fix(parameters): initialize the recursive param mutex in param_init()

The param access mutex must be recursive because a layered param lookup
re-enters the lock on the same thread (Layer::get() -> _parent->get()).

Initialize it in param_init() - which runs once before any thread accesses
parameters - via pthread_mutexattr_settype(PTHREAD_MUTEX_RECURSIVE), replacing
the previous pthread_once lazy init. This avoids the non-portable glibc static
initializer PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP, which is unavailable on QURT
and gated behind _GNU_SOURCE elsewhere (broke the voxl2 and SITL builds).

src/lib/parameters/atomic_transaction.cpp

@@ -34,10 +34,18 @@
 #include "atomic_transaction.h"
 
 #ifdef __PX4_POSIX
+// The mutex is recursive because a param lookup walks the layered param stack
+// holding the lock and delegates to the parent layer (Layer::get() ->
+// _parent->get()), which takes the same mutex again on the same thread.
+//
+// The recursive attribute is set at runtime rather than via a static initializer:
+// PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP is a glibc extension that is not portable
+// (unavailable on e.g. QURT, and gated behind _GNU_SOURCE elsewhere). initialize()
+// is called from param_init(), before any thread accesses parameters, so there is
+// no lazy-init race.
 static pthread_mutex_t _param_mutex;
-static pthread_once_t _param_mutex_once = PTHREAD_ONCE_INIT;
 
-static void _init_param_mutex()
+void AtomicTransaction::initialize()
 {
 	pthread_mutexattr_t attr;
 	pthread_mutexattr_init(&attr);
@@ -48,7 +56,6 @@ static void _init_param_mutex()
 
 pthread_mutex_t *AtomicTransaction::_get_mutex()
 {
-	pthread_once(&_param_mutex_once, _init_param_mutex);
 	return &_param_mutex;
 }
 #endif

src/lib/parameters/atomic_transaction.h

@@ -54,6 +54,12 @@ private:
 #endif
 
 public:
+#ifdef __PX4_POSIX
+	// Initialize the recursive param mutex. Must be called once before any thread
+	// accesses parameters (done from param_init()).
+	static void initialize();
+#endif
+
 	AtomicTransaction()
 	{
 		lock();

src/lib/parameters/parameters.cpp

@@ -130,6 +130,11 @@ static pthread_mutex_t file_mutex  =
 void
 param_init()
 {
+#ifdef __PX4_POSIX
+	// Set up the recursive param mutex before any thread can access parameters.
+	AtomicTransaction::initialize();
+#endif
+
 	param_export_perf = perf_alloc(PC_ELAPSED, "param: export");
 	param_find_perf = perf_alloc(PC_COUNT, "param: find");
 	param_get_perf = perf_alloc(PC_COUNT, "param: get");