perf(propagation): use a hex lookup table to decode traceparent

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- Decode `traceparent` using a hex lookup table in `go.opentelemetry.io/otel/propagation`, which rejects the specification-disallowed upper-case characters without a separate scan of the header. (#8739)
+
 ### Added
 
 - Add experimental `ProbabilitySampler` in `go.opentelemetry.io/otel/sdk/trace/x` that conforms to the [OpenTelemetry specification's threshold-based sampling algorithm](https://opentelemetry.io/docs/specs/otel/trace/sdk/#probabilitysampler). (#8123)

propagation/internal/hextable/hextable.go

@@ -0,0 +1,32 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package hextable provides a lookup table for decoding lowercase
+// hexadecimal.
+package hextable
+
+// Rev is a reverse hex lookup table for lowercase hex digits. Valid
+// hexadecimal characters are indexed at their respective values. All other
+// characters, including the upper-case A-F disallowed by the W3C trace
+// context specification, are represented with '\xff'.
+//
+// The '\xff' character is used as invalid because no valid character has the
+// upper 4 bits set. Meaning, an efficient validation can be performed over
+// multiple character parsing by checking these bits remain zero.
+const Rev = "" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\x0a\x0b\x0c\x0d\x0e\x0f\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"

propagation/trace_context.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"strings"
 
+	"go.opentelemetry.io/otel/propagation/internal/hextable"
 	"go.opentelemetry.io/otel/trace"
 )
 
@@ -126,27 +127,23 @@ func (TraceContext) extract(carrier TextMapCarrier) trace.SpanContext {
 	return sc
 }
 
-// upperHex detect hex is upper case Unicode characters.
-func upperHex(v string) bool {
-	for _, c := range v {
-		if c >= 'A' && c <= 'F' {
-			return true
-		}
-	}
-	return false
-}
-
 func extractPart(dst []byte, h *string, n int) bool {
 	part, left, _ := strings.Cut(*h, delimiter)
 	*h = left
-	// hex.Decode decodes unsupported upper-case characters, so exclude explicitly.
-	if len(part) != n || upperHex(part) {
+	if len(part) != n {
 		return false
 	}
-	if p, err := hex.Decode(dst, []byte(part)); err != nil || p != n/2 {
-		return false
+	// hextable.Rev maps every invalid character to 0xff, including the
+	// upper-case A-F the specification disallows. OR-ing every looked-up value
+	// together lets a single check detect any invalid character, because no
+	// valid value has the upper 4 bits set.
+	invalidMark := byte(0)
+	for i := 0; i < n; i += 2 {
+		hi, lo := hextable.Rev[part[i]], hextable.Rev[part[i+1]]
+		dst[i/2] = (hi << 4) | lo
+		invalidMark |= hi | lo
 	}
-	return true
+	return invalidMark&0xf0 == 0
 }
 
 // Fields returns the keys who's values are set with Inject.