[Prism] Fix panic in teststream

[shunping]

I have seen the following panic in one of the precommit test logs ( https://github.com/apache/beam/actions/runs/17888682703/job/50866217521?pr=36226).

 0: panic in ElementManager.Bundles watermark evaluation goroutine: assignment to entry in nil map
goroutine 5592 [running]:
runtime/debug.Stack()
	/home/runner/sdk/go1.24.4/src/runtime/debug/stack.go:26 +0x5e
github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.(*ElementManager).Bundles.func2.1()
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:385 +0x39
panic({0x169b4c0?, 0x2d0a4a0?})
	/home/runner/sdk/go1.24.4/src/runtime/panic.go:792 +0x132
github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.set[...].insert(...)
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:1131
github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.tsFinalEvent.Execute({{0xc153f7fc50?, 0x15c9f40?}}, 0xc212f6bd40)
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/teststream.go:256 +0xa5
github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.(*ElementManager).checkForQuiescence(0xc212f6bd40, 0xc212f6bd40?)
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:542 +0x17b
github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.(*ElementManager).Bundles.func2()
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:497 +0x251
created by github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.(*ElementManager).Bundles in goroutine 5582
	/runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:381 +0x156

Basically, there are a few cases where updateWatermarks will return a nil set(map). For example:

beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go

Line 2101 in 3659832

return nil

For most references of updateWatermarks, the returned map are read not written. The one in teststream, however, is an exception. Within there, we try to insert an element to this returned set(map), which leads to a panic if the set(map) is nil.

beam/sdks/go/pkg/beam/runners/prism/internal/engine/teststream.go

Lines 255 to 256 in 3659832

	kickSet := ss.updateWatermarks(em)
	kickSet.insert(ev.stageID)

Here is a note from go blog (https://go.dev/blog/maps#declaration-and-initialization):

A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don’t do that.

[gemini-code-assist]

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 addresses a critical bug that caused a runtime panic in the teststream runner, specifically within the ElementManager's watermark evaluation goroutine. The issue stemmed from an attempt to write to a nil map, which could be returned by the updateWatermarks function. The changes introduce a safeguard to initialize the map if it's nil, ensuring robust behavior and preventing application crashes during test execution.

Highlights

• Panic Fix: Resolved a runtime panic in the teststream component that occurred when attempting to insert an element into a nil map returned by the updateWatermarks function.
• Nil Map Handling: Introduced a check to ensure that the kickSet map is explicitly initialized if updateWatermarks returns a nil value, preventing illegal write operations.
• Documentation Update: Updated the documentation for the updateWatermarks function to clarify that it may return a nil set of stages.

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