osmet: avoid install performance regression in debug mode on Rust 1.64+

Debug builds on Rust 1.64+ were seeing a substantial performance hit in
osmet unpack, causing the kola testiso pxe-offline-install scenario to
time out.  This is due to an apparent Rust performance regression when
BufReader's buffer is larger than the blocks returned by the inner reader,
if the code is built without optimization.  In our case, osmet's
write_unpacked_image() produces data in 8 KiB chunks, which are passed
verbatim through the pipe to the 256 KiB PeekReader in write_image().

For now, work around this by having osmet consolidate writes through a
256 KiB BufWriter.  The extra buffer is harmless, but once the affected
Rust versions are no longer being used for development, it probably makes
sense to drop this code.

xref rust-lang/rust#102727

docs/release-notes.md

@@ -10,6 +10,8 @@ Major changes:
 
 Minor changes:
 
+- install: Avoid osmet performance regression in debug builds on Rust 1.64+
+
 Internal changes:
 
 Packaging changes:

src/osmet/unpacker.rs

@@ -14,7 +14,7 @@
 
 use std::ffi::OsStr;
 use std::fs::{File, OpenOptions};
-use std::io::{self, copy, ErrorKind, Read, Seek, SeekFrom, Write};
+use std::io::{self, copy, BufWriter, ErrorKind, Read, Seek, SeekFrom, Write};
 use std::os::unix::ffi::{OsStrExt, OsStringExt};
 use std::path::{Path, PathBuf};
 use std::thread;
@@ -107,11 +107,16 @@ fn osmet_unpack_to_writer(
     repo: PathBuf,
     writer: impl Write,
 ) -> Result<()> {
-    let mut w = WriteHasher::new_sha256(writer)?;
+    // Work around https://github.com/rust-lang/rust/issues/102727 by
+    // buffering writes into chunks the same size as the buffer
+    // used by write_image() when reading from us
+    let buffered = BufWriter::with_capacity(BUFFER_SIZE, writer);
+    let mut w = WriteHasher::new_sha256(buffered)?;
     let n = write_unpacked_image(&mut packed_image, &mut w, &osmet.partitions, &repo)?;
     if n != osmet.size {
         bail!("wrote {} bytes but expected {}", n, osmet.size);
     }
+    w.flush()?;
 
     let final_checksum: Sha256Digest = w.try_into()?;
     if final_checksum != osmet.checksum {