pjfanning opened a new pull request, #1226:
URL: https://github.com/apache/pekko-http/pull/1226
### Motivation
`LineParser` scanned its buffer one byte at a time:
```scala
bs(at) match {
case CR_BYTE if at < bs.length - 1 && bs(at + 1) == LF_BYTE => ...
```
The buffer is built as `buffer ++ grab(in)`, so it is a multi-fragment
`ByteString` whenever a line spans more than one chunk.
`ByteString$ByteStrings.apply` has no offset index or position cache - it walks
the fragment vector from the first fragment on every access - so scanning an
SSE line that arrives in N chunks costs O(bytes × N).
`ByteString.indexOf(byte, from)` is the opposite: it walks fragments once
and, inside a fragment, uses `SWARUtil` to test 8 bytes per step.
### Modification
Jump to the next CR or LF with `indexOf` instead of testing every byte. The
three identical line-emitting blocks are factored into a local `lineAt` helper.
Line termination semantics are unchanged - CR, LF, CRLF, a lone CR followed
by a non-LF byte, and a CRLF split across two chunks (the `lastCharWasCr` flag)
all behave exactly as before. The oversized-line handling (`maxLineSize` and
every `OversizedSseStrategy`) is untouched, just moved into the helper.
### Result
`LineParserBenchmark`, JDK 21, `-f 1 -wi 3 -i 3`:
| lineSize | chunkSize | before | after | |
|---|---|---|---|---|
| 1 KB | 512 | 0.265 ms/op | 0.189 ms/op | within noise |
| 1 KB | 8192 | 0.295 ms/op | 0.183 ms/op | within noise |
| 128 KB | 512 | 71.210 ms/op | 1.013 ms/op | ~70x |
| 128 KB | 8192 | 3.269 ms/op | 0.342 ms/op | ~10x |
| 1 MB | 512 | 9821.281 ms/op | 54.361 ms/op | ~180x |
Small lines are unaffected; the gain scales with how many chunks a line is
split across, which is exactly the shape of a real SSE stream over a network.
Note this is independent of the pending pekko-core change that gives
`ByteStrings.apply` a fragment hint. That hint helps monotonic forward scans,
but `indexOf` still avoids one virtual call per byte and tests 8 bytes at a
time, and this change also benefits the 1.x line, which is pinned to pekko
1.1.5.
### Tests
- `sbt "http-tests / Test / testOnly
org.apache.pekko.http.scaladsl.unmarshalling.sse.*"` - 48 passed (46 existing,
plus 2 new in `LineParserSpec`: a CRLF split across two chunks, and lines
parsed out of an explicitly multi-fragment `ByteString`, asserted non-compact
so the test really covers the rope path).
- `sbt "http-bench-jmh/Jmh/run …"` - numbers above, measured before and
after on the same machine.
- `scalafmt --mode diff-ref=upstream/main` - clean.
### References
None - found while auditing ByteString usage across the code base
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