tools/internal: wrap use of collators in mutexes

Despite being a process of reading from read-only state, somehow these
APIs turn out to be thread-unsafe, and parsing multiple PSLs concurrently
in the same process results in data races and panics.

Thankfully each use of the collator is quite short, so wrapping their use
in a global mutex, while unfortunate, is acceptable. And enabling parallel
parsing of PSLs speeds up online validations by a factor of 10x on many cored
machines.

tools/internal/domain/domain.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"slices"
 	"strings"
+	"sync"
 
 	"golang.org/x/net/idna"
 	"golang.org/x/text/collate"
@@ -203,10 +204,7 @@ func (l Label) Compare(m Label) int {
 	// If two labels aren't equal, we are free to order them however
 	// we want. We choose to order them with the English Unicode
 	// collation.
-	var buf collate.Buffer
-	kl := labelCollator.KeyFromString(&buf, l.label)
-	km := labelCollator.KeyFromString(&buf, m.label)
-	if res := bytes.Compare(kl, km); res != 0 {
+	if res := compareLabel(l, m); res != 0 {
 		return res
 	}
 
@@ -297,4 +295,20 @@ var domainValidator = idna.New(
 // byte compare. However, this option is buggy and silently ignored in
 // some cases (https://github.com/golang/go/issues/68379), so we do
 // this tie breaking ourselves in Label.Compare.
+var labelCollatorMu sync.Mutex
 var labelCollator = collate.New(language.English)
+
+func compareLabel(a, b Label) int {
+	// Somehow, despite this collation process being almost entirely
+	// reading from constant read-only tables compiled into the
+	// binary, x/text/collate's KeyFromString is not safe to use
+	// concurrently. Wrap its use in a mutex so that multiple
+	// concurrent parse operations serialize their collation key
+	// generation.
+	labelCollatorMu.Lock()
+	defer labelCollatorMu.Unlock()
+	var buf collate.Buffer
+	kl := labelCollator.KeyFromString(&buf, a.label)
+	km := labelCollator.KeyFromString(&buf, b.label)
+	return bytes.Compare(kl, km)
+}

tools/internal/parser/unicode.go

@@ -2,6 +2,7 @@ package parser
 
 import (
 	"bytes"
+	"sync"
 
 	"golang.org/x/text/collate"
 	"golang.org/x/text/language"
@@ -67,6 +68,15 @@ func compareCommentText(a string, b string) int {
 	// corresponding "sort keys", and then bytes.Compare those. There
 	// are more exhaustive tests for sort key computation, so there is
 	// higher confidence that it works correctly.
+	//
+	// Also, somehow, despite this collation process being almost
+	// entirely reading from constant read-only tables compiled into
+	// the binary, x/text/collate's KeyFromString is not safe to use
+	// concurrently. Wrap its use in a mutex so that multiple
+	// concurrent parse operations serialize their collation key
+	// generation.
+	commentCollatorMu.Lock()
+	defer commentCollatorMu.Unlock()
 	var buf collate.Buffer
 	ka := commentCollator.KeyFromString(&buf, a)
 	kb := commentCollator.KeyFromString(&buf, b)
@@ -77,3 +87,4 @@ func compareCommentText(a string, b string) int {
 // non-suffix text. See the comment at the start of this file for more
 // details.
 var commentCollator = collate.New(language.MustParse("en"))
+var commentCollatorMu sync.Mutex