fix: remote transcription posts to /transcribe, not /align (stem-splitter#17) (#959)

* fix: remote transcription posts to /transcribe, not /align (stem-splitter#17)

transcribe_vocals_remote() POSTed the vocal stem to /align. That endpoint is FORCED ALIGNMENT —
"here are the lyrics, tell me when each word is sung" — and its `text` field is required. We have
no lyrics; transcribing them is the entire point. So the server rejected every request with a 422
from FastAPI's validation layer, before its handler ever ran, and remote transcription has never
worked for anyone.

It now posts to /transcribe (added in feedBack-demucs-server#14), which takes only the audio.

`language` moves from the query string to the FORM BODY, where the server actually reads it
(Form("")). As a query param it was silently ignored, so an explicit hint did nothing and
Whisper's auto-detection quietly decided instead — loading the wrong wav2vec2 aligner. It
"worked", it was just wrong, which is the failure mode that hides for months.

Error bodies are no longer cut at 300 chars. The body IS the diagnosis: a 422's JSON names the
field it rejected, a 500's traceback answers on its LAST line. Both got decapitated — which is
part of why this stayed invisible for so long. The message explaining the bug was inside the part
that got cut.

Nothing caught any of this because every test of this module tested the MAPPER, fed a hand-written
dict. The mapper was always fine. The request was never exercised, and the request was the bug.
tests/test_lyrics_transcribe_remote.py now pins it: the endpoint, the form field, the multipart
upload, the bearer token, an instrumental returning no lyrics rather than an error, and a 404
saying the server is too old. Verified they FAIL against /align + params.

Signed-off-by: topkoa <topkoa@gmail.com>

* fix: make the error-body cap an actual bound; correct the docstring's endpoint

- _err_body() appended the truncation marker AFTER slicing to _MAX_ERR_BODY, so the result could
  exceed the cap it exists to enforce (4014 chars for a 4000 bound). A cap that is only a
  suggestion surprises exactly the callers who trust it — a log line, a job record persisted to
  disk and re-read on every load. The marker now fits inside the bound.

  It also stripped after measuring, so a short JSON body followed by kilobytes of trailing
  whitespace got truncated: real content cut to make room for blanks. Strip first, then measure.

- The public docstring still advertised /align — the exact contract this PR exists to change, in
  the one place a reader would look for it. It now says what the function does and why, and that
  an older server answers 404.

Found by Copilot and CodeRabbit on #959.

Signed-off-by: topkoa <topkoa@gmail.com>

* fix: keep the exception line when truncating — the tail is the answer

_err_body() kept only the HEAD of an over-long body. On a traceback the last line is the
diagnosis, and the docstring said exactly that while the code threw it away: a 4000-char window
holding "Traceback (most recent call last)" and none of the exception is a window onto nothing.
Same mistake as the 300-char cap it replaced, one level up — cutting off precisely the part the
function exists to preserve.

Head AND tail now, both inside the bound: two thirds head (what was being attempted), one third
tail (what actually went wrong), with the marker between them. Verified the test FAILS against
head-only truncation.

Found by Copilot on #959.

Signed-off-by: topkoa <topkoa@gmail.com>

* fix: every failure out of transcribe_vocals_remote() is a RuntimeError; 404 says why

The docstring promised one failure mode — RuntimeError — and the caller (_maybe_transcribe_lyrics)
catches exactly that so one song's failed lyrics don't take down the batch around it. But a DNS
failure, a timeout, a reset connection or an unreadable stem escaped as requests.RequestException
or OSError, walked straight past that handler, and turned "this song's lyrics failed" into "the
whole batch died".

A 404 now explains itself. Bare "404" sends someone hunting for a typo in their server URL; the
real answer is that their server predates /transcribe, and we are the only ones in a position to
know that.

Found by Copilot on #959.

Signed-off-by: topkoa <topkoa@gmail.com>

---------

Signed-off-by: topkoa <topkoa@gmail.com>

lib/lyrics_transcribe.py

@@ -23,9 +23,18 @@ Engine selection
 Two transcription paths share a common output:
 
 * `transcribe_vocals_remote(path, server_url, ...)` — POST the vocal
-  stem to the `/align` endpoint on a feedBack-demucs-server (got-feedBack's
-  reference server already hosts WhisperX alongside Demucs at the same
-  URL).
+  stem to the `/transcribe` endpoint on a feedBack-demucs-server
+  (got-feedBack's reference server already hosts WhisperX alongside
+  Demucs at the same URL).
+
+  It used to POST to `/align`, which is *forced alignment* — "here are
+  the lyrics, tell me when each word is sung". Its `text` field is
+  required and we have no lyrics (transcribing them is the point), so
+  the server answered 422 from FastAPI's validation layer before its
+  handler ran, and remote transcription never worked for anyone
+  (feedBack-plugin-stem-splitter#17). `/transcribe` takes only audio.
+  Requires feedBack-demucs-server ≥ the revision adding that endpoint;
+  an older server answers 404 and the error says so.
 
