feat(audio-cpp): AUDIOCPP_DEFAULT_BACKEND fallback for models without a backend option (#12133)

Models whose options carry no explicit backend: open their session on the
CPU backend even in accelerator images. The gallery entries carry
backend:best since #11892; this covers hand-written model configurations
the same way, per deployment: the environment variable supplies the
fallback, an explicit backend: option always wins (merged beside the
existing threads and maingpu fallbacks), and validation reuses the
option parser.

Assisted-by: Claude:claude-fable-5

Signed-off-by: Plamen K. Kosseff <p.kosseff@gmail.com>

backend/cpp/audio-cpp/grpc-server.cpp

@@ -596,6 +596,25 @@ public:
             if (!parsed.options.device_set && !request->maingpu().empty()) {
                 parsed.options.device = parse_device_index(request->maingpu());
             }
+            // AUDIOCPP_DEFAULT_BACKEND is the deployment's backend fallback:
+            // an accelerator image sets it (typically to "best") so models
+            // without an explicit backend: option use the compiled
+            // accelerator instead of the CPU default — gallery entries carry
+            // backend:best, hand-written model configs get the same fix
+            // here. An explicit backend: option wins, matching threads and
+            // maingpu above. Validation reuses the option parser itself.
+            if (!parsed.options.backend_set) {
+                const char * env = std::getenv("AUDIOCPP_DEFAULT_BACKEND");
+                if (env != nullptr && *env != '\0') {
+                    auto fallback = audiocpp_backend::parse_model_options(
+                        {std::string("backend:") + env});
+                    if (!fallback.error.empty()) {
+                        throw audiocpp_backend::ConfigError(
+                            "audio-cpp: AUDIOCPP_DEFAULT_BACKEND: " + fallback.error);
+                    }
+                    parsed.options.backend = fallback.options.backend;
+                }
+            }
 
             const std::string path = audiocpp_backend::resolve_model_path(
                 request->modelpath(), request->modelfile(), request->model());

backend/cpp/audio-cpp/model_options.cpp

@@ -120,6 +120,7 @@ ParsedOptions parse_model_options(const std::vector<std::string> &entries) {
                 return parsed;
             }
             parsed.options.backend = value;
+            parsed.options.backend_set = true;
         } else if (key == "model_spec_override") {
             parsed.options.model_spec_override = value;
         } else if (key == "device") {

backend/cpp/audio-cpp/model_options.h

@@ -19,6 +19,10 @@ struct ModelOptions {
     std::string task;
     // ggml backend: cpu, cuda, hip (or rocm), vulkan, metal, best.
     std::string backend = "cpu";
+    // True once a `backend:` entry has been seen: "cpu" is both the default
+    // and a legitimate explicit choice, so the value alone cannot tell them
+    // apart, and a caller merging in its own fallback needs the difference.
+    bool backend_set = false;
     int device = 0;
     // True once a `device:` entry has been seen. 0 is both the default and a
     // legitimate device index, so the value alone cannot tell an explicit

backend/cpp/audio-cpp/model_options_test.cpp

@@ -42,6 +42,10 @@ static void test_defaults() {
     check(r.options.family.empty(), "family defaults to empty");
     check(r.options.task.empty(), "task defaults to empty");
     check(r.options.backend == "cpu", "backend defaults to cpu");
+    // backend_set separates the "cpu" default from an explicit backend:cpu —
+    // grpc-server merges the AUDIOCPP_DEFAULT_BACKEND fallback only when the
+    // model's options chose nothing.
+    check(!r.options.backend_set, "backend_set defaults to false");
     check(r.options.device == 0, "device defaults to 0");
     check(r.options.threads == 0, "threads defaults to 0");
     check(r.options.busy_timeout_ms == 0, "busy_timeout_ms defaults to 0");
@@ -68,6 +72,7 @@ static void test_scalar_options() {
     check(r.options.family == "qwen3_tts", "family parsed");
     check(r.options.task == "tts", "task parsed");
     check(r.options.backend == "cuda", "backend parsed");
+    check(r.options.backend_set, "backend_set records the explicit option");
     check(r.options.device == 1, "device parsed");
     check(r.options.threads == 8, "threads parsed");
     check(r.options.busy_timeout_ms == 30000, "busy_timeout_ms parsed");

backend/cpp/audio-cpp/run.sh

@@ -10,6 +10,11 @@ set -e
 
 CURDIR=$(dirname "$(realpath "$0")")
 
+# The image is built for one accelerator; models whose options carry no
+# explicit backend: should use it rather than the wrapper's CPU default.
+# An explicit backend: option and a caller's own environment both win.
+export AUDIOCPP_DEFAULT_BACKEND="${AUDIOCPP_DEFAULT_BACKEND:-best}"
+
 if [ "$(uname -s)" = "Darwin" ]; then
 	export DYLD_LIBRARY_PATH="$CURDIR/lib:$CURDIR:$DYLD_LIBRARY_PATH"
 	exec "$CURDIR/grpc-server" "$@"