cxxrt guard: stop using cassert, check futex_wake result (#404)

compile_flags.txt

diff --git a/compile_flags.txt b/compile_flags.txt
index ae0cf0a..0ed75a0 100644
--- a/compile_flags.txt
+++ b/compile_flags.txt
@@ -22,6 +22,7 @@
 -DDEBUG_LOADER=true
 -DDEBUG_ALLOCATOR=true
 -DDEBUG_SCHEDULER=true
+-DDEBUG_CXXRT=true
 -DSAIL
 -DCPU_TIMER_HZ=2000
 -DTICK_RATE_HZ=10

sdk/lib/cxxrt/guard.cc

diff --git a/sdk/lib/cxxrt/guard.cc b/sdk/lib/cxxrt/guard.cc
index 04d7319..629a535 100644
--- a/sdk/lib/cxxrt/guard.cc
+++ b/sdk/lib/cxxrt/guard.cc
@@ -1,12 +1,14 @@
 // Copyright Microsoft and CHERIoT Contributors.
 // SPDX-License-Identifier: MIT
 
-#include <cassert>
 #include <cdefs.h>
+#include <debug.hh>
 #include <futex.h>
 #include <limits>
 #include <stdint.h>
 
+using Debug = ConditionalDebug<DEBUG_CXXRT, "cxxrt">;
+
 /**
  * The helper functions need to expose an unmangled name because the compiler
  * inserts calls to them.  Declare them using the asm label extension.
@@ -54,6 +56,8 @@
 
 		/**
 		 * Acquire the lock.
+		 *
+		 * This is safe only in IRQ-deferred context.
 		 */
 		void lock()
 		{
@@ -62,7 +66,7 @@
 			{
 				futex_wait(&high, LockBit);
 			}
-			assert(high == 0);
+			Debug::Assert(high == 0, "Corrupt guard word at {}", this);
 			high = LockBit;
 		}
 
@@ -71,9 +75,12 @@
 		 */
 		void unlock()
 		{
-			assert(high == LockBit);
-			high = 0;
-			futex_wake(&high, std::numeric_limits<uint32_t>::max());
+			Debug::Assert(high == LockBit, "Corrupt guard word at {}", this);
+			high    = 0;
+			int res = futex_wake(&high, std::numeric_limits<uint32_t>::max());
+			Debug::Assert(res >= 0,
+			              "futex_wake failed for guard {}; possible deadlock",
+			              this);
 		}
 
 		/**
@@ -109,8 +116,8 @@
 void __cxa_guard_release(uint64_t *guard)
 {
 	auto *g = reinterpret_cast<GuardWord *>(guard);
-	assert(!g->is_initialised());
-	assert(g->is_locked());
+	Debug::Assert(!g->is_initialised(), "Releasing uninitialized guard {}", g);
+	Debug::Assert(g->is_locked(), "Releasing unlocked guard {}", g);
 	g->set_initialised();
 	g->unlock();
 }