From 2848cdb18a1bcb0c1eb7ee4ac8807a5471432626 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Thu, 10 Nov 2022 16:54:39 +0800
Subject: [PATCH] fixes #13781; fixes #14901; add acyclic pragmas to FlowVar
 (#20804)

* add acyclic pragmas to FlowVar

* add testcases
---
 lib/pure/concurrency/threadpool.nim |  4 ++--
 tests/parallel/tlet_spawn.nim       | 11 ++++++++++-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/pure/concurrency/threadpool.nim b/lib/pure/concurrency/threadpool.nim
index e17fc38b7..9d773dac5 100644
--- a/lib/pure/concurrency/threadpool.nim
+++ b/lib/pure/concurrency/threadpool.nim
@@ -102,7 +102,7 @@ type
     idx: int
 
   FlowVarBase* = ref FlowVarBaseObj ## Untyped base class for `FlowVar[T] <#FlowVar>`_.
-  FlowVarBaseObj = object of RootObj
+  FlowVarBaseObj {.acyclic.} = object of RootObj
     ready, usesSemaphore, awaited: bool
     cv: Semaphore  # for 'blockUntilAny' support
     ai: ptr AwaitInfo
@@ -111,7 +111,7 @@ type
                    # be RootRef here otherwise the wrong GC keeps track of it!
     owner: pointer # ptr Worker
 
-  FlowVarObj[T] = object of FlowVarBaseObj
+  FlowVarObj[T] {.acyclic.} = object of FlowVarBaseObj
     blob: T
 
   FlowVar*[T] {.compilerproc.} = ref FlowVarObj[T] ## A data flow variable.
diff --git a/tests/parallel/tlet_spawn.nim b/tests/parallel/tlet_spawn.nim
index 62341d8f0..853ffc443 100644
--- a/tests/parallel/tlet_spawn.nim
+++ b/tests/parallel/tlet_spawn.nim
@@ -4,7 +4,7 @@ done999 999
 '''
 """
 
-import threadpool
+import std/[threadpool, os]
 
 proc foo(): int = 999
 
@@ -17,3 +17,12 @@ proc main =
   echo "done", f, " ", b
 
 main()
+
+# bug #13781
+proc thread(): string =
+  os.sleep(1000)
+  return "ok"
+
+var fv = spawn thread()
+sync()
+doAssert ^fv == "ok"