Some POSIX threads implementations (e.g. FreeBSD's libthr) allocate
memory on the heap when pthread_rwlock_init() is called. Every call to
that function must be accompanied by a corresponding call to
pthread_rwlock_destroy() or else the memory allocated for the rwlock
will leak.
jemalloc can be used for detecting memory allocations which are not
released by a process when it exits. Unfortunately, since jemalloc is
also the system allocator on FreeBSD and a special (profiling-enabled)
build of jemalloc is required for memory leak detection, this method
cannot be used for detecting leaked memory allocated by libthr on a
stock FreeBSD installation.
However, libthr's behavior can be emulated on any platform by
implementing alternative versions of libisc functions for creating and
destroying rwlocks that allocate memory using malloc() and release it
using free(). This enables using jemalloc for detecting missing
pthread_rwlock_destroy() calls on any platform on which it works
reliably.
When the newly introduced ISC_TRACK_PTHREADS_OBJECTS preprocessor macro
is set (and --enable-pthread-rwlock is used), allocate isc_rwlock_t
structures on the heap in isc_rwlock_init() and free them in
isc_rwlock_destroy(). Reuse existing functions defined in
lib/isc/rwlock.c for other operations, but rename them first, so that
they contain triple underscores (to indicate that these functions are
implementation-specific, unlike their mutex and condition variable
counterparts, which always use the pthreads implementation). Define the
isc__rwlock_init() macro so that it is a logical counterpart of
isc__mutex_init() and isc__condition_init(); adjust isc___rwlock_init()
accordingly. Remove a redundant function prototype for
isc__rwlock_lock() and rename that (static) function to rwlock_lock() in
order to avoid having to use quadruple underscores.
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