Free all possibly held mutexes in error handlers

Now that we have 3 mutexes lying around, it's important that we check
and free these if necessary if error() is called in any thread that can
hold them.  To do this, we now have flexthread.c, which defines a
flexthread_mutex struct.  This is a wrapper around a pthread_mutex_t and
a pthread_t.  The idea is that in the error handler, the thread can
check whether it holds the mutex and can free it if and only if it does.
This is important because pthread fast mutexes can be freed by *any*
thread, not just the thread which holds them.

Note: it is only ever safe for a thread to check if it holds the mutex
itself.  It is *never* safe to check if another thread holds a mutex
without first locking that mutex, which makes the whole operation rather
pointless.

src/flexthread.h

@@ -0,0 +1,29 @@
+#ifndef FLEXTHREAD_H
+#define FLEXTHREAD_H
+
+#include <pthread.h>
+
+/* Define a mutex wrapper object.  This wrapper allows us to easily
+ * track whether or not we currently hold the wrapped mutex.  If we hold
+ * the mutex when we destroy it, then we first release it.
+ *
+ * These are specifically for the case where an ERROR_* handler gets
+ * called when we might (or might not) have a mutex held.  The
+ * flexthread_mutex_held() function will tell you if your thread
+ * currently holds the given mutex.  It's not safe to make any other
+ * comparisons.
+ */
+
+struct flexthread_mutex {
+	pthread_mutex_t mutex;
+	pthread_t holder;
+};
+
+struct flexthread_mutex * flexthread_mutex_create(void);
+void flexthread_mutex_destroy( struct flexthread_mutex * );
+
+int flexthread_mutex_lock( struct flexthread_mutex * );
+int flexthread_mutex_unlock( struct flexthread_mutex * );
+int flexthread_mutex_held( struct flexthread_mutex * );
+
+#endif