semaphore-utils🔗 i

A small library for working with semaphores.

This is useful when you two functions that share a semaphore and one of them needs to call the other – it lets you tell the inner one not to use the semaphore since the outer one is already doing so.

;Note: The following code is intended for simplicity of example. In real use it would be

;better to, e.g., not share mutable state between threads and, at a minimum, to expose

;separate versions of the get-* functions that do not allow passing a semaphore in. Also,

;handle the case where a user is not already in the hashes.

;

> (require racket/splicingsemaphore-utils)

> (splicing-let([sema(make-semaphore 1)]

[name->dept-id(make-hash (list (cons 'alice1)(cons 'bill1)(cons 'charlie2)))]

[dept-id->names(make-hash (list (cons 1(set 'alice'bob))(cons 2(set 'charlie))))])

(define (get-dept-idname[semasema])

(call/semasema(thunk (hash-ref name->dept-idname))))

(define (get-usersdept-id[semasema])

(call/semasema(thunk (hash-ref dept-id->namesdept-id))))

(define (add-user!namedept-id)

(call/semasema

(thunk

(hash-set! name->dept-idnamedept-id)

(hash-set! dept-id->names

dept-id

;(set-add (get-users dept-id) name))))) ; WRONG! This will deadlock! `sema` is already in use!

;Pass #f as the semaphore so that we don't deadlock

(set-add (get-usersdept-id#f)name))))))

;

;If the following functions were running in different threads, the call/sema code would ensure that the 'get-*' functions

;did not interleave with a call to 'add-user!' and thereby see inconsistent state

> (define alice-dept-id(get-dept-id'alice))

> (get-usersalice-dept-id)

#<set: alice bob>

> (add-user!'evanalice-dept-id)

> (get-usersalice-dept-id)

#<set: evan alice bob>