perf: return relaxed header values to the fast scanner

Author: mcollina

src/llhttp/http.ts

@@ -863,9 +863,13 @@ export class HTTP {
       .peek('\n', span.headerValue.end(n('header_value_almost_done')))
       .skipTo(n('header_value_lenient'));
 
+    // After consuming a relaxed byte, return to the ordinary fast path.
+    // Staying in this state would duplicate the entire header-value scanning
+    // loop (including its vectorized blocks) on the hot state machine.
     n('header_value_relaxed')
-      .match(RELAXED_HEADER_CHARS, n('header_value_relaxed'))
-      .otherwise(n('header_value_otherwise'));
+      .match(RELAXED_HEADER_CHARS, n('header_value'))
+      .otherwise(span.headerValue.end(p.error(ERROR.INVALID_HEADER_TOKEN,
+        'Invalid header value char')));
 
     n('header_value_almost_done')
       .match('\n', n('header_value_lws'))

test/request/lenient-header-value-relaxed.md

@@ -35,6 +35,60 @@ off=43 headers complete method=1 v=1/1 flags=0 content_length=0
 off=43 message complete
 ```
 
+## Consecutive control chars return to the ordinary header scanner
+
+<!-- meta={"type": "request-lenient-header-value-relaxed"} -->
+```http
+GET /url HTTP/1.1
+Header1: hello\f\fworld
+
+
+```
+
+```log
+off=0 message begin
+off=0 len=3 span[method]="GET"
+off=3 method complete
+off=4 len=4 span[url]="/url"
+off=9 url complete
+off=9 len=4 span[protocol]="HTTP"
+off=13 protocol complete
+off=14 len=3 span[version]="1.1"
+off=17 version complete
+off=19 len=7 span[header_field]="Header1"
+off=27 header_field complete
+off=28 len=12 span[header_value]="hello\f\fworld"
+off=42 header_value complete
+off=44 headers complete method=1 v=1/1 flags=0 content_length=0
+off=44 message complete
+```
+
+## Bare CR after a relaxed control char is still rejected
+
+<!-- meta={"type": "request-lenient-header-value-relaxed"} -->
+```http
+GET /url HTTP/1.1
+Header1: hello\f\rworld
+
+
+```
+
+```log
+off=0 message begin
+off=0 len=3 span[method]="GET"
+off=3 method complete
+off=4 len=4 span[url]="/url"
+off=9 url complete
+off=9 len=4 span[protocol]="HTTP"
+off=13 protocol complete
+off=14 len=3 span[version]="1.1"
+off=17 version complete
+off=19 len=7 span[header_field]="Header1"
+off=27 header_field complete
+off=28 len=6 span[header_value]="hello\f"
+off=35 error code=3 reason="Missing expected LF after header value"
+```
+
 ## Control char in header value (strict)
 
 Control characters should be rejected in strict mode.