Add synchronization to createPlan

[SemionPar]

Description

createPlan incremented concurrentQueries without acquiring the lock

Additional context and related issues

Fixes #23582

Before the change:

After the change:

Release notes

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## Section
* Fix some things. ({issue}`issuenumber`)

[martint]

What is the purpose of this lock?

concurrentQueries is an AtomicInteger, so it's safe to update outside of a lock (plus, this is a read lock, so it wouldn't protect from concurrent modifications anyway)

[SemionPar]

concurrentQueries is an AtomicInteger, so it's safe to update outside of a lock

True to a T! Please note, however, that

spansValid = concurrentQueries.incrementAndGet() == 1;

is a compound operation which is not atomic, but even this isn't the root cause.

What I believe was happening here (and I am talking about flakiness of BaseBigQueryConnectorTest.testBigQueryMaterializedView which we see when the whole class is ran concurrently) is this:

testBigQueryMaterializedView(){
  executeExclusively(...) // <----- gets write lock and executes two queries sequentially, each time calling getSpans
}

// in some other thread, concurrently
testSomeOtherThing(){
  QueryAssert.someAssertCallingCreatePlan(...)  // <---- without locking, it goes ahead and sets spansValid == false
}

Adding the lock here means someAssertCallingCreatePlan won't start until executeExclusively releases the write lock - I assume this is the intended behavior. Why a read lock and not a write lock? TBH I wasn't sure myself, but not looking far, there is executeInternal() which does use read lock in the same way.

[martint]

Ok, so the lock is to prevent running concurrently when executeExclusively, not for protecting access to the concurrentQueries counter. Please change the description in the PR and add an explanation in the commit message explaining this.