1 Basic Disposable API and Concepts
A disposable is a producer of values that allocates external resources when producing a value and provides a way to deallocate those resources when a produced value is no longer needed. A value produced by a disposable is called a disposable value.
Disposables are closely related to custodians, but differ in what sort of resources they manage and how those resources are reclaimed. Broadly speaking, resources requiring management can be divided into two categories:
System resources —
resources acquired from a program’s execution environment such as memory, file descriptors, operating system threads, handles to other local processes, etc. External resources —
resources managed by arbitrary external systems that a program communicates with, such as test data in a database, network connections with graceful termination protocols, machines used to execute distributed data processing, temporary access credentials acquired from a remote key management service, etc.
These two kinds of resources have very different implications for programs that use them. Due to the distributed nature of external resources and the inherent unreliability of network communication, it is impossible to guarantee that a given external resource is released. However, system resources are very rarely impossible to release. Custodians are designed to mangage system resources and assume reliable reclamation is possible, placing several restrictions on how programs can use custodians as a result:
It must be possible to forcibly reclaim system resources during program termination as occurs when custodian-shutdown-all is called. This prevents reclaiming resources by communicating over a network because it’s impossible to guarantee successful distributed communication.
Placing a user-defined resource under the management of a custodian requires using the ffi/unsafe/custodian module. This is an unsafe API because custodian shutdown actions are executed in atomic mode, resulting in possible deadlocks if shutdown actions perform asynchronous IO.
Custodian shutdown callbacks normally should not strongly reference the values they’re meant to clean up, as these shutdown callbacks frequently double as finalizers that run when the garbage collector determines the value to finalize is no longer reachable. This defeats certain composition patterns for shutdown callbacks, particularly composition that uses closures to reference both composed callbacks and managed values.
These restrictions make managing external resources with custodians inappropriate. Instead, an external resource should be produced by a disposable resulting in a disposable value that can be safely managed in a distributed system.
All of the bindings documented in this section are provided by disposable with the exception of acquire!, which is provided by disposable/unsafe.
1.1 Data Definition
Concretely, a disposable is implemented as a thunk that when called allocates a new disposable value and returns that value alongside another thunk that deallocates the external resources associated with that particular disposable value.
procedure
(disposable alloc dealloc) → disposable?
alloc : (-> any/c) dealloc : (-> any/c void?)
procedure
(make-disposable proc) → disposable?
proc : (-> (values any/c (-> void?)))
procedure
(disposable? v) → boolean?
v : any/c
procedure
(disposable/c c) → contract?
c : contract?
1.2 Consuming Disposable Values
Disposable values can be acquired in a variety of ways, ranging from unsafe low level access with acquire! to automated per-thread allocation with acquire-virtual.
syntax
(with-disposable ([id disp-expr] ...) body ...+)
disp-expr : disposable?
> (with-disposable ([x example-disposable] [y example-disposable]) (- x y))
Allocated 55
Allocated 94
Deallocated 55
Deallocated 94
39
> (with-disposable ([n example-disposable]) (error "uh oh!"))
Allocated 27
Deallocated 27
uh oh!
procedure
(call/disposable disp proc) → any
disp : disposable? proc : (-> any/c any)
> (call/disposable example-disposable (λ (n) (* n n)))
Allocated 64
Deallocated 64
4096
> (call/disposable example-disposable (λ (_) (error "uh oh!")))
Allocated 24
Deallocated 24
uh oh!
procedure
disp : disposable? evt : evt? = (thread-dead-evt (current-thread))
> (sync (thread (λ () (define n (acquire example-disposable)) (printf "Acquired ~v\n" n))))
Allocated 99
Acquired 99
Deallocated 99
#<thread>
procedure
(acquire-global disp [#:plumber plumber]) → any/c
disp : disposable? plumber : plumber? = (current-plumber)
> (define plumb (make-plumber)) > (define n (acquire-global example-disposable #:plumber plumb)) Allocated 61
> (add1 n) 62
> (plumber-flush-all plumb) Deallocated 61
procedure
(acquire-virtual disp) → (-> any/c)
disp : disposable?
> (define virtual-example (acquire-virtual example-disposable))
> (define (spawn) (thread (λ () (printf "Acquired ~v\n" (virtual-example))))) > (sync (spawn) (spawn) (spawn))
Allocated 87
Acquired 87
Allocated 45
Acquired 45
Allocated 47
Acquired 47
Deallocated 47
Deallocated 45
Deallocated 87
#<thread>
1.3 Unsafe Allocation of Disposables
(require disposable/unsafe) | package: disposable |
The disposable/unsafe module provides a single export, acquire!, which provides an unsafe building block upon which safe allocation abstractions can be built.
procedure
(acquire! disp) →
any/c (-> void?) disp : disposable?
> (define-values (n dispose!) (acquire! example-disposable)) Allocated 51
> (printf "Acquired ~v unsafely\n" n) Acquired 51 unsafely
> (dispose!) Deallocated 51
1.4 Monadic Composition of Disposables
procedure
(disposable-pure v) → disposable?
v : any/c
> (with-disposable ([n (disposable-pure 42)]) n) 42
procedure
(disposable-apply f disp ...) → disposable?
f : procedure? disp : disposable?
> (struct posn (x y) #:transparent)
> (define disposable-posn (disposable-apply posn example-disposable example-disposable))
> (with-disposable ([p disposable-posn]) (printf "Acquired ~v\n" p))
Allocated 10
Allocated 78
Acquired (posn 78 10)
Deallocated 10
Deallocated 78
procedure
(disposable-chain disp f) → disposable?
disp : disposable? f : (-> any/c disposable?)
> (define (add-example x) (disposable-apply (λ (y) (+ x y)) example-disposable)) > (define sum-disp (disposable-chain example-disposable add-example))
> (with-disposable ([v sum-disp]) (printf "Acquired ~v\n" v))
Allocated 70
Allocated 92
Acquired 162
Deallocated 92
Deallocated 70
1.5 Asynchronous Cleanup
procedure
(disposable/async-dealloc disp) → disposable?
disp : disposable?
> (with-disposable ([n (disposable/async-dealloc example-disposable)]) (printf "Acquired ~v\n" n))
Allocated 44
Acquired 44
Deallocated 44
> (sleep 0.1)
1.6 Disposables with Associated Custodians
procedure
(disposable/custodian disp cust) → disposable?
disp : disposable? cust : custodian?
Added in version 0.4 of package disposable.
1.7 Memoization and Disposables
procedure
(disposable/memoize f [ #:make-dict make-dict]) → (disposable/c procedure?) f : (unconstrained-domain-> disposable?) make-dict : (-> (and/c dict? dict-mutable?)) = make-hash
> (define (color+ex-disp c) (disposable-apply list (disposable-pure c) example-disposable))
> (with-disposable ([color+ex (disposable/memoize color+ex-disp)]) (displayln (color+ex "red")) (displayln (color+ex "blue")) (displayln (color+ex "red")))
Allocated 52
(red 52)
Allocated 16
(blue 16)
(red 52)
Deallocated 16
Deallocated 52
Reuse of values by the memoized f is achieved by mapping arguments (both positional and keyword) to allocated values in a mutable dictionary. Allocation of the memoized f creates that mutable dictionary using make-dict, which by default produces a mutable hash table that holds keys and values strongly.
Added in version 0.5 of package disposable.