fix(cava): throw instead of exit() on a bad config

loadConfig() called exit(EXIT_FAILURE) when the cava config failed to load or
no input source was available, killing the whole bar. Throw std::runtime_error
instead: the factory/bar catch it at construction and disable only the cava
module. The read_thread_ also calls loadConfig() at runtime, so contain the
throw there too, logging instead of terminating.

Fixes #4456

src/modules/cava/cava_backend.cpp

@@ -2,6 +2,8 @@
 
 #include <spdlog/spdlog.h>
 
+#include <stdexcept>
+
 std::shared_ptr<waybar::modules::cava::CavaBackend> waybar::modules::cava::CavaBackend::inst(
     const Json::Value& config) {
   static auto* backend = new CavaBackend(config);
@@ -20,7 +22,13 @@ waybar::modules::cava::CavaBackend::CavaBackend(const Json::Value& config) : con
       spdlog::warn("Cava backend. Read source error: {0}", e.what());
     }
     read_thread_.sleep_for(fetch_input_delay_);
+    // loadConfig() now throws on failure; contain it so a runtime config error
+    // logs instead of terminating the process (#4456).
+    try {
       loadConfig();
+    } catch (const std::exception& e) {
+      spdlog::error("{}", e.what());
+    }
   };
   // Write outcoming data. Emit signals
   out_thread_ = [this] {
@@ -175,8 +183,9 @@ void waybar::modules::cava::CavaBackend::loadConfig() {
   error_.length = 0;
 
   if (!load_config(cfgPath, &prm_, &error_)) {
-    spdlog::error("cava backend. Error loading config. {0}", error_.message);
-    exit(EXIT_FAILURE);
+    // Throw, don't exit(): a bad config must disable only the cava module, not
+    // kill the whole bar (#4456). Caught by the factory (ctor) / read_thread_.
+    throw std::runtime_error(std::string{"cava backend: error loading config: "} + error_.message);
   }
 
   // Override cava parameters by the user config
@@ -252,8 +261,7 @@ void waybar::modules::cava::CavaBackend::loadConfig() {
   input_source_ = get_input(&audio_data_, &prm_);
 
   if (!input_source_) {
-    spdlog::error("cava backend API didn't provide input audio source method");
-    exit(EXIT_FAILURE);
+    throw std::runtime_error("cava backend: API didn't provide an input audio source");
   }
 
   prm_.output = ::cava::output_method::OUTPUT_RAW;