feat(whisper-cpp): implement streaming transcription (#9751)

* test(whisper): wire e2e streaming transcription target

Adds test-extra-backend-whisper-transcription, mirroring the existing
llama-cpp / sherpa-onnx / vibevoice-cpp targets. The generic
AudioTranscriptionStream spec at tests/e2e-backends/backend_test.go:644
fails today because backend/go/whisper has no streaming impl - this
target is the failing TDD gate that the next phase makes pass.

Confirmed RED locally: 3 Passed (health, load, offline transcription),
1 Failed (streaming spec hits its 300s context deadline because the
base implementation returns 'unimplemented' but doesn't close the
result channel, leaving the gRPC stream open until the client times
out).

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* feat(whisper-cpp): expose new_segment_callback to the Go side

Adds set_new_segment_callback() and a C-side trampoline that whisper.cpp
invokes once per new text segment during whisper_full(). The trampoline
dispatches (idx_first, n_new, user_data) to a Go function pointer
registered via purego.NewCallback - text and timings are pulled by Go
through the existing get_segment_text/get_segment_t0/get_segment_t1
getters.

Wires the hook only when streaming is actually requested, to avoid a
per-segment function-pointer dispatch on the offline path.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* feat(whisper-cpp): implement AudioTranscriptionStream

Wires whisper.cpp's new_segment_callback through purego back to Go so
the streaming transcription RPC produces real, time-correlated deltas
while whisper_full() is still decoding. Each segment becomes one
TranscriptStreamResponse{Delta}; whisper_full's return is the
TranscriptStreamResponse{FinalResult} carrying the full segment list,
language, and duration.

Per-call state is tracked in a sync.Map keyed by an atomic counter; the
Go callback registered via purego.NewCallback is a singleton, dispatched
through user_data. SingleThread today means only one entry is ever live,
but the map shape matches the sherpa-onnx TTS callback pattern.

The streaming path's final.Text is the literal concat of every emitted
delta (a strings.Builder accumulated by onNewSegment) so the e2e
invariant `final.Text == concat(deltas)` holds exactly. The first delta
has no leading space; subsequent deltas are space-prefixed. The offline
AudioTranscription path is unchanged.

Closes the gap with sherpa-onnx, vibevoice-cpp, llama-cpp, and tinygrad,
which already implement AudioTranscriptionStream.

Verified GREEN locally: make test-extra-backend-whisper-transcription
passes 4/4 specs (3 Passed initially under RED, +1 streaming spec now).

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* test(whisper-cpp): assert progressive multi-segment streaming

Drives AudioTranscriptionStream against a real long-audio fixture and
asserts len(deltas) >= 2. The generic e2e spec at
tests/e2e-backends/backend_test.go:644 only checks len(deltas) >= 1
which is satisfied by both real and faked streaming - this spec is the
guardrail that a future "fake" impl can't sneak past.

Skipped by default (env-gated, like the cancellation spec); set
WHISPER_LIBRARY, WHISPER_MODEL_PATH, and WHISPER_AUDIO_PATH to a 30+
second clip to run.

Verified locally with a 55s 5x-JFK concat against ggml-base.en.bin:
1 Passed in 7.3s, deltas >= 2, finalSegmentCount >= 2,
concat(deltas) == final.Text.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* ci(whisper-cpp): add transcription gRPC e2e job

Mirrors tests-sherpa-onnx-grpc-transcription /
tests-llama-cpp-grpc-transcription. Runs make
test-extra-backend-whisper-transcription whenever the whisper backend
or the run-all switch fires, so a pin-bump or refactor that breaks
streaming transcription gets caught before merge.

The whisper output on detect-changes is already emitted by
scripts/changed-backends.js (it iterates allBackendPaths); this PR
just exposes it as a workflow output and consumes it.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(whisper-cpp): silence errcheck on AudioTranscriptionStream defers

golangci-lint runs with new-from-merge-base=origin/master, so the
identical defer patterns in the existing offline AudioTranscription
path are grandfathered while the new ones in AudioTranscriptionStream
trip errcheck. Wrap both defers in `func() { _ = ... }()` to match what
errcheck wants without altering behavior. The errors from os.RemoveAll
and *os.File.Close are not actionable inside a defer here (we're
already returning), matching the offline path's contract.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

.github/workflows/test-extra.yml

@@ -43,6 +43,7 @@ jobs:
       insightface: ${{ steps.detect.outputs.insightface }}
       speaker-recognition: ${{ steps.detect.outputs.speaker-recognition }}
       sherpa-onnx: ${{ steps.detect.outputs.sherpa-onnx }}
+      whisper: ${{ steps.detect.outputs.whisper }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6
@@ -583,6 +584,27 @@ jobs:
       - name: Build sherpa-onnx backend image and run streaming ASR gRPC e2e tests
         run: |
           make test-extra-backend-sherpa-onnx-transcription
+  # End-to-end transcription via the e2e-backends gRPC harness against
+  # the whisper.cpp backend. Drives AudioTranscription (offline) and
+  # AudioTranscriptionStream (real, segment-callback-driven deltas) on
+  # ggml-base.en + the JFK 11s clip.
+  tests-whisper-grpc-transcription:
+    needs: detect-changes
+    if: needs.detect-changes.outputs.whisper == 'true' || needs.detect-changes.outputs.run-all == 'true'
+    runs-on: ubuntu-latest
+    timeout-minutes: 90
+    steps:
+      - name: Clone
+        uses: actions/checkout@v6
+        with:
+          submodules: true
+      - name: Setup Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.25.4'
+      - name: Build whisper backend image and run transcription gRPC e2e tests
+        run: |
+          make test-extra-backend-whisper-transcription
   # VITS TTS via the sherpa-onnx backend. Drives both TTS (file write) and
   # TTSStream (PCM chunks) on the e2e-backends harness.
   tests-sherpa-onnx-grpc-tts: