asr: reject undersized streaming transcript buffers

tools/omnivoice/tests/ffi-parakeet-asr.cpp

@@ -166,10 +166,19 @@ int main() {
             CHECK(count_words(fin) + 3 >= count_words(text) && count_words(fin) <= count_words(text) + 3,
                   "concurrent stream word count %zu far from batch %zu", count_words(fin), count_words(text));
             std::printf("concurrent feed/partial: %d polls, %zu words\n", polls, count_words(fin));
-            // a tiny buffer: truncated, NUL-terminated, length < capacity
+            // Insufficient output space must not return a successful prefix.
             char tiny[5];
             rc = eliza_inference_asr_stream_partial(cs, tiny, sizeof(tiny), nullptr, nullptr, &err);
-            CHECK(rc >= 0 && rc < (int) sizeof(tiny) && tiny[rc] == '\0', "tiny buffer rc %d", rc);
+            CHECK(rc < 0, "tiny partial buffer must fail, got %d", rc);
+            eliza_inference_free_string(err);
+            err = nullptr;
+            rc = eliza_inference_asr_stream_finish(cs, tiny, sizeof(tiny), nullptr, nullptr, &err);
+            CHECK(rc < 0, "tiny final buffer must fail, got %d", rc);
+            eliza_inference_free_string(err);
+            err = nullptr;
+            char retry[4096];
+            rc = eliza_inference_asr_stream_finish(cs, retry, sizeof(retry), nullptr, nullptr, &err);
+            CHECK(rc > 0 && std::strcmp(fin, retry) == 0, "retry after small buffer lost transcript");
             eliza_inference_asr_stream_close(cs);
         }
     }

tools/parakeet/include/parakeet.h

@@ -105,7 +105,8 @@ pk_stream * pk_stream_open(pk_model * model, pk_stream_params params);
 int pk_stream_feed(pk_stream * st, const float * pcm16k, size_t n_samples);
 /* Decode if at least min_hop_ms of new audio arrived since the last decode,
  * then write committed + tentative text. Returns bytes written (excluding
- * NUL) or a negative pk_status. */
+ * NUL) or a negative pk_status. An insufficient output buffer returns
+ * PK_E_INVALID without partial text; retry with more space. */
 int pk_stream_partial(pk_stream * st, char * out, size_t cap, pk_cancel_cb cancel, void * cancel_user);
 /* Decode the remaining audio with full right context and write the final
  * transcript. */

tools/parakeet/src/pk-stream.cpp

@@ -345,14 +345,13 @@ static int write_text(pk_stream * s, char * out, size_t cap) {
     all.insert(all.end(), s->tentative.begin(), s->tentative.end());
     const std::string t = detokenize(s->m, all.data(), all.size());
     if (!out || cap == 0) return (int) t.size();
-    size_t n = std::min(t.size(), cap - 1);
-    // never cut a UTF-8 sequence: back off over continuation bytes
-    if (n < t.size()) {
-        while (n > 0 && ((unsigned char) t[n] & 0xC0) == 0x80) --n;
+    if (t.size() >= cap) {
+        out[0] = '\0';
+        return PK_E_INVALID;
     }
-    std::memcpy(out, t.data(), n);
-    out[n] = '\0';
-    return (int) n;
+    std::memcpy(out, t.data(), t.size());
+    out[t.size()] = '\0';
+    return (int) t.size();
 }
 
 extern "C" int pk_stream_partial(pk_stream * s, char * out, size_t cap, pk_cancel_cb cancel, void * cu) {