Unfreed (but still reachable) allocations in std::io::stdin are reported by Valgrind if non-default `--show-leak-kinds=all` option is specified

[boydjohnson]

There was a memory leak introduced in between nightly-2020-09-27 and nightly-2020-09-28.

If you build this code with two different toolchains one will have a memory leak and the other not.

use std::io::stdin;
use std::io::Read;

fn main() {
    let mut buf = vec![];

    if let Err(e) = stdin().read(&mut buf) {
        println!("{}", e)
    }
}

this code built with nightly-2020-09-28

valgrind output

==11076== HEAP SUMMARY:
==11076==     in use at exit: 8,232 bytes in 2 blocks
==11076==   total heap usage: 13 allocs, 11 frees, 10,409 bytes allocated

while this code built with nightly-2020-09-27:

valgrind output

==11601== HEAP SUMMARY:
==11601==     in use at exit: 0 bytes in 0 blocks
==11601==   total heap usage: 18 allocs, 18 frees, 10,577 bytes allocated

valgrind --leak-check=full --show-leak-kinds=all

==11920== HEAP SUMMARY:
==11920==     in use at exit: 8,232 bytes in 2 blocks
==11920==   total heap usage: 13 allocs, 11 frees, 10,409 bytes allocated
==11920== 
==11920== 40 bytes in 1 blocks are still reachable in loss record 1 of 2
==11920==    at 0x4C31B0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11920==    by 0x120934: alloc (alloc.rs:74)
==11920==    by 0x120934: alloc_impl (alloc.rs:153)
==11920==    by 0x120934: alloc (alloc.rs:212)
==11920==    by 0x120934: exchange_malloc (alloc.rs:302)
==11920==    by 0x120934: new<std::io::buffered::BufReader<std::io::stdio::StdinRaw>> (mutex.rs:222)
==11920==    by 0x120934: {{closure}} (stdio.rs:299)
==11920==    by 0x120934: {{closure}}<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0> (lazy.rs:245)
==11920==    by 0x120934: {{closure}}<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0,!> (lazy.rs:372)
==11920==    by 0x120934: std::sync::once::Once::call_once_force::{{closure}} (once.rs:321)
==11920==    by 0x120F84: std::sync::once::Once::call_inner (once.rs:419)
==11920==    by 0x1211BB: call_once_force<closure-0> (once.rs:321)
==11920==    by 0x1211BB: std::lazy::SyncOnceCell<T>::initialize (lazy.rs:371)
==11920==    by 0x11E2CB: get_or_try_init<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0,!> (lazy.rs:292)
==11920==    by 0x11E2CB: get_or_init<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0> (lazy.rs:245)
==11920==    by 0x11E2CB: std::io::stdio::stdin (stdio.rs:298)
==11920==    by 0x10D902: mem_leak::main (main.rs:7)
==11920==    by 0x10DCAA: core::ops::function::FnOnce::call_once (function.rs:227)
==11920==    by 0x10D11D: std::sys_common::backtrace::__rust_begin_short_backtrace (backtrace.rs:137)
==11920==    by 0x10DBB0: std::rt::lang_start::{{closure}} (rt.rs:66)
==11920==    by 0x123F7F: call_once<(),Fn<()>> (function.rs:259)
==11920==    by 0x123F7F: do_call<&Fn<()>,i32> (panicking.rs:381)
==11920==    by 0x123F7F: try<i32,&Fn<()>> (panicking.rs:345)
==11920==    by 0x123F7F: catch_unwind<&Fn<()>,i32> (panic.rs:382)
==11920==    by 0x123F7F: std::rt::lang_start_internal (rt.rs:51)
==11920==    by 0x10DB86: std::rt::lang_start (rt.rs:65)
==11920==    by 0x10DB39: main (in /home/boydjohnson/mem-leak/target/debug/mem-leak)
==11920== 
==11920== 8,192 bytes in 1 blocks are still reachable in loss record 2 of 2
==11920==    at 0x4C31B0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11920==    by 0x120910: alloc (alloc.rs:74)
==11920==    by 0x120910: alloc_impl (alloc.rs:153)
==11920==    by 0x120910: alloc (alloc.rs:212)
==11920==    by 0x120910: allocate_in<u8,alloc::alloc::Global> (raw_vec.rs:188)
==11920==    by 0x120910: with_capacity_in<u8,alloc::alloc::Global> (raw_vec.rs:163)
==11920==    by 0x120910: with_capacity<u8> (raw_vec.rs:93)
==11920==    by 0x120910: with_capacity<u8> (vec.rs:363)
==11920==    by 0x120910: with_capacity<std::io::stdio::StdinRaw> (buffered.rs:103)
==11920==    by 0x120910: {{closure}} (stdio.rs:299)
==11920==    by 0x120910: {{closure}}<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0> (lazy.rs:245)
==11920==    by 0x120910: {{closure}}<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0,!> (lazy.rs:372)
==11920==    by 0x120910: std::sync::once::Once::call_once_force::{{closure}} (once.rs:321)
==11920==    by 0x120F84: std::sync::once::Once::call_inner (once.rs:419)
==11920==    by 0x1211BB: call_once_force<closure-0> (once.rs:321)
==11920==    by 0x1211BB: std::lazy::SyncOnceCell<T>::initialize (lazy.rs:371)
==11920==    by 0x11E2CB: get_or_try_init<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0,!> (lazy.rs:292)
==11920==    by 0x11E2CB: get_or_init<std::sync::mutex::Mutex<std::io::buffered::BufReader<std::io::stdio::StdinRaw>>,closure-0> (lazy.rs:245)
==11920==    by 0x11E2CB: std::io::stdio::stdin (stdio.rs:298)
==11920==    by 0x10D902: mem_leak::main (main.rs:7)
==11920==    by 0x10DCAA: core::ops::function::FnOnce::call_once (function.rs:227)
==11920==    by 0x10D11D: std::sys_common::backtrace::__rust_begin_short_backtrace (backtrace.rs:137)
==11920==    by 0x10DBB0: std::rt::lang_start::{{closure}} (rt.rs:66)
==11920==    by 0x123F7F: call_once<(),Fn<()>> (function.rs:259)
==11920==    by 0x123F7F: do_call<&Fn<()>,i32> (panicking.rs:381)
==11920==    by 0x123F7F: try<i32,&Fn<()>> (panicking.rs:345)
==11920==    by 0x123F7F: catch_unwind<&Fn<()>,i32> (panic.rs:382)
==11920==    by 0x123F7F: std::rt::lang_start_internal (rt.rs:51)
==11920==    by 0x10DB86: std::rt::lang_start (rt.rs:65)
==11920==    by 0x10DB39: main (in /home/boydjohnson/mem-leak/target/debug/mem-leak)
==11920== 
==11920== LEAK SUMMARY:
==11920==    definitely lost: 0 bytes in 0 blocks
==11920==    indirectly lost: 0 bytes in 0 blocks
==11920==      possibly lost: 0 bytes in 0 blocks
==11920==    still reachable: 8,232 bytes in 2 blocks
==11920==         suppressed: 0 bytes in 0 blocks

