cacache-11.5.2 crashes with SIGBUS when disk full and data size <= 512KB

[komar007]

cacache-11.5.2 crashes a program with SIGBUS when working under disk-full conditions on linux.
Running ubuntu-19.10 with linux-5.3.0-64-generic.

$ rustc --version
rustc 1.69.0 (84c898d65 2023-04-16)

Reproduction:

• create a filesystem with limited space, for example mkdir /tmp/ram && mount -ttmpfs -osize=5m tmpfs /tmp/ram
• run the following program:

  fn main() {
      for i in 0..12 {
          println!("{}", i);
          let data: Vec<_> = (0..512*1024).map(|_| rand::random::<u8>()).collect();
          println!("{:?}", cacache::write_hash_sync("/tmp/ram/cache", &data));
      }
  }
  
  // in Cargo.toml
  // ...
  // [dependencies]
  // cacache = "11.5.2"
  // rand = "0.8"

• result:

  0
  Writer::new size=Some(524288)
  Ok(Integrity { hashes: [Hash { algorithm: Sha256, digest: "QQ9CVmHX6CzNPkuGFAhp/k8wSkmEVexMp6ARULmLdMM=" }] })
  1
  Writer::new size=Some(524288)
  Ok(Integrity { hashes: [Hash { algorithm: Sha256, digest: "JAI/yZ2LjUfpKko8L4RFV7g7DzNxHvq7jfYhX/9mQ4o=" }] })
  [...]
  9
  Writer::new size=Some(524288)
  Bus error (core dumped)
  

The problem is caused by the optimization where if the binary blob to cache is no more than 512KB, then it is written via mmap. Since the file obtained from tempfile may be sparse, writing to it may result in allocation of more blocks on the fs and failure with SIGBUS (I'm not sure this is standard/defined behavior or not, it seems legit though - what else can the OS do?). The call to std::fs::File::set_len only results in calling the truncate syscall, which does not guarantee file allocation and does not return an error on not enough space on device.

Calling posix_fallocate on the fd of the file obtained from tempfile fixes the issue for me, but I am not sure posix_fallocate guarantees file allocation or it just happens to work ok.