Fix a bug: buffer_size_ should not regard as bytes_buffered_ + buffer…

…_pos_

cpp/src/arrow/io/buffered.cc

@@ -51,7 +51,7 @@ class BufferedBase {
     return !is_open_;
   }
 
-  // Reset the `buffer_` to a new buffer of size `buffer_size_`
+  // Allocate buffer_ if needed, and resize it to buffer_size_ if required.
   Status ResetBuffer() {
     if (!buffer_) {
       // On first invocation, or if the buffer has been released, we allocate a
@@ -284,9 +284,9 @@ class BufferedInputStream::Impl : public BufferedBase {
   }
 
   // Resize internal read buffer. Note that the internal buffer-size
-  // should be not larger than the raw_read_bound_.
-  //
-  // SetBufferSize will not change the buffer_pos_ and bytes_buffered_.
+  // should not be larger than the raw_read_bound_.
+  // It might change the buffer_size_, but will not change buffer states
+  // buffer_pos_ and bytes_buffered_.
   Status SetBufferSize(int64_t new_buffer_size) {
     if (new_buffer_size <= 0) {
       return Status::Invalid("Buffer size should be positive");
@@ -301,13 +301,14 @@ class BufferedInputStream::Impl : public BufferedBase {
       // No need to reserve space for more than the total remaining number of bytes.
       if (bytes_buffered_ == 0) {
         // Special case: we can not keep the current buffer because it does not
-        // contains any required data.
+        // contain any required data.
         new_buffer_size = std::min(new_buffer_size, raw_read_bound_ - raw_read_total_);
       } else {
-        // We should keeping the current buffer because it contains data that
+        // We should keep the current buffer because it contains data that
         // can be read.
         new_buffer_size =
-            std::min(new_buffer_size, buffer_size_ + (raw_read_bound_ - raw_read_total_));
+            std::min(new_buffer_size,
+                     buffer_pos_ + bytes_buffered_ + (raw_read_bound_ - raw_read_total_));
       }
     }
     return ResizeBuffer(new_buffer_size);
@@ -325,7 +326,7 @@ class BufferedInputStream::Impl : public BufferedBase {
     }
 
     // Increase the buffer size if needed.
-    if (nbytes > buffer_->size() - buffer_pos_) {
+    if (nbytes > buffer_size_ - buffer_pos_) {
       RETURN_NOT_OK(SetBufferSize(nbytes + buffer_pos_));
       DCHECK(buffer_->size() - buffer_pos_ >= nbytes);
     }
@@ -364,7 +365,7 @@ class BufferedInputStream::Impl : public BufferedBase {
   }
 
   Status DoBuffer() {
-    // Refill the buffer from the raw stream with `buffer_size_` bytes.
+    // Fill the buffer from the raw stream with at most `buffer_size_` bytes.
     if (!buffer_) {
       RETURN_NOT_OK(ResetBuffer());
     }
@@ -458,8 +459,8 @@ class BufferedInputStream::Impl : public BufferedBase {
   // The default -1 indicates that it is unbounded
   int64_t raw_read_bound_;
 
-  // Number of remaining bytes in the buffer, to be reduced on each read from
-  // the buffer
+  // Number of remaining valid bytes in the buffer, to be reduced on each read
+  // from the buffer.
   int64_t bytes_buffered_;
 };
 

cpp/src/arrow/io/buffered_test.cc

@@ -491,17 +491,28 @@ TEST_F(TestBufferedInputStream, BufferSizeLimit) {
   }
 }
 
-TEST_F(TestBufferedInputStream, PeekPastBufferedBytesTwice) {
+TEST_F(TestBufferedInputStream, PeekPastBufferedBytes) {
+  // GH-43949: Peek and SetBufferSize should not affect the
+  // buffered bytes.
   MakeExample1(/*buffer_size=*/10, default_memory_pool(), /*raw_read_bound=*/15);
-  ASSERT_OK(buffered_->Read(9));
+  ASSERT_OK_AND_ASSIGN(auto bytes, buffered_->Read(9));
+  EXPECT_EQ(std::string_view(bytes->data_as<char>(), bytes->size()),
+            kExample1.substr(0, 9));
   ASSERT_EQ(1, buffered_->bytes_buffered());
   ASSERT_EQ(10, buffered_->buffer_size());
-  ASSERT_OK(buffered_->Peek(6));
-  ASSERT_EQ(6, buffered_->bytes_buffered());
-  ASSERT_EQ(15, buffered_->buffer_size());
-  ASSERT_OK(buffered_->Peek(6));
+  ASSERT_OK_AND_ASSIGN(auto view, buffered_->Peek(3));
+  EXPECT_EQ(view, kExample1.substr(9, 3));
+  ASSERT_EQ(3, buffered_->bytes_buffered());
+  ASSERT_EQ(12, buffered_->buffer_size());
+  ASSERT_OK_AND_ASSIGN(view, buffered_->Peek(10));
+  EXPECT_EQ(view, kExample1.substr(9, 6));
   ASSERT_EQ(6, buffered_->bytes_buffered());
   ASSERT_EQ(15, buffered_->buffer_size());
+  // Do read
+  ASSERT_OK_AND_ASSIGN(bytes, buffered_->Read(6));
+  EXPECT_EQ(std::string_view(bytes->data_as<char>(), bytes->size()),
+            kExample1.substr(9, 6));
+  ASSERT_EQ(0, buffered_->bytes_buffered());
 }
 
 class TestBufferedInputStreamBound : public ::testing::Test {