MDEV-12679 purge_sys_t::~purge_sys_t(): Assertion `latch.magic_n == 0' failed...

MDEV-12679 purge_sys_t::~purge_sys_t(): Assertion `latch.magic_n == 0' failed on --bootstrap (CMAKE_BUILD_TYPE=Debug)

rw_lock_free_func(): Assign lock->magic_n = 0 in debug builds,
because sometimes, the assignment in the explicit destructor call
is being optimized away.
This was verified when using GCC 6.3.0 with -O3 -g -mtune=native
on a Debian GNU/Linux 9.0 system on AMD64.

storage/innobase/sync/sync0rw.cc

@@ -277,6 +277,9 @@ rw_lock_free_func(
 
 	/* We did an in-place new in rw_lock_create_func() */
 	ut_d(lock->~rw_lock_t());
+	/* Sometimes (maybe when compiled with GCC -O3) the above call
+	to rw_lock_t::~rw_lock_t() will not actually assign magic_n=0. */
+	ut_d(lock->magic_n = 0);
 }
 
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