Fix a deadlock situation for BlockOnSplit test case. by shunping · Pull Request #36226 · apache/beam

shunping · 2025-09-21T04:00:15Z

A deadlock can occur in the TestDynamicSplit/BlockOnSplit test case due to a race condition between two go routines, the processPlan and splitPlan goroutines:

processPlan - This is the routine where ProcessElement, GetRestriction and TryClaim are called.

beam/sdks/go/pkg/beam/core/runtime/exec/dynsplit_test.go

Line 442 in 65dfd30

func (fn *splitTestSdf) ProcessElement(rt *splitTestRTracker, _ int, emit func(offsetrange.Restriction, int)) {
splitPlan - This is the routine where TrySplit is called.

The splitBlockingDriver uses channels for synchronization.

beam/sdks/go/pkg/beam/core/runtime/exec/dynsplit_test.go

Lines 178 to 182 in 65dfd30

    
           <-rt.split 
        
           // Start processing and start a claim, that'll be waiting for the mutex. 
        
           <-sdf.proc 
        
           <-rt.claim

However, a specific execution order can lead to a deadlock:

Driver executes and starts processPlan and splitPlan.
Driver waits for a signal on the sdf.proc channel from ProcessElement in processPlan routine
TrySplit in splitPlan routine executes first, acquires the rt.mu lock and then blocks while waiting on the rt.blocksplit channel.

beam/sdks/go/pkg/beam/core/runtime/exec/dynsplit_test.go

Lines 399 to 401 in 65dfd30

rt.mu.Lock()

defer rt.mu.Unlock()

rt.blockSplit <- struct{}{}
ProcessElement in processPlan routine executes GetRestriction where it tries to acquire rt.mu but it is already-held.

beam/sdks/go/pkg/beam/core/runtime/exec/dynsplit_test.go

Line 422 in 65dfd30

rt.mu.Lock()
Unfortunately, ProcessElement can only send the signal to the sdf.proc channel after GetRestriction is finished.

beam/sdks/go/pkg/beam/core/runtime/exec/dynsplit_test.go

Lines 443 to 444 in 65dfd30

i := rt.GetRestriction().(offsetrange.Restriction).Start

fn.proc <- struct{}{}

This creates a circular dependency and a deadlock. This change resolves the issue by moving the signal on rt.blockSplit to before the mutex is locked, preventing the deadlock.

fixes #36224

gemini-code-assist · 2025-09-21T04:00:27Z

Summary of Changes

Hello @shunping, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical deadlock scenario identified in the TestDynamicSplit/BlockOnSplit test case. The deadlock stemmed from a specific timing issue where two goroutines, processPlan and splitPlan, contended for a shared mutex while waiting on each other's signals. By carefully reordering the mutex acquisition and channel signaling within the splitTestRTracker's TryClaim and TrySplit methods, this change eliminates the circular dependency, ensuring the test case can execute reliably without encountering the previously observed deadlock.

Highlights

Deadlock Resolution: Addresses a deadlock in the TestDynamicSplit/BlockOnSplit test case caused by a race condition between processPlan and splitPlan goroutines. The issue arose when TrySplit acquired a mutex and blocked, while ProcessElement simultaneously attempted to acquire the same mutex, leading to a circular dependency.
Synchronization Logic Adjustment: The fix involves reordering operations within the splitTestRTracker's TryClaim and TrySplit methods. Specifically, the rt.blockSplit signal is now sent before acquiring the rt.mu mutex, which effectively breaks the circular dependency and prevents the deadlock.

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github-actions · 2025-09-21T04:37:31Z

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LGTM

shunping · 2025-09-23T03:18:44Z

FYI, I ran the test 1000x with race detector and could not see a stuck test.

go test -v -race -count=1000 github.com/apache/beam/sdks/v2/go/pkg/beam/core/runtime/exec -run TestDynamicSplit/BlockOnSplit

Previously, the test would get stuck when running 10x with race detector.

Fix a deadlock situation for BlockOnSplit test case.

450bfbf

github-actions Bot added the go label Sep 21, 2025

shunping self-assigned this Sep 21, 2025

github-actions Bot added the Next Action: Reviewers label Sep 21, 2025

jrmccluskey approved these changes Sep 22, 2025

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jrmccluskey merged commit 9146029 into apache:master Sep 22, 2025
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Fix a deadlock situation for BlockOnSplit test case. - #36226

Fix a deadlock situation for BlockOnSplit test case.#36226
jrmccluskey merged 1 commit into
apache:masterfrom
shunping:issue-36224

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	<-rt.split

	// Start processing and start a claim, that'll be waiting for the mutex.
	<-sdf.proc
	<-rt.claim

	i := rt.GetRestriction().(offsetrange.Restriction).Start
	fn.proc <- struct{}{}

	rt.mu.Lock()
	defer rt.mu.Unlock()
	rt.blockSplit <- struct{}{}

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shunping commented Sep 21, 2025

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