I’m facing an issue with BufReader in Rust when attempting to read data into two separate temporary buffers under different conditions.

Below is a breakdown along with the added code.

• First Buffer Read (first_tmp_buf): It successfully reads 500 bytes of data.
  • Output example: [44, 32, 49, 50, 47, 55, ..., 55] ✅
• Second Buffer Read (second_tmp_buf): During the first iteration, it’s intended to read 1500 bytes. However, only the first 1000 bytes are valid, and the last 500 bytes are unexpectedly zeros. I suspect this issue arises from the initial 500-byte read into first_tmp_buf, which may be influencing the read position.
  • Output example: [44, 32, 49, 50, ..., 0, ..., 0] ❌
• Subsequent Reads (second_tmp_buf): These operations perform as expected, consistently returning 1500 valid bytes and completely filling the buffer.
  • Output example: [44, 32, 49, 50, 47, 55, ..., 55] ✅

I suspect the problem is linked to how the read position is managed across different buffer contexts, but I’m not certain how to address this effectively.

use std::io::{BufReader, Read, Cursor};

fn main() -> std::io::Result<()> {
    let huge_string_data = vec![0_u8; 200_00_00]; // Simulating a large dataset
    let src_buf = Reader::from_vec(huge_string_data);

    let mut first_tmp_buf = vec![0_u8; 500];
    // First read into `first_tmp_buf`:
    // ✅ Successfully returns 500 bytes of data.
    // Eg output: [44, 32, 49, 50, 47, 55, ..., 55]
    let bytes_read = src_buf.inner.read(&mut first_tmp_buf)?;

    let mut second_tmp_buf = vec![0_u8; 1500];
    let mut iter = 0;
    while condition {
        let bytes_read = src_buf.inner.read(&mut second_tmp_buf)?;

        // Second read into `second_tmp_buf` during first iteration:
        // When `(iter=0)`
        // ❌ Returns 1000 bytes of valid data, but last 500 bytes are zeros, not good.
        // Eg output: [44, 32, 49, 50, ..., 0, 0, 0, ..., 0]


        // Third read into `second_tmp_buf`:
        // When `(iter=1)`
        // ✅ Successfully returns 1500 bytes of data.
        // Eg output: [44, 32, 49, 50, 47, 55, ..., 55]
        
        
        iter += 1;
    }

    Ok(())
}

struct Reader {
    pub inner: BufReader<Box<dyn Read>>,
}

impl Reader {
    fn from_vec(v: Vec<u8>) -> Self {
        let cursor = Cursor::new(v);
        let boxed_reader = Box::new(cursor);

        Self {
            inner: BufReader::with_capacity(0x4000, boxed_reader),
        }
    }
}

Could anyone shed light on why BufReader behaves this way and suggest the best approach to ensure consistent reads across varying buffer sizes and contexts? Your insights would be very helpful!

Thank you!