From 37af1e8726e6e94321a7319cabc5ed5d76428c72 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Tue, 25 Oct 2016 16:59:00 -0400
Subject: [PATCH] Cope with transient pulses of demand in during/actor

---
 racket/syndicate/actor.rkt                    | 27 +++++++++++++++++--
 .../actor/example-assertion-spike.rkt         | 27 +++++++++++++++++++
 racket/syndicate/protocol/instance.rkt        | 13 +++++++++
 3 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 racket/syndicate/protocol/instance.rkt

diff --git a/racket/syndicate/actor.rkt b/racket/syndicate/actor.rkt
index a1f3d3f..c067e4f 100644
--- a/racket/syndicate/actor.rkt
+++ b/racket/syndicate/actor.rkt
@@ -92,6 +92,7 @@
 (require "support/hash.rkt")
 (require "pretty.rkt")
 (require "functional-queue.rkt")
+(require "protocol/instance.rkt")
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Data Definitions and Structures
@@ -443,9 +444,31 @@
        (analyze-pattern E-stx #'P))
      (quasisyntax/loc stx
        (on #,E-stx
-           (let ((p #,instantiated))
+           (let* ((id (gensym 'during/actor))
+                  (p #,instantiated) ;; this is the concrete assertion corresponding to demand
+                  (inst (instance id p))) ;; this is the assertion representing supply
+             (react (stop-when (asserted inst)
+                               ;; Supply (inst) appeared before demand (p) retracted.
+                               ;; Transition to a state where we monitor demand, but also
+                               ;; express interest in supply: this latter acts as a signal
+                               ;; to the supply that it should stick around. We could, if
+                               ;; we liked, react to retraction of supply before
+                               ;; retraction of demand, interpreting it perhaps as a crash
+                               ;; of some kind. Once demand is retracted, this facet
+                               ;; terminates, retracting its interest in supply, thereby
+                               ;; signalling to the supply that it is no longer wanted.
+                               (react (stop-when (retracted inst)) ;; NOT OPTIONAL
+                                      (stop-when (retracted p))))
+                    (stop-when (retracted p)
+                               ;; Demand (p) retracted before supply (inst) appeared. We
+                               ;; MUST wait for the supply to fully appear so that we can
+                               ;; reliably tell it to shut down. We must maintain interest
+                               ;; in supply until we see supply, and then terminate, thus
+                               ;; signalling to supply that it is no longer wanted.
+                               (react (stop-when (asserted inst)))))
              (w.wrapper #:name name.N
-              (stop-when (retracted p))
+              (assert inst)
+              (stop-when (retracted (observe inst)))
               O ...))))]))
 
 (define-syntax (begin/dataflow stx)
diff --git a/racket/syndicate/examples/actor/example-assertion-spike.rkt b/racket/syndicate/examples/actor/example-assertion-spike.rkt
index 693a17d..d892682 100644
--- a/racket/syndicate/examples/actor/example-assertion-spike.rkt
+++ b/racket/syndicate/examples/actor/example-assertion-spike.rkt
@@ -1,4 +1,31 @@
 #lang syndicate/actor
+;;
+;; Test case for a problem written up on 25 Oct 2016 in my research
+;; journal.
+;;
+;; When the problem exists, we see only "starting", and not
+;; "stopping", because by the time the spawn action producing the 'up
+;; actor is processed, 'up has already been retracted (by the
+;; termination of 'asserter).
+;;
+;; This is an expectation/operation mismatch.
+;;
+;; The approach I've chosen is to *label* each instance of a
+;; `during/actor` using the new syndicate/protocol/instance, and to
+;; change the lifetime-control protocol between the spawner and
+;; spawnee to match.
+;;
+;; PROBLEM OUTPUT:
+;;
+;; got done
+;; starting
+;;
+;; EXPECTED OUTPUT:
+;;
+;; got done
+;; starting
+;; stopping
+;;
 
 (require/activate syndicate/drivers/timer)
 
diff --git a/racket/syndicate/protocol/instance.rkt b/racket/syndicate/protocol/instance.rkt
new file mode 100644
index 0000000..91e815e
--- /dev/null
+++ b/racket/syndicate/protocol/instance.rkt
@@ -0,0 +1,13 @@
+#lang racket/base
+;; "Instance" protocol for discriminating among
+;; otherwise-indistinguishable entities.
+
+(provide (struct-out instance))
+
+;; (instance Any Any), assertion or message
+;;
+;; In cases where `spec` can have multiple instantiations, serves to
+;; distinguish between them. Each `id` should be unique within its
+;; scope.
+;;
+(struct instance (id spec) #:prefab)