fix: return closed non-nil channel for empty streams instead of (nil, nil)

[kharkevich]

Summary

Fixes the public streaming API returning (nil, nil) for empty streams. When a provider's chunk channel closes before the first chunk, CheckFirstStreamChunkForError returns a nil channel with a nil error, and executeRequestWithRetries assigned that straight into the public *StreamRequest result — so integrators received a nil channel with no error and blocked forever on the receive (a nil-channel receive never returns).

Closes #5555.

Changes

• core/bifrost.go (executeRequestWithRetries): when the checked stream comes back nil (empty stream), substitute a closed, non-nil channel as the result. Zero-chunk semantics are preserved — range exits immediately, a receive yields (nil, false) — while the implicit contract "nil error ⇒ usable channel" now holds.
• The substitution deliberately happens at the call site, not inside CheckFirstStreamChunkForError, and the empty case keeps isStreamChan == false, for two reasons:
  1. Span lifecycle: an empty stream has no surviving provider goroutine to complete a deferred span. Keeping the synchronous span-completion path (pre-existing behavior) avoids stranding a deferred span in the TraceStore or racing the provider goroutine's finalizer.
  2. Large-payload passthrough: SetupStreamingPassthrough providers intentionally return an already-closed placeholder channel; changing the helper's return would have silently moved every passthrough request onto the deferred-span path with no completer.
• CheckFirstStreamChunkForError itself is unchanged — its nil return is now genuinely consumed by the caller as the empty-stream signal, matching its doc comment.

Behavior note (intended): through the HTTP transport, an empty stream previously hit the stream == nil guard and produced an error response; it now returns a well-formed SSE stream with zero data events, which is the truthful representation of "the provider ended the stream without chunks".

Type of change

• Bug fix

Affected areas

• Core (Go)

How to test

cd core
go test -run 'TestExecuteRequestWithRetries_EmptyStreamReturnsClosedChannel' .
go test ./providers/utils/
go build ./...

TestExecuteRequestWithRetries_EmptyStreamReturnsClosedChannel is a caller-level regression test: it fails on the previous behavior (nil channel) and passes with this change. Existing TestCheckFirstStreamChunk_* tests are untouched and still green.

Breaking changes

• No

Related issues

Closes #5555

Security considerations

None. One extra channel allocation only on the empty-stream path.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: e8034b35-7383-4023-b2d1-5ee113587db8

📥 Commits

Reviewing files that changed from the base of the PR and between 2952aae and dba0928.

📒 Files selected for processing (2)

• core/bifrost.go
• core/bifrost_test.go

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Fixed streaming requests that return no data so they now complete safely with a closed stream instead of hanging indefinitely.
  • Improved completion handling for empty streaming responses.

• Tests

  • Added coverage to verify empty streams are non-nil, closed, and safe to read or iterate.

Walkthrough

Empty provider streams now return a closed, non-nil channel instead of (nil, nil). Streaming span completion is handled synchronously for empty streams, and a regression test verifies immediate channel consumption.

Changes

Empty stream handling

Layer / File(s)	Summary
Normalize empty streaming results core/bifrost.go, core/bifrost_test.go	Empty streams are replaced with closed non-nil channels, span completion is performed synchronously, and tests verify non-blocking receives and ranging.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• maximhq/bifrost#5491: Updates streaming span handling for streams that fail before producing their first chunk.

Suggested reviewers: tejasghatte, roroghost17, akshaydeo

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the bug fix: returning a closed non-nil channel for empty streams instead of nil.
Description check	✅ Passed	The description covers summary, changes, type, affected area, testing, breaking changes, related issue, and security notes.
Linked Issues check	✅ Passed	The code matches #5555 by turning empty streams into a closed non-nil channel while preserving zero-chunk behavior.
Out of Scope Changes check	✅ Passed	The patch stays focused on the empty-stream fix and its regression test, with no unrelated code changes visible.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (2.12.2)

level=error msg="[linters_context] typechecking error: pattern ./...: directory prefix . does not contain main module or its selected dependencies"

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