Fix deadlock and couple more problems in DefaultConnectionPool
#699
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This commit fixes two problems in the `OpenConcurrencyLimiter`: - deadlock in `DefaultConnectionPool`; - potential removal from `OpenConcurrencyLimiter.desiredConnectionSlots` when its empty. Deadlock a) Before introducing OpenConcurrencyLimiter, "AsyncGetter" thread was used only to do blocking `ConcurrentPool.get`. b) After, I started to additionally use "AsyncGetter" to do blocking `OpenConcurrencyLimiter.waitUntilOpenPermitAvailable`. As a result, we may have a thread that gets the last connection (`maxSize` is reached) from `ConcurrentPool` and submits `waitUntilOpenPermitAvailable` to "AsyncGetter". Concurrently with this happening, the "AsyncGetter" tries to get from `ConcurrentPool` and is blocked because there are no more connections available. In such an execution, `waitUntilOpenPermitAvailable` cannot be completed by "AsyncGetter" because "AsyncGetter" is blocked doing a different task, which itself cannot be completed. a solution is to do `ConcurrentPool.get` and `waitUntilOpenPermitAvailable` in different threads. This way these two different kinds of tasks will not block each other by waiting in the same queue to be done by a single thread. Potential removal from `OpenConcurrencyLimiter.desiredConnectionSlots` when its empty. If `acquirePermitOrGetAvailableOpenedConnection` is called with `true` as `tryGetAvailable`, and `getPooledConnectionImmediately` throws an exception, then `expressDesireToGetAvailableConnection` is not called but `giveUpOnTryingToGetAvailableConnection` is still called, which is incorrect. JAVA-3928
stIncMale
commented
Apr 25, 2021
| private static Stream<Arguments> concurrentUsageArguments() { | ||
| return Stream.of( | ||
| Arguments.of(0, 1, 8, true, false, 0.02f), | ||
| Arguments.of(0, 1, 8, false, true, 0.02f), |
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Of all 5 parameter sets, only this one reproduced the deadlock.
stIncMale
commented
Apr 25, 2021
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| /** | ||
| * Package-private methods are thread-safe, | ||
| * Package-access methods are thread-safe, |
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"package-private" is not a term the JLS uses, while "package-access" is.
stIncMale
commented
Apr 25, 2021
| } finally { | ||
| try { | ||
| if (tryGetAvailable && availableConnection == null) {//the desired connection slot has not yet been removed | ||
| if (expressedDesireToGetAvailableConnection && availableConnection == null) { |
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This fixes the second problem I discovered while implementing pausable pool. getPooledConnectionImmediately there may throw an exception, which in turn may result in this code calling giveUpOnTryingToGetAvailableConnection despite expressDesireToGetAvailableConnection not being called.
This is an example of how one should try not to rely if possible on methods not throwing exceptions.
JAVA-3927
stIncMale
commented
Apr 25, 2021
| * spuriously or because of receiving a signal. | ||
| */ | ||
| private long awaitNanos(final long timeoutNanos) { | ||
| private long awaitNanos(final Condition condition, final long timeoutNanos) throws MongoInterruptedException { |
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This change makes the behavior of this method consistent with Timeout and allows to see explicitly what condition it waits for on the calling side.
stIncMale
changed the title
DefaultConnectionPoolDefaultConnectionPool
On one hand `ConcurrentPool.ensureMinSize` tries to not require the caller to do what can be done by the method itself, e.g., it releases the connection to the pool itself. On the other hand, always releasing in `ensureMinSize` may lead to releasing a permit for the same connection twice, thus not respecting the `maxSize`. It is not easy (requires an additional knob and logic) to prevent the caller (`DefaultConnectionPool`) from releasing a connection when initialization fails. It is, therefore, seems better to mandate that the caller releases a connection if initialization fails. JAVA-3927
stIncMale
changed the title
DefaultConnectionPoolDefaultConnectionPool
rozza
approved these changes
Apr 30, 2021
driver-core/src/main/com/mongodb/internal/connection/ConcurrentPool.java
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jyemin
reviewed
Apr 30, 2021
driver-core/src/main/com/mongodb/internal/connection/DefaultConnectionPool.java
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I discovered 3 problems in
DefaultConnectionPoolandConcurrentPoolintroduced by me in #685.OpenConcurrencyLimiter.desiredConnectionSlotsdespite not putting anything there previously. See this commit message for the details.ConcurrentPool.ensureMinSizeexplicitly requires a caller to release resources ifinitializethrows an exception, but then releases the permit fornewItemitself, which leads to double release. This was by far the hardest problem to investigate and find the cause. See this commit message for more details.Commits in this PR can be reviewed one by one.
JAVA-3927