventov/code-review-checklist-java-concurrency-49398c326154).
* Prohibit assigning concurrent maps into Map-types variables and fields; Fix a race condition in CoordinatorRuleManager; improve logic in DirectDruidClient and ResourcePool
* Enforce that if compute(), computeIfAbsent(), computeIfPresent() or merge() is called on a ConcurrentHashMap, it's stored in a ConcurrentHashMap-typed variable, not ConcurrentMap; add comments explaining get()-before-computeIfAbsent() optimization; refactor Counters; fix a race condition in Intialization.java
* Remove unnecessary comment
* Checkstyle
* Fix getFromExtensions()
* Add a reference to the comment about guarded computeIfAbsent() optimization; IdentityHashMap optimization
* Fix UriCacheGeneratorTest
* Workaround issue with MaterializedViewQueryQueryToolChest
* Strengthen Appenderator's contract regarding concurrency
Not putting this to 0.13 milestone because the found bugs are not critical (one is a harmless DI config duplicate, and another is in a benchmark.
Change in `DumpSegment` is just an indentation change.
* Rename io.druid to org.apache.druid.
* Fix META-INF files and remove some benchmark results.
* MonitorsConfig update for metrics package migration.
* Reorder some dimensions in inner queries for some reason.
* Fix protobuf tests.
* Various changes about druid-services module
* Patch improvements from reviewer
* Add ToArrayCallWithZeroLengthArrayArgument & ArraysAsListWithZeroOrOneArgument into inspection profile
* Fix ArraysAsListWithZeroOrOneArgument
* Fix conflict
* Fix ToArrayCallWithZeroLengthArrayArgument
* Fix AliEqualsAvoidNull
* Remove blank line
* Remove unused import clauses
* Fix code style in TopNQueryRunnerTest
* Fix conflict
* Don't use Collections.singletonList when converting the type of array type
* Add argLine into maven-surefire-plugin in druid-process module & increase the timeout value for testMoveSegment testcase
* Roll back the latest commit
* Add java.io.File#toURL() into druid-forbidden-apis
* Using Boolean.parseBoolean instead of Boolean.valueOf for CliCoordinator#isOverlord
* Add a new regexp element into stylecode xml file
* Fix style error for new regexp
* Set the level of ArraysAsListWithZeroOrOneArgument as WARNING
* Fix style error for new regexp
* Add option BY_LEVEL for ToArrayCallWithZeroLengthArrayArgument in inspection profile
* Roll back the level as ToArrayCallWithZeroLengthArrayArgument as ERROR
* Add toArray(new Object[0]) regexp into checkstyle config file & fix them
* Set the level of ArraysAsListWithZeroOrOneArgument as ERROR & Roll back the level of ToArrayCallWithZeroLengthArrayArgument as WARNING until Youtrack fix it
* Add a comment for string equals regexp in checkstyle config
* Fix code format
* Add RedundantTypeArguments as ERROR level inspection
* Fix cannot resolve symbol datasource
* Remove some unnecessary use of boxed types.
* Fix some incorrect format strings.
* Enable IDEA's MalformedFormatString inspection.
* Add a Checkstyle check for finding uses of incorrect logging packages.
* Fix some incorrect usages of the metamx logger.
* Bypass incorrect logger Checkstyle check where using the correct logger is not simple.
* Fix some more places where the wrong number of arguments are provided to format strings.
* Suppress `MalformedFormatString` inspection on legacy logging test.
* Use @SuppressWarnings rather than a noinspection suppression comment.
* Fix some more incorrect format strings.
* Suppress some more incorrect format string warnings where the incorrect string is intentional.
* Log the aggregator when closing it fails.
* Remove some unneeded log lines.
* Enable most IntelliJ 'Probable bugs' inspections
* Fix in RemoteTestNG
* Fix IndexSpec's equals() and hashCode() to include longEncoding
* Fix inspection errors
* Extract global isntance of natural().nullsFirst(); address comments
* Fix
* Use noinspection comments instead of SuppressWarnings on method for IntelliJ-specific inspections
* Prohibit Ordering.natural().nullsFirst() using Checkstyle
Roman Leventov
Benedict Jin
Gian Merlino
Benedict Jin
Jihoon Son
Chris Gavin