[
https://issues.apache.org/jira/browse/GROOVY-12343?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18110919#comment-18110919
]
ASF GitHub Bot commented on GROOVY-12343:
-----------------------------------------
paulk-asert opened a new pull request, #2867:
URL: https://github.com/apache/groovy/pull/2867
A deadline can now be a branch of a ChannelSelect rather than an exception
thrown around it by orTimeout. AsyncChannel.after(millis) and after(Duration)
create a capacity-1 channel that delivers the Instant it fired and then closes,
in the manner of Go's time.After. Being an ordinary built-in channel it takes
part in the claim protocol, so a losing timer is withdrawn rather than
consumed, and it composes with per-branch deadlines, when() guards, fair() and
random().
The clock starts at creation. A timer that has delivered its value closes,
so a second receive fails with ChannelClosedException instead of waiting
forever. Closing a timer before it fires cancels the scheduled task; a negative
delay fires at once, which suits deadlines computed from an absolute time
already passed.
> Provide AsyncChannel.after
> --------------------------
>
> Key: GROOVY-12343
> URL: https://issues.apache.org/jira/browse/GROOVY-12343
> Project: Groovy
> Issue Type: Improvement
> Reporter: Paul King
> Assignee: Paul King
> Priority: Major
>
> h2. Summary
> A {{ChannelSelect}} cannot have a timeout branch. Every comparable system has
> one — it is how "wait for
> work, but not forever" is written — and so, already, does the OTHER half of
> this library: an actor can
> schedule a message to itself, a select cannot wait on a deadline.
> There is a workaround, and it is worth being precise about how far it goes,
> because it goes further than it
> first appears.
> h2. What a user writes today
> {{Awaitable.orTimeout}} wraps the whole select:
> {code:groovy}
> try {
> def r = await alt.select().orTimeoutMillis(100)
> ...
> } catch (TimeoutException e) {
> ... // the timeout branch
> }
> {code}
> This is *safe*. {{AsyncSupport.completeOnTimeout}}/{{orTimeout}} cancel the
> source when the deadline fires,
> and {{Winner.cancel}} is {{claim.tryCommitCancel() && super.cancel(…)}}, so
> the cancellation wins the claim
> and no offer commits. GROOVY-12320's protocol already makes the outer timeout
> correct — nothing is consumed
> and lost. That is worth saying plainly, since it is the first thing a
> reviewer will worry about.
> What it does not give:
> * *Exceptions for expected control flow.* A timeout that is part of the
> design is signalled by
> {{TimeoutException}} rather than by a branch.
> * *No fallback form.* {{completeOnTimeout}} needs a fallback value, and
> {{ChannelSelect.Result}}'s
> constructor is package-private — a caller cannot construct one. So only the
> throwing form is usable here.
> * *One deadline for the whole select.* Different deadlines on different
> branches need nested selects or
> extra tasks.
> * *No guarded timeout.* The deadline sits outside the select, so {{Offer.when
> \{ … \}}} (GROOVY-12326)
> cannot reach it. "Time out only while the buffer is empty" — never
> mid-transaction — is not expressible.
> * *No composition with the choice policy.* The timeout is not a branch, so it
> is outside {{fair()}} /
> {{random()}} rotation and outside the {{select(boolean...)}} mask.
> h2. What it enables
> Ordinary, and common:
> * wait for work, and on silence emit a heartbeat, retry, log, or shut down;
> * serve requests while a periodic tick flushes a batch, emits metrics, or
> polls for shutdown.
> Both are achievable today with try/catch. The two that are genuinely awkward
> are per-branch deadlines and
> the guarded timeout above.
> h2. The library is already inconsistent about this
> The strongest argument is not what other ecosystems do; it is that
> {{groovy.concurrent}} has already decided
> this question once, on the other side of the library. {{ActorContext}}
> carries:
> {code:java}
> Cancellable scheduleOnce(T message, java.time.Duration delay)
> Cancellable scheduleAtFixedRate(T message, java.time.Duration initial,
> java.time.Duration period)
> {code}
> So an actor can say "deliver me this in 100ms" and interleave it with the
> messages it is already handling —
> which is the actor spelling of exactly the shape a timer branch gives a
> select. The CSP half has no
> equivalent. {{AsyncChannel.after(…)}} is therefore not a new capability for
> the library so much as parity
> between its two halves, and the absence is more likely to read as an
> oversight than as a decision.
> The same asymmetry shows in the bounded forms: {{ActorOptions}} has
> {{withBoundedMailbox(n, Overflow)}} and
> {{withStashBound(n, StashOverflow)}}, so the actor side is already explicit
> about time and about bounds,
> while the channel side is explicit about neither.
