Fix wrong Tell() result from BufferedOutputStream in an edge case (#73) (#74)

Co-authored-by: Hongze Zhang <hongze.zhang@intel.com>

Author: zhouyuan

cpp/src/arrow/io/buffered.cc

@@ -144,6 +144,8 @@ class BufferedOutputStream::Impl : public BufferedBase {
       RETURN_NOT_OK(FlushUnlocked());
       DCHECK_EQ(buffer_pos_, 0);
       if (nbytes >= buffer_size_) {
+        // Invalidate cached raw pos
+        raw_pos_ = -1;
         // Direct write
         if (buffer) {
           return raw_->Write(buffer);

cpp/src/arrow/io/buffered_test.cc

@@ -296,6 +296,19 @@ TEST_F(TestBufferedOutputStream, Tell) {
   AssertTell(0);
 }
 
+TEST_F(TestBufferedOutputStream, Tell2) {
+  OpenBuffered();
+  AssertTell(0);
+
+  const std::string datastr = GenerateRandomData(10000);
+  const char* d = datastr.data();
+
+  ASSERT_OK(buffered_->Write(d, 5000));
+  AssertTell(5000);
+
+  ASSERT_OK(buffered_->Close());
+}
+
 TEST_F(TestBufferedOutputStream, TruncatesFile) {
   OpenBuffered();