From 6affbd66d446723acd3e78683dec3d0d1c048eee Mon Sep 17 00:00:00 2001
From: Julien Portalier <julien@portalier.com>
Date: Tue, 13 Jun 2017 11:13:00 +0200
Subject: [PATCH] Fix: IO::FileDescriptor#seek from current position

Trying to seek from the current position failed because the system
position of a buffered IO is different from the actual position in
the stream.
---
 spec/std/file_spec.cr     | 7 +++++++
 src/io/file_descriptor.cr | 8 +++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/spec/std/file_spec.cr b/spec/std/file_spec.cr
index e039e332e7f5..4a08b7d10110 100644
--- a/spec/std/file_spec.cr
+++ b/spec/std/file_spec.cr
@@ -621,6 +621,13 @@ describe "File" do
     file.gets(4).should eq("ello")
   end
 
+  it "seeks from the current position" do
+    file = File.new("#{__DIR__}/data/test_file.txt")
+    file.gets(5)
+    file.seek(-4, IO::Seek::Current)
+    file.tell.should eq(1)
+  end
+
   it "raises if invoking seek with a closed file" do
     file = File.new("#{__DIR__}/data/test_file.txt")
     file.close
diff --git a/src/io/file_descriptor.cr b/src/io/file_descriptor.cr
index 7122f216a936..24406aa48b19 100644
--- a/src/io/file_descriptor.cr
+++ b/src/io/file_descriptor.cr
@@ -131,7 +131,13 @@ class IO::FileDescriptor
     check_open
 
     flush
-    seek_value = LibC.lseek(@fd, offset, whence)
+
+    if whence.current?
+      seek_value = LibC.lseek(@fd, tell + offset, Seek::Set)
+    else
+      seek_value = LibC.lseek(@fd, offset, whence)
+    end
+
     if seek_value == -1
       raise Errno.new "Unable to seek"
     end