feat: transcribe chunks with configurable parallel workers

Process up to eight chunks concurrently by default and add --jobs 1..64,
with --jobs 1 retaining sequential operation. Give each worker separate
encoding and request files, release payloads after completion, and merge
responses by original chunk index regardless of completion order.

Use a bounded worker scheduler that handles jobs finishing before wait is
called. Stop scheduling on an observed worker failure and terminate worker
process groups on failure or interruption before removing temporary files.
Include jobs interrupted between launch and scheduler registration in cleanup.
Preserve all-or-nothing stdout for both text and SRT output.

Document the Bash 5.1 requirement, concurrency option, resource usage, and
limits of cancelling requests already accepted by a provider.

Test Plan:
- python3 -B -m unittest -v: all 52 tests passed.
- Mocked provider tests verified eight concurrent requests by default,
  configurable limits, rolling worker replacement, isolated payloads,
  ordered text, and identical sequential/parallel SRT output.
- Verified immediate encoder failure, concurrent API failure, SIGINT and
  SIGTERM, child-process cancellation, cleanup, and no partial stdout.
- bash -n transcribe, ./transcribe --help, and
  git diff --cached --check: passed.
- No live provider request; credentials are unavailable in this environment.

README.md

@@ -1,6 +1,6 @@
 # Transcribe
 
-Requires Bash, FFmpeg, curl, jq, Python 3, and standard coreutils. Keep
+Requires Bash 5.1 or newer, FFmpeg, curl, jq, Python 3, and standard coreutils. Keep
 `chunks.py`, `subtitles.py`, and `transcript.py` beside the script. The API key is read using
 the 1Password CLI (`op`) from `op://API Keys/openrouter-mai-transcribe-2/credential`.
 
@@ -11,6 +11,8 @@ the 1Password CLI (`op`) from `op://API Keys/openrouter-mai-transcribe-2/credent
 ./transcribe --no-diarization recording.m4a > transcript.txt
 ./transcribe --srt --no-diarization video.mp4 > subtitles.srt
 ./transcribe --chunk-minutes 10 too_long.webm > transcript.txt
+./transcribe --jobs 4 too_long.webm > transcript.txt
+./transcribe --jobs 1 --srt too_long.webm > subtitles.srt
 ```
 
 Accepts m4a, mp4, aac, flac, opus, mp3, ogg, wav, and other formats your
@@ -88,9 +90,20 @@ identify a speaker across requests.
 
 Audio is first decoded to temporary mono 16 kHz PCM (about 115 MB per hour),
 so all cuts share the same sample clock even when container duration metadata
-is missing. Only one MP3 chunk and its request payload are staged at a time.
-Requests run sequentially. A failed request stops the run; there is no automatic
-retry or resume, and rerunning sends the earlier chunks again.
+is missing. Up to **eight chunks** are encoded and transcribed concurrently
+by default. Use `--jobs N` (an integer from 1 to 64) to change the limit;
+`--jobs 1` runs sequentially. Each worker uses separate temporary files and
+releases its MP3 and request payload when finished, so staged upload data is
+bounded by the number of workers. Lower `--jobs` to reduce simultaneous CPU,
+memory, disk, or provider demand.
+
+Responses may finish in any order; merging and speaker numbering always follow
+the recording's original order. A failed worker stops further scheduling and
+cancels the remaining workers. Interruptions also stop workers and their child
+processes before temporary files are removed. Requests already accepted by the
+provider may still be processed and billed. There is no automatic retry or
+resume, and rerunning sends earlier chunks again. No transcript is emitted
+unless every worker and the final conversion succeed.
 
 `--srt` requests word timestamps using OpenRouter's `verbose_json` response.
