Reuse Rng during temp name creation

The fastrand Rng should be created manually to avoid TLS deallocation
issues. The fastrand::alphanumeric function uses a TLS storage
internally.

With current Rust and fastrand implementation, this is no problem.

Yet, we can speed up the process a bit by not accessing the storage
for every single random char.

src/util.rs

@@ -4,20 +4,7 @@ use std::{io, iter::repeat_with};
 
 use crate::error::IoResultExt;
 
-fn tmpname(prefix: &OsStr, suffix: &OsStr, rand_len: usize) -> OsString {
- let capacity = prefix
- .len()
- .saturating_add(suffix.len())
- .saturating_add(rand_len);
- let mut buf = OsString::with_capacity(capacity);
- buf.push(prefix);
- let mut char_buf = [0u8; 4];
- for c in repeat_with(fastrand::alphanumeric).take(rand_len) {
- buf.push(c.encode_utf8(&mut char_buf));
- }
- buf.push(suffix);
- buf
-}
+use fastrand::Rng;
 
 pub fn create_helper<R>(
  base: &Path,
@@ -27,12 +14,29 @@ pub fn create_helper<R>(
  permissions: Option<&std::fs::Permissions>,
  mut f: impl FnMut(PathBuf, Option<&std::fs::Permissions>) -> io::Result<R>,
 ) -> io::Result<R> {
+ let capacity = prefix
+ .len()
+ .saturating_add(random_len)
+ .saturating_add(suffix.len());
+ let mut buf = OsString::with_capacity(capacity);
+
  if random_len == 0 {
- let path = base.join(tmpname(prefix, suffix, random_len));
+ buf.push(prefix);
+ buf.push(suffix);
+ let path = base.join(buf);
  f(path, permissions)
  } else {
+ let mut char_buf = [0u8; 4];
+ let mut rng = Rng::new();
+
  for _ in 0..crate::NUM_RETRIES {
- let path = base.join(tmpname(prefix, suffix, random_len));
+ buf.push(prefix);
+ for c in repeat_with(|| rng.alphanumeric()).take(random_len) {
+ buf.push(c.encode_utf8(&mut char_buf));
+ }
+ buf.push(suffix);
+ let path = base.join(&buf);
+ buf.clear();
  return match f(path, permissions) {
  Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => continue,
  // AddrInUse can happen if we're creating a UNIX domain socket and