From 01a62320f03ebd3282a7def231351c4f84c319d2 Mon Sep 17 00:00:00 2001
From: Clyybber <darkmine956@gmail.com>
Date: Fri, 23 Oct 2020 03:26:50 +0200
Subject: [PATCH] Fix #12410 (#15685)

* Fix #12410, big thanks to @pmetras for this fix

* Add testcase
---
 lib/pure/streams.nim      |  2 +-
 tests/stdlib/tstreams.nim | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/lib/pure/streams.nim b/lib/pure/streams.nim
index 77f384a79a..dc00271a5d 100644
--- a/lib/pure/streams.nim
+++ b/lib/pure/streams.nim
@@ -242,7 +242,7 @@ proc readDataStr*(s: Stream, buffer: var string, slice: Slice[int]): int =
     result = s.readDataStrImpl(s, buffer, slice)
   else:
     # fallback
-    result = s.readData(addr buffer[0], buffer.len)
+    result = s.readData(addr buffer[slice.a], slice.b + 1 - slice.a)
 
 template jsOrVmBlock(caseJsOrVm, caseElse: untyped): untyped =
   when nimvm:
diff --git a/tests/stdlib/tstreams.nim b/tests/stdlib/tstreams.nim
index 354bdf60f5..6979bae389 100644
--- a/tests/stdlib/tstreams.nim
+++ b/tests/stdlib/tstreams.nim
@@ -6,6 +6,7 @@ Nice name: Arne
 fs is: nil
 
 threw exception
+_heh_
 '''
   nimout: '''
 I
@@ -49,3 +50,15 @@ block tstreams3:
     for line in s.lines:
       echo line
     s.close
+
+# bug #12410
+
+var a = newStringStream "hehohihahuhyh"
+a.readDataStrImpl = nil
+
+var buffer = "_ooo_"
+
+doAssert a.readDataStr(buffer, 1..3) == 3
+
+echo buffer
+