 Words are grouped into cues of at most two lines (42 characters per line,

test_transcribe.py

@@ -2,8 +2,10 @@ import json
 import os
 from pathlib import Path
 import shutil
+import signal
 import subprocess
 import tempfile
+import time
 import unittest
 
 from test_chunks import make_audio
@@ -29,18 +31,159 @@ class TranscribeTests(unittest.TestCase):
                         TRANSCRIBE_TEST_LOG=str(self.log), TRANSCRIBE_TEST_MODE="ok")
         self.audio = self.path / "input with spaces.wav"
 
-    def run_cli(self, *args, mode="ok", duration=70):
+    def run_cli(self, *args, mode="ok", duration=70, env_overrides=None):
         make_audio(self.audio, duration)
         return subprocess.run([str(ROOT / "transcribe"), *args, str(self.audio)],
-                              env=dict(self.env, TRANSCRIBE_TEST_MODE=mode),
+                              env=dict(self.env, TRANSCRIBE_TEST_MODE=mode, **(env_overrides or {})),
                               capture_output=True, text=True, timeout=30)
 
     def requests(self):
-        return [json.loads(line) for line in self.log.read_text().splitlines()]
+        return sorted((json.loads(line) for line in self.log.read_text().splitlines()),
+                      key=lambda request: request["index"])
+
+    def events(self):
+        path = self.log.with_suffix(".events.jsonl")
+        return [json.loads(line) for line in path.read_text().splitlines()] if path.exists() else []
+
+    def reset_logs(self):
+        self.log.unlink(missing_ok=True)
+        self.log.with_suffix(".events.jsonl").unlink(missing_ok=True)
+
+    def assert_processes_stopped(self):
+        for event in self.events():
+            for pid in (event["pid"], event.get("child_pid")):
+                if pid is None:
+                    continue
+                try:
+                    # A killed descendant can await reaping by init; it must
+                    # not still be running or holding our temporary files.
+                    state = Path(f"/proc/{pid}/stat").read_text().rsplit(")", 1)[1].split()[0]
+                    self.assertEqual(state, "Z", f"Process {pid} survived worker cleanup")
+                except FileNotFoundError:
+                    pass
 
     def assert_cleaned(self):
         self.assertTrue(all(not Path(r["staging"]).exists() for r in self.requests()))
 
+    def test_parallel_default_and_job_limit_with_ordered_output(self):
+        for options, limit, duration in (((), 8, 500), (("--jobs", "2"), 2, 180),
+                                         (("--jobs", "1"), 1, 130)):
+            with self.subTest(options=options):
+                self.reset_logs()
+                result = self.run_cli(
+                    *options, "--chunk-minutes", "1", mode="parallel", duration=duration,
+                    env_overrides={"TRANSCRIBE_TEST_BARRIER": str(limit)},
+                )
+                self.assertEqual(result.returncode, 0, result.stderr)
+                requests = self.requests()
+                self.assertEqual(result.stdout.split(), [f"Part{i}." for i in range(len(requests))])
+                active = maximum = 0
+                events = sorted(self.events(), key=lambda event: event["time"])
+                for event in events:
+                    active += 1 if event["event"] == "start" else -1
+                    maximum = max(maximum, active)
+                self.assertEqual(active, 0)
+                self.assertEqual(maximum, limit)
+                self.assertEqual(len({r["work"] for r in requests}), len(requests))
+                if limit > 1:
+                    first_finished = next(e for e in events if e["event"] == "finish")
+                    self.assertNotEqual(first_finished["index"], 0)
+                    refilled = next(e["time"] for e in events if e["event"] == "start" and e["index"] == limit)
+                    slow_finished = next(e["time"] for e in events if e["event"] == "finish" and e["index"] == 0)
+                    self.assertLess(refilled, slow_finished)
+                self.assert_processes_stopped()
+                self.assert_cleaned()
+
+    def test_parallel_srt_matches_sequential_output(self):
+        serial = self.run_cli("--jobs", "1", "--chunk-minutes", "1", "--srt", mode="parallel", duration=130)
+        self.assertEqual(serial.returncode, 0, serial.stderr)
+        self.reset_logs()
+        parallel = self.run_cli(
+            "--jobs", "3", "--chunk-minutes", "1", "--srt", mode="parallel", duration=130,
