MINOR: debug: add a new BUG_ON macro

We do have some code paths testing for impossible errors that tend to
be quite confusing, first for maintenance (what to do on such errors,
and how far to guess the bug), second for developers as it tends to
hide the main purpose and expectations of these call places. Also
most of the time impossible errors are ignored by the callers so the
tests are not even usable during debugging.

Let's instead implement a BUG_ON macro which takes a condition, which
if true, will cause a message to be emitted and optionally to crash the
process. Additionally, these calls inserted at various places server as
hints and documentation for developers to know that such conditions
must absolutely not happen.

This is only enabled when DEBUG_STRICT or DEBUG_STRICT_NOCRASH are set.
As its name implies, DEBUG_STRICT_NOCRASH only performs the test but
does not crash, which can be useful to track some checkpoints.

At the moment nothing uses this code.

include/common/debug.h

@@ -42,6 +42,32 @@
  */
 #define ABORT_NOW() (*(volatile int*)1=0)
 
+/* BUG_ON: complains if <cond> is true when DEBUG_STRICT or DEBUG_STRICT_NOCRASH
+ * are set, does nothing otherwise. With DEBUG_STRICT in addition it immediately
+ * crashes using ABORT_NOW() above.
+ */
+#if defined(DEBUG_STRICT) || defined(DEBUG_STRICT_NOCRASH)
+#if defined(DEBUG_STRICT)
+#define CRASH_NOW() ABORT_NOW()
+#else
+#define CRASH_NOW()
+#endif
+
+#define BUG_ON(cond) _BUG_ON(cond, __FILE__, __LINE__)
+#define _BUG_ON(cond, file, line) __BUG_ON(cond, file, line)
+#define __BUG_ON(cond, file, line)                                             \
+	do {                                                                   \
+		if (unlikely(cond)) {					       \
+			const char msg[] = "\nFATAL: bug condition \"" #cond "\" matched at " file ":" #line "\n"; \
+			(void)write(2, msg, strlen(msg));                      \
+			CRASH_NOW();                                           \
+		}                                                              \
+	} while (0)
+#else
+#undef CRASH_NOW
+#define BUG_ON(cond)
+#endif
+
 /* this one is provided for easy code tracing.
  * Usage: TRACE(strm||0, fmt, args...);
  *        TRACE(strm, "");