From 072a991bc2eafb283cd25eaad1f4fd2f70c0f8b9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Fri, 13 Oct 2017 11:47:41 +0200
Subject: Reads that draw from both buffer and file must work

---
 lib/kernel/test/file_SUITE.erl | 10 ++++++++++
 1 file changed, 10 insertions(+)

(limited to 'lib/kernel')

diff --git a/lib/kernel/test/file_SUITE.erl b/lib/kernel/test/file_SUITE.erl
index 6139d2161e..3cb249c565 100644
--- a/lib/kernel/test/file_SUITE.erl
+++ b/lib/kernel/test/file_SUITE.erl
@@ -3161,6 +3161,16 @@ read_ahead(Config) when is_list(Config) ->
     Data1Data2Data3 = Data1++Data2++Data3,
     {ok, Data1Data2Data3} = ?FILE_MODULE:read(Fd5, 3*Size+1),
     ok = ?FILE_MODULE:close(Fd5),
+
+    %% Ensure that a read that draws from both the buffer and the file won't
+    %% return anything wonky.
+    SplitData = << <<(I rem 256)>> || I <- lists:seq(1, 1024) >>,
+    file:write_file(File, SplitData),
+    {ok, Fd6} = ?FILE_MODULE:open(File, [raw, read, binary, {read_ahead, 256}]),
+    {ok, <<1>>} = file:read(Fd6, 1),
+    <<1, Shifted:512/binary, _Rest/binary>> = SplitData,
+    {ok, Shifted} = file:read(Fd6, 512),
+
     %%
     [] = flush(),
     ok.
-- 
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