[jonas-schievink]

rustc 1.48.0-nightly (623fb90 2020-09-26)
rustc 1.48.0-nightly (7f7a1cb 2020-09-27)

Commits in that range: 623fb90...7f7a1cb

[jonas-schievink]

#77154 looks like it might cause this. @m-ou-se can you tell if this is an actual leak or a false positive?

[m-ou-se]

I'm a bit busy right now. I'll look at it later, but I think this is a false positive (leaked on purpose).

[m-ou-se]

Before that change, the buffer for standard input was deallocated in an atexit() handler. After that change, it was simply not deallocated to simplify the code, since it'd always live to the end of the program anyway. So this leak is indeed intentional.

[joshtriplett]

@m-ou-se That's still somewhat problematic; people use valgrind to track down leaks, and intentional leaks make that harder.

We could address this via a patch to valgrind's standard "suppressions" mechanism.

[nnethercote]

First, this memory is still reachable, so it's not a true leak. You have to run Valgrind with --show-leak-kinds=all/--show-leak-kinds=reachable to even see these listed individually, and you have to run Valgrind with --errors-for-leak-kinds=all/--errors-for-leak-kinds=reachable to have then considered as actual errors. Only people who are very particular about not having any unfreed memory at termination are likely to use those options. I suspect the number of people like that is relatively small, so this is not as bad as first sounds. I have changed the title accordingly.

Having said that, Valgrind has a number of built-in suppressions for still-reachable "leaks" of this kind in system libraries, so it would be reasonable to add one for this case. I am happy to do that, so I have assigned this issue to me. Note that the fix will be in Valgrind, rather than in Rust. Also, Valgrind releases are unfortunately somewhat infrequent, so this change will almost certainly hit a Rust release before any fix makes it into a Valgrind release.