add unit test

src/IO/ReadBufferFromFileDescriptor.cpp

@@ -234,6 +234,7 @@ void ReadBufferFromFileDescriptor::rewind()
     working_buffer.resize(0);
     pos = working_buffer.begin();
     file_offset_of_buffer_end = 0;
+    buffer_is_dirty = true;
 }
 
 

src/IO/tests/gtest_seek_backwards.cpp

@@ -0,0 +1,36 @@
+#include <gtest/gtest.h>
+#include <Common/filesystemHelpers.h>
+#include <IO/WriteBufferFromFile.h>
+#include <IO/ReadBufferFromFile.h>
+#include <IO/WriteHelpers.h>
+#include <IO/ReadHelpers.h>
+
+using namespace DB;
+
+TEST(ReadBufferFromFile, seekBackwards)
+{
+    static constexpr size_t N = 256;
+    static constexpr size_t BUF_SIZE = 64;
+
+    auto tmp_file = createTemporaryFile("/tmp/");
+
+    {
+        WriteBufferFromFile out(tmp_file->path());
+        for (size_t i = 0; i < N; ++i)
+            writeIntBinary(i, out);
+    }
+
+    ReadBufferFromFile in(tmp_file->path(), BUF_SIZE);
+    size_t x;
+
+    /// Read something to initialize the buffer.
+    in.seek(BUF_SIZE * 10, SEEK_SET);
+    readIntBinary(x, in);
+
+    /// Check 2 consecutive  seek calls without reading.
+    in.seek(BUF_SIZE * 2, SEEK_SET);
+    in.seek(BUF_SIZE, SEEK_SET);
+
+    readIntBinary(x, in);
+    ASSERT_EQ(x, 8);
+}