> h2. Prior art — everyone has this
> || System || How the timeout appears || Timer is… ||
> | Go | {{case <-time.After(d):}} | a channel |
> | Rust (crossbeam-channel) | {{recv(after(timeout)) -> _ =>}} | a channel
> ({{after(Duration) -> Receiver<Instant>}}) |
> | Clojure core.async | {{(alts! \[ch (timeout 1000)\])}} | a channel |
> | Kotlin coroutines | {{select \{ onTimeout(ms) \{ … \} \}}} | a select
> clause |
> | Erlang | {{receive … after Timeout ->}} | language syntax |
> | Ada | {{select … or delay 5.0;}} | language syntax |
> | JCSP | {{CSTimer}} in the {{Guard\[\]}} | a guard kind |
> | *Groovy 6 today* | *— (outer wrapper only)* | *—* |
> Kotlin is the closest precedent for Groovy's situation: it has BOTH the outer
> wrapper
> ({{withTimeout}}/{{withTimeoutOrNull}}) and the branch ({{onTimeout}}).
> Having the wrapper is not treated as
> making the branch redundant.
> Note what Go, Rust and Clojure share: *the timer is a channel*, not a special
> branch kind. That shapes the
> proposal below.
> h2. Option A (preferred) — a timer CHANNEL
> {code:java}
> public static AsyncChannel<java.time.Instant> after(long millis)
> public static AsyncChannel<java.time.Instant> after(java.time.Duration
> duration)
> {code}
> A capacity-1 channel that delivers one element — the instant it fired — after
> the given delay.
> {code:groovy}
> def alt = ChannelSelect.from(work, AsyncChannel.after(100))
> def r = await alt.select()
> if (r.index == 1) { ... timed out ... }
> {code}
> *Why this one.* It needs *no change to {{ChannelSelect}} at all*, and it
> composes with everything already
> there for free: {{from(…)}} and {{offers(receive(…))}}, {{Offer.when \{ …
> \}}} for the guarded timeout,
> {{fair()}} / {{random()}} rotation, the {{select(boolean...)}} mask, and
> {{Result.getChannel()}} (which
> identifies the timer like any other branch, rather than having to return
> null). It is also useful *outside*
> a select — {{await AsyncChannel.after(50).first()}} as a delay, or as one arm
> of {{Awaitable.any}} — where a
> select-only offer would not reach. And it is the design three of the four
> comparable ecosystems chose.
> *Implementation.* A channel whose element is produced by one scheduled task,
> on the same scheduler
> {{AsyncSupport}} already uses for {{orTimeout}}. The single new concern is
> *cancellation*: when a select
> withdraws its losing branches the timer's {{ScheduledFuture}} should be
> cancelled too, or an abandoned
> select leaves a task pending until it fires. That is one thing to get right,
> against five for Option B.
> h2. Option B — a timer OFFER
> {code:java}
> public static ChannelSelect.Offer after(long millis)
> {code}
> Symmetric with the existing {{send(…)}} / {{receive(…)}} offers, and closest
> to JCSP's {{CSTimer}} guard.
> The cost is spread wider. {{Offer}} currently always has a channel, so it
> gains a nullable one plus a delay
> (or a subtype); {{Result.getChannel()}} must return null for a timer win, and
> say so; the registration loop
> in {{select(int\[\])}} gains a third path beside {{sendIfUnclaimed}} /
> {{receiveIfUnclaimed}} — though that
> path is largely written already, since a timer can compete for the claim
> exactly as a non-claimable foreign
> channel does, via {{winner.claim.tryCommit(future)}}. Withdrawal must still
> cancel the scheduled task.
> Option B is defensible if the symmetry with {{send}}/{{receive}} matters more
> than the reuse; it is listed
> so the choice is explicit rather than implied.
> h2. Open questions
> * *When does the clock start?* At creation, as in Go and crossbeam, is the
> obvious answer — and it means a
> fresh timer per loop iteration, so a long-running loop allocates one
> scheduled task per round. Go carries
> the same caveat for {{time.After}} in a loop and answers it with a
> resettable {{Timer}}. Worth deciding
> whether Groovy wants a resettable form now or later.
> * *Periodic ticks.* Go has {{time.Tick}} and crossbeam {{tick(d)}} — a
> channel that fires repeatedly. That
> is the natural companion for "every 100ms, flush", and is probably a
> separate issue rather than part of
> this one.
> * *Element type.* {{Instant}} matches crossbeam and is more useful than
> {{null}} or a token; Go sends the
> time too.
> h2. Provenance
> Found while porting Jon Kerridge's _Using Concurrency and Parallelism
> Effectively_ onto
> {{groovy.concurrent}} with a static verifier — the same exercise behind
> GROOVY-12320, GROOVY-12323,
> GROOVY-12324 and GROOVY-12326. Three of the book's chapters put a timer in an
> {{ALT}} (c05's scaling device,
> c14's hand-eye test, c17's sampling {{Sniffer}}), and none of them ports: the
> timeout is part of the choice
> in each, not a wrapper around it. With input, output and boolean guards now
> all present, the timer is the
> one member of JCSP's guard family Groovy's select still lacks.
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
