From 492e9afa4c32be278b61498be20ae0afc3ef3188 Mon Sep 17 00:00:00 2001
From: flywind <43030857+xflywind@users.noreply.github.com>
Date: Fri, 13 Nov 2020 15:11:00 +0800
Subject: [PATCH] fix #15941 (#15948)

* fix #15941

* add testcase

* update
---
 lib/pure/collections/sharedlist.nim | 11 +++++++----
 tests/stdlib/tsharedlist.nim        | 17 +++++++++++++++++
 2 files changed, 24 insertions(+), 4 deletions(-)
 create mode 100644 tests/stdlib/tsharedlist.nim

diff --git a/lib/pure/collections/sharedlist.nim b/lib/pure/collections/sharedlist.nim
index 790529b79a..c72477675b 100644
--- a/lib/pure/collections/sharedlist.nim
+++ b/lib/pure/collections/sharedlist.nim
@@ -65,11 +65,14 @@ iterator items*[A](x: var SharedList[A]): A =
 proc add*[A](x: var SharedList[A]; y: A) =
   withLock(x):
     var node: SharedListNode[A]
-    if x.tail == nil or x.tail.dataLen == ElemsPerNode:
-      node = cast[type node](allocShared0(sizeof(node[])))
-      node.next = x.tail
+    if x.tail == nil:
+      node = cast[typeof node](allocShared0(sizeof(node[])))
+      x.tail = node
+      x.head = node
+    elif x.tail.dataLen == ElemsPerNode:
+      node = cast[typeof node](allocShared0(sizeof(node[])))
+      x.tail.next = node
       x.tail = node
-      if x.head == nil: x.head = node
     else:
       node = x.tail
     node.d[node.dataLen] = y
diff --git a/tests/stdlib/tsharedlist.nim b/tests/stdlib/tsharedlist.nim
new file mode 100644
index 0000000000..a795be0f3a
--- /dev/null
+++ b/tests/stdlib/tsharedlist.nim
@@ -0,0 +1,17 @@
+import sharedlist
+
+var
+  list: SharedList[int]
+  count: int
+
+init(list)
+
+for i in 1 .. 250:
+  list.add i
+
+for i in list:
+  inc count
+
+doAssert count == 250
+
+deinitSharedList(list)