Excessive syscalls when using BufWriter

[meithecatte]

I am writing to a BufWriter<File>. Unfortunately, std::io::BufWriter::stream_position falls back to running a seek syscall to figure out the current stream position. This makes my serialization workload take ~300x longer than necessary.

As a workaround, NoSeek<BufWriter<File>> works for my usecase. In fact, the values returned by stream_position are never used by the generated code. Emitting them only when necessary should improve the performance in many cases.

[csnover]

Hi, sorry for the delay. Thanks for reporting this issue, it is symmetrical to the issue with std::io::BufReader where we provide a different implementation there for the same reason.

Could you provide an example of something that that emits a stream_position call where the value is never consumed? Mostly, stream position is emitted for the sake of error handling, so there should always be a consumer of the emitted value, even if it is never used most of the time because most of the time writes don’t fail.

Thanks!

[meithecatte]

Hi, sorry for taking a while to get back to you. In the following example, the output of cargo expand has 3 occurrences of stream_position, always assigned to an entirely dead variable:

use binrw::binwrite;

#[binwrite]
struct Meow {
    a: u32,
    b: u32,
}

fn main() {
    println!("Hello, world!");
}

Tested with binrw@0.12.0 (latest at the time of writing).