02/03: Support request timeouts in the thread pool

[Christopher Baines]

cbaines pushed a commit to branch master
in repository data-service.

commit 638e0442c380f256b4ca08e6144599c66cd1ad29
Author: Christopher Baines <mail@cbaines.net>
AuthorDate: Thu Apr 27 11:49:31 2023 +0200

    Support request timeouts in the thread pool
---
 guix-data-service/utils.scm | 51 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 39 insertions(+), 12 deletions(-)

diff --git a/guix-data-service/utils.scm b/guix-data-service/utils.scm
index c5c89a4..b7124d5 100644
--- a/guix-data-service/utils.scm
+++ b/guix-data-service/utils.scm
@@ -32,6 +32,7 @@
             prevent-inlining-for-tests
 
             thread-pool-channel
+            thread-pool-request-timeout
             make-thread-pool-channel
             parallel-via-thread-pool-channel
             par-map&
@@ -129,30 +130,56 @@
      (iota threads))
     channel))
 
+(define &thread-pool-request-timeout
+  (make-exception-type '&thread-pool-request-timeout
+                       &error
+                       '()))
+
+(define make-thread-pool-request-timeout-error
+  (record-constructor &thread-pool-request-timeout))
+
+(define thread-pool-request-timeout-error?
+  (record-predicate &thread-pool-request-timeout))
+
 (define thread-pool-channel
   (make-parameter #f))
 
+(define thread-pool-request-timeout
+  (make-parameter #f))
+
 (define (defer-to-thread-pool-channel thunk)
   (let ((reply (make-channel)))
     (spawn-fiber
      (lambda ()
-       (put-message (thread-pool-channel)
-                    (list reply
-                          (get-internal-real-time)
-                          thunk))))
+       (let ((val
+              (perform-operation
+               (let ((put
+                      (wrap-operation
+                       (put-operation (thread-pool-channel)
+                                      (list reply
+                                            (get-internal-real-time)
+                                            thunk))
+                       (const 'success))))
+                 (or
+                  (and=> (thread-pool-request-timeout)
+                         (lambda (timeout)
+                           (choice-operation
+                            put
+                            (wrap-operation (sleep-operation timeout)
+                                            (const 'request-timeout)))))
+                  put)))))
+         (when (eq? val 'request-timeout)
+           (put-message reply val)))))
     reply))
 
-(define (fetch-result-of-defered-thunk reply-channel)
-  (match (get-message reply-channel)
-    (('worker-thread-error . exn)
-     (raise-exception exn))
-    (result
-     (apply values result))))
-
 (define (fetch-result-of-defered-thunks . reply-channels)
-  (let ((responses (map get-message reply-channels)))
+  (let ((responses (map get-message
+                        reply-channels)))
     (map
      (match-lambda
+       ('request-timeout
+        (raise-exception
+         (make-thread-pool-request-timeout-error)))
        (('worker-thread-error . exn)
         (raise-exception exn))
        (result