1 files changed, 49 insertions, 0 deletions
diff --git a/lib/gitlab/ci/trace_reader.rb b/lib/gitlab/ci/trace_reader.rb
new file mode 100644
index 00000000000..37e51536e8f
--- /dev/null
+++ b/lib/gitlab/ci/trace_reader.rb
@@ -0,0 +1,49 @@
+module Gitlab
+  module Ci
+    # This was inspired from: http://stackoverflow.com/a/10219411/1520132
+    class TraceReader
+      BUFFER_SIZE = 4096
+
+      attr_accessor :path, :buffer_size
+
+      def initialize(new_path, buffer_size: BUFFER_SIZE)
+        self.path = new_path
+        self.buffer_size = Integer(buffer_size)
+      end
+
+      def read(last_lines: nil)
+        if last_lines
+          read_last_lines(last_lines)
+        else
+          File.read(path)
+        end
+      end
+
+      def read_last_lines(max_lines)
+        File.open(path) do |file|
+          chunks = []
+          pos = lines = 0
+          max = file.size
+
+          # We want an extra line to make sure fist line has full contents
+          while lines <= max_lines && pos < max
+            pos += buffer_size
+
+            buf = if pos <= max
+                    file.seek(-pos, IO::SEEK_END)
+                    file.read(buffer_size)
+                  else # Reached the head, read only left
+                    file.seek(0)
+                    file.read(buffer_size - (pos - max))
+                  end
+
+            lines += buf.count("\n")
+            chunks.unshift(buf)
+          end
+
+          chunks.join.lines.last(max_lines).join
+        end
+      end
+    end
+  end
+end