feat(go): added channel-based streaming flow API

[apascal07]

Adds genkitx.DefineStreamingFlow (experimental) as an alternative to the callback-based genkit.DefineStreamingFlow. The channel-based API is more idiomatic Go for streaming use cases and will replace the callback-based version in the next major version release.

Usage

import genkitx "github.com/firebase/genkit/go/genkit/x"

jokeFlow := genkitx.DefineStreamingFlow(g, "jokeFlow",
    func(ctx context.Context, topic string, streamCh chan<- string) (string, error) {
        for chunk, err := range genkit.GenerateStream(ctx, g,
            ai.WithPrompt("Tell me a joke about %s.", topic),
        ) {
            if err != nil {
                return "", err
            }
            select {
            case streamCh <- chunk.Text():
            case <-ctx.Done():
                return "", ctx.Err()
            }
        }
        return "done", nil
    })

Changes

• genkit/x/genkit.go: New DefineStreamingFlow function with channel-based signature
• core/flow.go: Fix panic when consumer breaks out of streaming loop early

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated: added docs for channel-based streaming flow API  genkit-ai/docsite#209

[gemini-code-assist]

Summary of Changes

Hello @apascal07, 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 enhances the Genkit framework by introducing a new, more Go-idiomatic channel-based API for defining streaming flows. This provides developers with a familiar and efficient way to handle streaming data, complementing the existing callback-based mechanism. Additionally, it includes a crucial bug fix to gracefully manage early termination by streaming consumers, preventing potential panics and improving overall stability.

Highlights

• New Channel-Based Streaming Flow API: Introduced x.DefineStreamingFlow as an alternative to the existing callback-based genkit.DefineStreamingFlow. This new API leverages Go channels for streaming, offering a more idiomatic approach for Go developers.
• Improved Streaming Consumer Termination Handling: Implemented a fix in core/flow.go to prevent a panic that could occur when a streaming consumer breaks out of a loop early, ensuring more robust error handling during streaming operations.

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[gemini-code-assist]

Code Review

This pull request introduces a new channel-based streaming flow API, x.DefineStreamingFlow, which provides a more idiomatic Go approach for streaming use cases compared to the existing callback-based API. The implementation is well-structured, and the accompanying tests are thorough, covering various edge cases like early consumer termination. The fix in core/flow.go to prevent a panic on early loop exit is also a good improvement. I have one suggestion to prevent a potential goroutine leak in the non-streaming path. Overall, this is a valuable and well-implemented feature.

[gemini-code-assist]

In the non-streaming case (sendChunk == nil), if the user-provided function fn panics, close(discardCh) will not be called. This will cause the goroutine that ranges over discardCh to leak because it will block forever.

Using defer ensures that the channel is closed even if a panic occurs, thus preventing the goroutine leak.

Suggested change

	output, err := fn(ctx, input, discardCh)
	close(discardCh)
	return output, err
	defer close(discardCh)
	return fn(ctx, input, discardCh)