From e43c265870b8abb4d9c69463e0eb4563553e23b2 Mon Sep 17 00:00:00 2001
From: Christopher Baines <mail@cbaines.net>
Date: Wed, 19 Jan 2022 13:15:05 +0000
Subject: Add letpar&

Mostly copied from the Guix Data Service.
---
 guix-build-coordinator/utils/fibers.scm | 58 ++++++++++++++++++++++++++++++++-
 1 file changed, 57 insertions(+), 1 deletion(-)

(limited to 'guix-build-coordinator/utils')

diff --git a/guix-build-coordinator/utils/fibers.scm b/guix-build-coordinator/utils/fibers.scm
index a8b6738..5481191 100644
--- a/guix-build-coordinator/utils/fibers.scm
+++ b/guix-build-coordinator/utils/fibers.scm
@@ -10,7 +10,9 @@
 
             call-with-sigint
 
-            run-server/patched))
+            run-server/patched
+
+            letpar&))
 
 (define %worker-thread-args
   (make-parameter #f))
@@ -147,3 +149,57 @@ If already in the worker thread, call PROC immediately."
       (set-nonblocking! socket)
       (sigaction SIGPIPE SIG_IGN)
       (spawn-fiber (lambda () (socket-loop socket handler))))))
+
+(define (defer-to-fiber thunk)
+  (let ((reply (make-channel)))
+    (spawn-fiber
+     (lambda ()
+       (put-message
+        reply
+        (with-exception-handler
+            (lambda (exn)
+              (cons 'worker-fiber-error exn))
+          (lambda ()
+            (with-exception-handler
+                (lambda (exn)
+                  (simple-format
+                   (current-error-port)
+                   "worker fiber: exception: ~A\n"
+                   exn)
+                  (backtrace)
+                  (raise-exception exn))
+              (lambda ()
+                (call-with-values
+                    thunk
+                  (lambda vals
+                    vals)))))
+          #:unwind? #t)))
+     #:parallel? #t)
+    reply))
+
+(define (fetch-result-of-defered-thunks . reply-channels)
+  (let ((responses (map get-message reply-channels)))
+    (map
+     (match-lambda
+       (('worker-thread-error . exn)
+        (raise-exception exn))
+       (result
+        (apply values result)))
+     responses)))
+
+(define-syntax parallel-via-fibers
+  (lambda (x)
+    (syntax-case x ()
+      ((_ e0 ...)
+       (with-syntax (((tmp0 ...) (generate-temporaries (syntax (e0 ...)))))
+         #'(let ((tmp0 (defer-to-fiber
+                         (lambda ()
+                           e0)))
+                 ...)
+             (apply values (fetch-result-of-defered-thunks tmp0 ...))))))))
+
+(define-syntax-rule (letpar& ((v e) ...) b0 b1 ...)
+  (call-with-values
+      (lambda () (parallel-via-fibers e ...))
+    (lambda (v ...)
+      b0 b1 ...)))
-- 
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