RFR: 8372198: Avoid closing PlainHttpConnection while holding a lock

[Daniel Jeliński]

On Thu, 20 Nov 2025 13:10:53 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

> An experimental change to SelectorManager::shutdown unveiled a potential deadlock between the SelectorManager thread trying to stop the HttpClientImpl, and an executor thread concurrently trying to return a connection to the pool.
> 
> The issue seems to be caused by the ConnectionPool::returnToPool trying to close the returned connection when stopping, while holding the ConnectionPool state lock, and the SelectorManager thread trying to close a pooled connection, holding the connection stateLock and trying to close the channel, which caused the CleanupTrigger to fire and attempt to remove the connection from the pool.
> 
> This problem was observed once with the java/net/httpclient/ThrowingSubscribersAsLimitingAsync.java test.
> 
> To avoid the problem, the proposed fix is to wait until the ConnectionPool::stateLock has been released before actually closing the connection, and to wait until the PlainHttpConnection::stateLock has been released before actually closing the channel. Indeed, there should be no need to close those while holding the lock.
> 
> This PR was recreated due to a bad merge pushed to https://github.com/openjdk/jdk/pull/28421

Marked as reviewed by djelinski (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28430#pullrequestreview-3499998719