+            env_overrides={"TRANSCRIBE_TEST_BARRIER": "3"},
+        )
+        self.assertEqual(parallel.returncode, 0, parallel.stderr)
+        self.assertEqual(parallel.stdout, serial.stdout)
+        self.assertEqual([block.splitlines()[0] for block in parallel.stdout.strip().split("\n\n")],
+                         ["1", "2", "3"])
+        self.assert_cleaned()
+
+    def test_parallel_failure_cancels_workers_and_their_children(self):
+        result = self.run_cli(
+            "--jobs", "2", "--chunk-minutes", "1", mode="parallel-error", duration=180,
+            env_overrides={"TRANSCRIBE_TEST_BARRIER": "2"},
+        )
+        self.assertNotEqual(result.returncode, 0)
+        self.assertEqual(result.stdout, "")
+        self.assertIn("simulated concurrent failure", result.stderr)
+        self.assertIn("stopping remaining workers", result.stderr)
+        self.assertEqual({e["index"] for e in self.events() if e["event"] == "start"}, {0, 1})
+        self.assertTrue(any(e["event"] == "child" for e in self.events()))
+        self.assert_processes_stopped()
+        self.assert_cleaned()
+
+    def test_interruptions_stop_workers_before_removing_staging(self):
+        for signum in (signal.SIGINT, signal.SIGTERM):
+            with self.subTest(signal=signum):
+                self.reset_logs()
+                make_audio(self.audio, 180)
+                process = subprocess.Popen(
+                    [str(ROOT / "transcribe"), "--jobs", "2", "--chunk-minutes", "1", str(self.audio)],
+                    env=dict(self.env, TRANSCRIBE_TEST_MODE="blocked"),
+                    stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True, start_new_session=True,
+                )
+                try:
+                    deadline = time.monotonic() + 5
+                    while time.monotonic() < deadline:
+                        events = self.events()
+                        if (sum(e["event"] == "start" for e in events) >= 2 and
+                                any(e["event"] == "child" for e in events)):
+                            break
+                        time.sleep(0.01)
+                    else:
+                        self.fail("Workers did not start")
+                    process.send_signal(signum)
+                    stdout, stderr = process.communicate(timeout=5)
+                    self.assertEqual(process.returncode, 128 + signum, stderr)
+                    self.assertEqual(stdout, "")
+                    self.assert_processes_stopped()
+                    self.assertTrue(all(not Path(e["staging"]).exists() for e in self.events()))
+                finally:
+                    if process.poll() is None:
+                        process.kill()
+                    for event in self.events():
+                        try:
+                            os.killpg(event["group"], signal.SIGKILL)
+                        except ProcessLookupError:
+                            pass
+                    process.communicate(timeout=5)
+
+    def test_immediate_encoder_failure_is_not_lost_by_worker_waiting(self):
+        encoder = self.bin / "ffmpeg"
+        encoder.write_text(
+            "#!/usr/bin/env python3\nimport os, sys\n"
+            "if '-ss' in sys.argv:\n    sys.exit(42)\n"
+            f"os.execv({shutil.which('ffmpeg')!r}, ['ffmpeg', *sys.argv[1:]])\n"
+        )
+        encoder.chmod(0o755)
+        result = self.run_cli("--jobs", "64", "--chunk-minutes", "1", duration=500)
+        self.assertEqual(result.returncode, 42, result.stderr)
+        self.assertEqual(result.stdout, "")
+        self.assertIn("stopping remaining workers", result.stderr)
+        self.assertFalse(self.log.exists())
+
+    def test_invalid_jobs_fail_before_provider_access(self):
+        for value in ("0", "65", "-1", "1.5", "abc", "9999999999999999999999"):
+            with self.subTest(value=value):
+                result = self.run_cli("--jobs", value, duration=1)
+                self.assertNotEqual(result.returncode, 0)
+                self.assertIn("integer between 1 and 64", result.stderr)
+        result = subprocess.run([str(ROOT / "transcribe"), "--jobs"], env=self.env,
+                                capture_output=True, text=True, timeout=5)
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("integer between 1 and 64", result.stderr)
+        self.assertFalse(self.log.exists())
+
     def assert_diarization(self, enabled):
