A Lisp object.
A function designator for a function of no arguments, or
A Lisp form, or
with-ensuring-gethash gets the value for the key key from the hash table hash-table, and if this fails constructs a new value, puts it in the table and returns it.
with-ensuring-gethash does this in a thread-safe way, which means that all threads calling it with the same key and hash-table return the same value (as long as nothing removes it from the table).
Only one of constructor-form or constructor can be non-nil. When key is not found, constructor-form or constructor is used to construct the new value. If constructor is non-nil, it is called without arguments. If constructor-form is non-nil, it is executed. If both are
nil, the new value is
nil unless the in-lock-constructor or in-lock-constructor-form construct it. The call or execution of the constructor or constructor-form is done without any lock. The result may be discarded if, by the time it returned, there is a match for the key in the table.
Only one of in-lock-constructor or in-lock-constructor-form can be non-nil, which is used when the key is not found after constructing the new value. If in-lock-constructor-form is non-nil, it is executed and the result is the actual value to use (the result of the construction by constructor-form or constructor is ignored). If in-lock-constructor is non-nil, it is called with the result of the construction by constructor-form or constructor, and the result is used as the new value. In either case, the call or execution is done with hash-table locked, and the result is guaranteed to be put in hash-table and returned. If both in-lock-constructor and in-lock-constructor-form are
nil, the result of the construction is used.
nil, gethash-ensuring is probably simpler and better.
LispWorks User Guide and Reference Manual - 13 Feb 2015