[Bug][Prism]: Prism gets stuck when trying to flatten 2 unbounded pcollections

[avery-nguyen-verily]

What happened?

What happened?

If you try running a pipeline that has multiple unbounded sources where 1 or more do not emit any data within the window and try to flatten the pcollections together Prism will not execute any subsequent ParDos until all sources emit something even after the window ends.

Expected Behaviour

Prism should finish processing the data once the window ends, even if not all the sources that are being flattened have emitted data.

Minimal Example

package main

import (
        "context"
        "log"
        "math"
        "time"

        "github.com/apache/beam/sdks/v2/go/pkg/beam"
        "github.com/apache/beam/sdks/v2/go/pkg/beam/core/graph/window"
        beamLog "github.com/apache/beam/sdks/v2/go/pkg/beam/log"
        "github.com/apache/beam/sdks/v2/go/pkg/beam/register"
        "github.com/apache/beam/sdks/v2/go/pkg/beam/transforms/periodic"
        "github.com/apache/beam/sdks/v2/go/pkg/beam/x/beamx"
)

func init() {
        register.Function3x0(count)
        register.Function3x0(initCount)
        register.Emitter1[int]()
}

func initCount(ctx context.Context, _ []byte, emit func(int)) {
        beamLog.Info(ctx, "Starting count")
        emit(1)
}

func count(ctx context.Context, currCount int, emit func(int)) {
        beamLog.Infof(ctx, "Current Count = %d", currCount)
        emit(currCount + 1)
}

func main() {
        beam.Init()

        ctx := context.Background()
        pipeline, scope := beam.NewPipelineWithRoot()

        duration := 5 * time.Second
        fixedWindow := window.NewFixedWindows(duration)
        unboundedSource1 := periodic.Impulse(scope, time.Now(), time.UnixMilli(math.MaxInt64), duration, false)
        unboundedSource2 := periodic.Impulse(scope, time.UnixMilli(math.MaxInt32), time.UnixMilli(math.MaxInt64), duration, false)
        trigger1 := beam.WindowInto(scope, fixedWindow, unboundedSource1)
        trigger2 := beam.WindowInto(scope, fixedWindow, unboundedSource2)

        s1 := scope.Scope("source")
        c1 := beam.ParDo(s1, initCount, trigger1)
        c2 := beam.ParDo(s1, initCount, trigger2)
        s := beam.Flatten(s1, c1, c2) // prism will never run any ParDo that depends on this
        s2 := scope.Scope("count")
        c3 := beam.ParDo(s2, count, s)
        c4 := beam.ParDo(s2, count, c3)
        beam.ParDo(s2, count, c4)

        if err := beamx.Run(ctx, pipeline); err != nil {
                log.Fatalf("Failed to execute job: %v", err)
        }
}

Output

Prism only ever runs initCount, none of the count ParDo functions are ever run because it is waiting for data from c2

2025/01/30 15:55:59 INFO log from SDK worker worker.ID=job-001[go-job-1-1738252559500852940]_go worker.endpoint=localhost:38569 sdk.transformID=e12 sdk.location=/home/user/workspace/beam-starter-go/main.go:25 sdk.time=2025-01-30T15:55:59.830Z sdk.msg="Starting count"
2025/01/30 15:55:59 INFO log from SDK worker worker.ID=job-001[go-job-1-1738252559500852940]_go worker.endpoint=localhost:38569 sdk.transformID=e12 sdk.location=/home/user/workspace/beam-starter-go/main.go:25 sdk.time=2025-01-30T15:55:59.830Z sdk.msg="Starting count"
2025/01/30 15:55:59 INFO log from SDK worker worker.ID=job-001[go-job-1-1738252559500852940]_go worker.endpoint=localhost:38569 sdk.transformID=e12 sdk.location=/home/user/workspace/beam-starter-go/main.go:25 sdk.time=2025-01-30T15:55:59.830Z sdk.msg="Starting count"
2025/01/30 15:55:59 INFO log from SDK worker worker.ID=job-001[go-job-1-1738252559500852940]_go worker.endpoint=localhost:38569 sdk.transformID=e12 sdk.location=/home/user/workspace/beam-starter-go/main.go:25 sdk.time=2025-01-30T15:55:59.830Z sdk.msg="Starting count"
2025/01/30 15:55:59 INFO log from SDK worker worker.ID=job-001[go-job-1-1738252559500852940]_go worker.endpoint=localhost:38569 sdk.transformID=e12 sdk.location=/home/user/workspace/beam-starter-go/main.go:25 sdk.time=2025-01-30T15:55:59.830Z sdk.msg="Starting count"

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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