 * `transcribe_vocals_local(path, ...)` — load WhisperX in-process. Heavy
   (~3 GB of model weights for `large-v2` + the wav2vec2 aligner) and
@@ -416,6 +425,38 @@ def transcribe_vocals_local(
 
 # ── Remote transcription ────────────────────────────────────────────────────
 
+_MAX_ERR_BODY = 4000
+
+
+def _err_body(resp) -> str:
+    """The server's error body, whole if it plausibly is one, and marked when it isn't.
+
+    This was capped at 300 chars, which is enough for "Internal Server Error" and not much else.
+    The bodies carrying the most diagnosis are the long ones — a FastAPI validation body naming
+    the field it rejected, a 500 whose traceback answers on its LAST line — and those are exactly
+    the ones a 300-char cap decapitates. The cap survives so a server answering with a 2 MB HTML
+    error page can't dump a novel into a log line.
+    """
+    # Strip FIRST, then measure: a body that is 300 chars of JSON and 3900 of trailing whitespace
+    # is not a long body, and truncating it would cut real content to make room for blanks.
+    text = (getattr(resp, "text", "") or "").strip()
+    if len(text) <= _MAX_ERR_BODY:
+        return text
+
+    # Keep the HEAD **and the TAIL**. Head-only truncation throws away the exception line — and
+    # on a traceback the exception line is the answer. This docstring said as much while the code
+    # did the opposite: it cut off precisely the part it exists to preserve, which is the same
+    # mistake, one level up, as the 300-char cap it replaced.
+    #
+    # The marker sits inside the bound, not past it: otherwise _MAX_ERR_BODY is a suggestion, and
+    # the callers who trust it (a log line, a job record persisted to disk) are the ones surprised.
+    marker = f"\n… [truncated, {len(text)} chars total] …\n"
+    budget = max(0, _MAX_ERR_BODY - len(marker))
+    head = budget * 2 // 3          # context: what was being attempted
+    tail = budget - head            # verdict: what actually went wrong
+    return text[:head].rstrip() + marker + text[len(text) - tail:].lstrip()
+
+
 def transcribe_vocals_remote(
     vocals_path: Path,
     server_url: str,
@@ -426,7 +467,17 @@ def transcribe_vocals_remote(
     min_word_score: float = 0.35,
     progress_cb: ProgressCB = None,
 ) -> list[dict]:
-    """POST the vocal stem to `{server_url}/align` and parse the response.
+    """POST the vocal stem to `{server_url}/transcribe` and parse the response.
+
+    NOT `/align` — that endpoint is forced alignment ("here are the lyrics,
+    tell me when each word is sung") and its `text` field is required. We
+    have no lyrics; producing them is the point. Posting there returned a
+    422 from FastAPI's validation layer before the server's handler ran, so
+    remote transcription never worked at all
+    (feedBack-plugin-stem-splitter#17).
+
+    Requires a feedBack-demucs-server carrying `/transcribe`; an older one
+    answers 404 and the raised error says so.
 
     Expects the server to respond with a JSON object carrying a `words` (or
     `segments`) field in WhisperX's native shape; `_whisperx_to_sloppak`
@@ -454,21 +505,53 @@ def transcribe_vocals_remote(
     if api_key:
         headers["Authorization"] = f"Bearer {api_key}"
 
-    params: dict[str, str] = {}
+    # POST to /transcribe, not /align.
+    #
+    # /align is FORCED ALIGNMENT: "here are the lyrics, tell me when each word is sung". Its
+    # `text` field is required, and we have no lyrics — transcription is the whole point. So the
+    # server rejected every request with a 422 in FastAPI's validation layer, before its handler
+    # ever ran, and remote transcription has never worked for anyone. /transcribe answers the
+    # question we are actually asking and takes only the audio.
+    # (feedBack-plugin-stem-splitter#17; endpoint added in feedBack-demucs-server#14.)
+    #
+    # `language` goes in the FORM BODY, not the query string: the server reads it with
+    # Form(""), and a query param would be silently ignored — so an explicit language hint would
+    # do nothing and Whisper's auto-detection would quietly decide instead, which is exactly the
+    # kind of "it works but it's wrong" that hides for months.
+    form: dict[str, str] = {}
     if language:
-        params["language"] = language
+        form["language"] = language
 
-    with open(vocals_path, "rb") as f:
-        resp = requests.post(
-            f"{server_url}/align",
-            files={"file": (vocals_path.name, f, "audio/ogg")},
-            params=params,
-            headers=headers or None,
-            timeout=timeout,
+    # Everything that can go wrong out here comes back as RuntimeError, which is what the
+    # docstring promises and what the caller catches. A DNS failure, a timeout, a reset
+    # connection or an unreadable stem file would otherwise surface as requests.RequestException
+    # or OSError and escape the one handler written to log-and-continue — turning "this song's
+    # lyrics failed" into "the whole batch died".
+    try:
+        with open(vocals_path, "rb") as f:
+            resp = requests.post(
+                f"{server_url}/transcribe",
+                files={"file": (vocals_path.name, f, "audio/ogg")},
+                data=form or None,
+                headers=headers or None,
+                timeout=timeout,
+            )
+    except requests.RequestException as e:
+        raise RuntimeError(f"could not reach the WhisperX server at {server_url}: {e}") from e
+    except OSError as e:
+        raise RuntimeError(f"could not read the vocal stem {vocals_path.name}: {e}") from e
+
+    if resp.status_code == 404:
+        # The endpoint isn't there. Say what that means, because "404" on its own sends someone
+        # hunting for a typo in their URL when the real answer is that their server predates the
+        # feature. (feedBack-demucs-server#14 added /transcribe.)
+        raise RuntimeError(
+            f"the WhisperX server at {server_url} has no /transcribe endpoint (404) — it "
+            f"predates remote transcription support. Update the server, or use 'Check for "
+            f"update' if it is the plugin-managed one."
         )
-
     if resp.status_code != 200:
-        raise RuntimeError(f"WhisperX server error ({resp.status_code}): {resp.text[:300]}")
+        raise RuntimeError(f"WhisperX server error ({resp.status_code}): {_err_body(resp)}")
 
     data = resp.json()