From 31917cc2d415879a5051fcfc525c33431b8b5e75 Mon Sep 17 00:00:00 2001
From: Hongze Zhang <hongze.zhang@intel.com>
Date: Fri, 7 Jan 2022 18:57:00 +0800
Subject: [PATCH] Fix wrong Tell() result from BufferedOutputStream in an edge
 case (#73)

---
 cpp/src/arrow/io/buffered.cc      |  2 ++
 cpp/src/arrow/io/buffered_test.cc | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/cpp/src/arrow/io/buffered.cc b/cpp/src/arrow/io/buffered.cc
index 7804c130ca1d9..ccfe9a360ab98 100644
--- a/cpp/src/arrow/io/buffered.cc
+++ b/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);
diff --git a/cpp/src/arrow/io/buffered_test.cc b/cpp/src/arrow/io/buffered_test.cc
index ede18f20d4ce6..5e812b124e365 100644
--- a/cpp/src/arrow/io/buffered_test.cc
+++ b/cpp/src/arrow/io/buffered_test.cc
@@ -297,6 +297,19 @@ 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();