Parse Content-Length as digits with surrounding whitespace only

[pjfanning]

Motivation

ContentLengthParser skips whitespace anywhere in the field value:

if (DIGIT(c)) { ... } else if (WSP(c)) recurse(ix + 1, result)

So Content-Length: 1 2 is read as 12, Content-Length: 5 5 as 55, and an empty Content-Length: as 0. The field value is 1*DIGIT surrounded by optional whitespace (RFC 9110, section 8.6 and RFC 9112, section 5).

The other ways two implementations could disagree about a body length are already closed here — two Content-Length headers with different values are rejected, Transfer-Encoding accepts only a single chunked, and chunked together with a Content-Length is rejected — which is what makes this one worth tightening: it is the remaining case where pekko-http reads a length that a proxy or another server in the request path reads differently, or rejects.

Modification

Skip whitespace before and after the digits, require at least one digit, and end the value at the first non-digit.

Result

A value with whitespace between digits, or with no digit at all, is rejected with the same "Illegal `Content-Length` header value" error that other malformed values already produce. Leading and trailing whitespace still parse as before.

This rejects two inputs that were accepted before: whitespace inside the digits, and an empty value that used to be read as 0. Both are malformed per the grammar, but it is a behaviour change for anyone who was relying on the lenient reading.

Tests

• sbt "http-core/testOnly org.apache.pekko.http.impl.engine.parsing.RequestParserCRLFSpec org.apache.pekko.http.impl.engine.parsing.RequestParserLFSpec org.apache.pekko.http.impl.engine.parsing.ResponseParserSpec" - pass, with 2 new tests, one for whitespace inside the value and one for an empty value; both fail without the change. Leading whitespace stays covered by the existing Content-length: 17 tests, so the accepted cases are pinned too.
• sbt http-core/test - pass (1218 tests)
• sbt http-tests/test - pass (1483 tests)
• sbt http-core/mimaReportBinaryIssues - pass
• sbt http-core/scalafmt http-core/Test/scalafmt - clean

References

None - tightens Content-Length parsing to the grammar