         for request in self.requests():
             self.assertEqual(request["provider"], {

tests/fake_provider.py

@@ -7,6 +7,7 @@ import os
 from pathlib import Path
 import subprocess
 import sys
+import time
 
 
 if Path(sys.argv[0]).name == "op":
@@ -14,18 +15,34 @@ if Path(sys.argv[0]).name == "op":
     sys.exit(0)
 
 args = sys.argv[1:]
-request = json.loads(Path(args[args.index("--data-binary") + 1][1:]).read_text())
+request_path = Path(args[args.index("--data-binary") + 1][1:])
+request = json.loads(request_path.read_text())
 output = Path(args[args.index("-o") + 1])
 index = int(output.stem.split("-")[1])
 manifest = json.loads((output.parent / "chunks.json").read_text())
 chunk = manifest[index]
+mode = os.environ.get("TRANSCRIBE_TEST_MODE", "ok")
+events_path = Path(os.environ["TRANSCRIBE_TEST_LOG"]).with_suffix(".events.jsonl")
+
+
+def event(kind, **extra):
+    record = dict(event=kind, index=index, pid=os.getpid(), group=os.getpgrp(),
+                  time=time.monotonic(), staging=str(output.parent), **extra)
+    with events_path.open("a") as log:
+        log.write(json.dumps(record) + "\n")
+
+
+event("start")
+if mode in ("parallel-error", "blocked") and index == 0:
+    child = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(30)"])
+    event("child", child_pid=child.pid)
 assert request["model"] == "microsoft/mai-transcribe-2"
 assert request["input_audio"]["format"] == "mp3"
 if len(manifest) > 1:
     assert request["response_format"] == "verbose_json"
     assert request["timestamp_granularities"] == ["segment", "word"]
 
-audio = output.parent / "uploaded.mp3"
+audio = output.parent / f"uploaded-{index}.mp3"
 audio.write_bytes(base64.b64decode(request["input_audio"]["data"], validate=True))
 info = json.loads(subprocess.check_output([
     "ffprobe", "-v", "error", "-show_entries",
@@ -36,13 +53,33 @@ assert abs(float(info["format"]["duration"]) - (chunk["end"] - chunk["start"]))
 with open(os.environ["TRANSCRIBE_TEST_LOG"], "a") as log:
     log.write(json.dumps({
         "index": index, "chunk": chunk, "staging": str(output.parent),
+        "work": str(request_path.parent),
         "upload_duration": float(info["format"]["duration"]),
         "provider": request.get("provider"),
         "response_format": request.get("response_format"),
         "timestamp_granularities": request.get("timestamp_granularities"),
     }) + "\n")
 
-mode = os.environ.get("TRANSCRIBE_TEST_MODE", "ok")
+if mode in ("parallel", "parallel-error"):
+    deadline = time.monotonic() + 5
+    barrier = int(os.environ.get("TRANSCRIBE_TEST_BARRIER", "1"))
+    while True:
+        events = [json.loads(line) for line in events_path.read_text().splitlines()]
+        enough = sum(e["event"] == "start" for e in events) >= barrier
+        child_ready = mode != "parallel-error" or any(e["event"] == "child" for e in events)
+        if enough and child_ready:
+            break
+        if time.monotonic() >= deadline:
+            raise RuntimeError("Worker concurrency barrier timed out")
+        time.sleep(0.01)
+    if mode == "parallel-error" and index == 1:
+        output.write_text('{"error":{"message":"simulated concurrent failure"}}')
+        event("finish")
+        sys.exit(22)
+    time.sleep(30 if mode == "parallel-error" else (0.8 if index == 0 else 0.05))
+elif mode == "blocked":
+    time.sleep(30)
+
 if index == 1:
     if mode == "http-error":
         output.write_text('{"error":{"message":"simulated timeout"}}')
@@ -78,5 +115,9 @@ else:
                 if mode == "speakers" or speaker == 0:
                     word["speaker"] = (speaker if len(manifest) == 1 else
                                        int(word["start"] + chunk["start"] >= 57.3))
+    if mode == "parallel":
+        middle = (chunk["end"] - chunk["start"]) / 2
+        words = [{"word": f"Part{index}.", "start": middle, "end": middle + 0.4}]
     response = {"text": " ".join(w["word"] for w in words), "words": words}
 output.write_text(json.dumps(response))
+event("finish")

transcribe

@@ -5,17 +5,27 @@ set -euo pipefail
 srt=false
 diarization=true
 chunk_minutes=
+jobs=8
 usage() {
-    echo "Usage: ${0##*/} [--srt] [--no-diarization] [--chunk-minutes 1..30] AUDIO_FILE"
+    echo "Usage: ${0##*/} [--srt] [--no-diarization] [--chunk-minutes 1..30] [--jobs 1..64] AUDIO_FILE"
     echo 'Transcribe audio using OPENROUTER_API_KEY; print wrapped text with speaker markers to stdout.'
