BUG/MINOR: threads: fix multiple use of argument inside HA_ATOMIC_CAS()

When threads are disabled, HA_ATOMIC_CAS() becomes a simple compound
expression. However this expression presents a problem, which is that
its arguments are evaluated multiple times, once for the comparison
and once again for the assignement. This presents a risk of performing
some side-effect operations twice in the non-threaded case (e.g. in
case of auto-increment or function return).

The macro was rewritten using local copies for arguments like the
other macros do.

Fortunately a complete inspection of the code indicates that this case
currently never happens. It was however responsible for the strict-aliasing
warning emitted when building fd.c without threads but with 64-bit CAS.

This may be backported as far as 1.8 though it will not fix any existing
bug and is more of a long-term safety measure in case a future fix would
depend on this behavior.

include/common/hathreads.h

@@ -77,7 +77,12 @@ extern THREAD_LOCAL struct thread_info *ti; /* thread_info for the current threa
 #define __decl_rwlock(lock)
 #define __decl_aligned_rwlock(lock)
 
-#define HA_ATOMIC_CAS(val, old, new) ({((*val) == (*old)) ? (*(val) = (new) , 1) : (*(old) = *(val), 0);})
+#define HA_ATOMIC_CAS(val, old, new)                                    \
+	({                                                              \
+		typeof(val) _v = (val);                                 \
+		typeof(old) _o = (old);                                 \
+		(*_v == *_o) ? ((*_v = (new)), 1) : ((*_o = *_v), 0);   \
+	})
 
 /* warning, n is a pointer to the double value for dwcas */
 #define HA_ATOMIC_DWCAS(val, o, n)				       \