     echo '  --srt  Print SRT subtitles in short cues using word timestamps instead.'
     echo '  --no-diarization  Disable speaker diarization (enabled by default).'
     echo '  --chunk-minutes N  Maximum upload duration in minutes (default: 10; 30 with --no-diarization).'
+    echo '  --jobs N  Maximum chunks processed in parallel (default: 8; 1 for sequential).'
 }
 while [[ $# -gt 0 ]]; do
     case $1 in
         --srt) srt=true; shift ;;
         --no-diarization) diarization=false; shift ;;
+        --jobs)
+            if [[ ! ${2:-} =~ ^([1-9]|[1-5][0-9]|6[0-4])$ ]]; then
+                echo '--jobs must be an integer between 1 and 64' >&2
+                exit 1
+            fi
+            jobs=$2
+            shift 2
+            ;;
         --chunk-minutes)
             if [[ ! ${2:-} =~ ^([1-9]|[12][0-9]|30)$ ]]; then
                 echo '--chunk-minutes must be an integer between 1 and 30' >&2
@@ -42,6 +52,10 @@ if [[ ! -f $1 || ! -r $1 ]]; then
     echo "Cannot read audio file: $1" >&2
     exit 1
 fi
+if (( BASH_VERSINFO[0] < 5 || (BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] < 1) )); then
+    echo 'Bash 5.1 or newer is required for parallel chunk processing.' >&2
+    exit 1
+fi
 export OPENROUTER_API_KEY
 OPENROUTER_API_KEY="$(op read "op://API Keys/openrouter-mai-transcribe-2/credential")"
 : "${OPENROUTER_API_KEY:?Set OPENROUTER_API_KEY first}"
@@ -54,7 +68,25 @@ for tool in "${dependencies[@]}"; do
 done
 
 tmp=$(mktemp -d)
-trap 'rm -rf -- "$tmp"' EXIT
+declare -A workers=()
+cleanup() {
+    trap - EXIT
+    trap '' INT TERM
+    local pid
+    local -a pending
+    # Include a job interrupted between launch and registration in workers.
+    mapfile -t pending < <(jobs -p)
+    # Each worker owns a process group, including its encoder and curl. Stop
+    # the whole group and reap workers before removing files they might use.
+    for pid in "${pending[@]}"; do
+        kill -KILL -- "-$pid" 2>/dev/null || true
+    done
+    for pid in "${pending[@]}"; do
+        wait "$pid" 2>/dev/null || true
+    done
+    rm -rf -- "$tmp"
+}
+trap cleanup EXIT
 trap 'exit 130' INT
 trap 'exit 143' TERM
 script_dir=$(dirname -- "$(realpath -- "$0")")
@@ -70,32 +102,37 @@ if "$diarization"; then timestamps=true; fi
 if (( count > 1 )); then
     timestamps=true
     echo "Splitting audio into $count overlapping chunks (maximum $chunk_minutes minutes each)." >&2
+    echo "Processing up to $jobs chunks in parallel." >&2
 fi
 
-for ((i = 0; i < count; i++)); do
+transcribe_chunk() {
+    trap - EXIT INT TERM
+    local i=$1 start duration work
+    work=$tmp/chunk-$i
+    mkdir -- "$work"
     read -r start duration < <(jq -r --argjson i "$i" \
         '.[$i] | "\(.start) \(.end - .start)"' "$tmp/chunks.json")
     if (( count > 1 )); then
         echo "Transcribing chunk $((i + 1))/$count (start ${start}s, duration ${duration}s)..." >&2
     fi
     ffmpeg -nostdin -hide_banner -loglevel error -y -ss "$start" -i "$tmp/audio.wav" \
-        -t "$duration" -map 0:a:0 -ac 1 -ar 16000 -c:a libmp3lame -b:a 64k "$tmp/audio.mp3"
+        -t "$duration" -map 0:a:0 -ac 1 -ar 16000 -c:a libmp3lame -b:a 64k "$work/audio.mp3"
 
     # Keep large audio payloads out of shell arguments (which have a size limit).
-    base64 <"$tmp/audio.mp3" | tr -d '\n\r' >"$tmp/audio.b64"
-    jq -n --rawfile data "$tmp/audio.b64" --argjson timestamps "$timestamps" \
+    base64 <"$work/audio.mp3" | tr -d '\n\r' >"$work/audio.b64"
+    jq -n --rawfile data "$work/audio.b64" --argjson timestamps "$timestamps" \
         --argjson diarization "$diarization" \
         '{model: "microsoft/mai-transcribe-2", input_audio: {data: $data, format: "mp3"},
           provider: {options: {azure: {diarization: {enabled: $diarization}}}}}
          + (if $timestamps then {response_format: "verbose_json", timestamp_granularities: ["segment", "word"]}
             else {} end)' \
-        >"$tmp/request.json"
+        >"$work/request.json"
 
     curl --silent --show-error --fail-with-body \
         https://openrouter.ai/api/v1/audio/transcriptions \
         -H "Authorization: Bearer $OPENROUTER_API_KEY" \
         -H 'Content-Type: application/json' \
-        --data-binary @"$tmp/request.json" -o "$tmp/response-$i.json" || {
+        --data-binary @"$work/request.json" -o "$tmp/response-$i.json" || {
         echo "Transcription failed for chunk $((i + 1))/$count." >&2
         if [[ -f $tmp/response-$i.json ]]; then
             cat "$tmp/response-$i.json" >&2
@@ -113,7 +150,53 @@ for ((i = 0; i < count; i++)); do
             echo "Warning: chunk $((i + 1))/$count returned some words without speaker labels." >&2
         fi
     fi
+    rm -rf -- "$work"
+}
+
+wait_for_worker() {
+    local pid finished status=0
+    # wait -n skips jobs that finished before it was called. Reap those by PID
+    # first, so even an immediate encoder/request failure cannot disappear.
+    for pid in "${!workers[@]}"; do
+        if ! kill -0 "$pid" 2>/dev/null; then
+            wait "$pid" || status=$?
+            unset 'workers[$pid]'
+            return "$status"
+        fi
+    done
+    wait -n -f -p finished || status=$?
+    if [[ -z ${finished:-} ]]; then
+        # All remaining jobs may have exited between the probe and wait -n.
+        for pid in "${!workers[@]}"; do
+            status=0
+            wait "$pid" || status=$?
+            finished=$pid
+            break
+        done
+    fi
+    unset 'workers[$finished]'
+    return "$status"
+}
+
+# Job control gives every background worker a separate process group. Their
+# response filenames retain recording order regardless of completion order.
+set -m
+next=0
+while (( next < count || ${#workers[@]} > 0 )); do
+    while (( next < count && ${#workers[@]} < jobs )); do
+        transcribe_chunk "$next" &
+        workers[$!]=$next
+        next=$((next + 1))
+    done
+    if wait_for_worker; then
+        :
+    else
+        status=$?
+        echo 'Transcription failed; stopping remaining workers.' >&2
+        exit "$status"
+    fi
 done
+set +m
 
 # Only emit a transcript after every chunk succeeds, including SRT conversion.
 if (( count > 1 )); then