crypto/x509: add CertPool.Equal

Fixes #46057

Change-Id: Id3af101c54108d6fd5b65946c4358872358eefcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/388915
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/crypto/x509/cert_pool.go

@@ -249,3 +249,16 @@ func (s *CertPool) Subjects() [][]byte {
 	}
 	return res
 }
+
+// Equal reports whether s and other are equal.
+func (s *CertPool) Equal(other *CertPool) bool {
+	if s.systemPool != other.systemPool || len(s.haveSum) != len(other.haveSum) {
+		return false
+	}
+	for h := range s.haveSum {
+		if !other.haveSum[h] {
+			return false
+		}
+	}
+	return true
+}

src/crypto/x509/cert_pool_test.go

@@ -0,0 +1,58 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package x509
+
+import "testing"
+
+func TestCertPoolEqual(t *testing.T) {
+	a, b := NewCertPool(), NewCertPool()
+	if !a.Equal(b) {
+		t.Error("two empty pools not equal")
+	}
+
+	tc := &Certificate{Raw: []byte{1, 2, 3}, RawSubject: []byte{2}}
+	a.AddCert(tc)
+	if a.Equal(b) {
+		t.Error("empty pool equals non-empty pool")
+	}
+
+	b.AddCert(tc)
+	if !a.Equal(b) {
+		t.Error("two non-empty pools not equal")
+	}
+
+	otherTC := &Certificate{Raw: []byte{9, 8, 7}, RawSubject: []byte{8}}
+	a.AddCert(otherTC)
+	if a.Equal(b) {
+		t.Error("non-equal pools equal")
+	}
+
+	systemA, err := SystemCertPool()
+	if err != nil {
+		t.Fatalf("unable to load system cert pool: %s", err)
+	}
+	systemB, err := SystemCertPool()
+	if err != nil {
+		t.Fatalf("unable to load system cert pool: %s", err)
+	}
+	if !systemA.Equal(systemB) {
+		t.Error("two empty system pools not equal")
+	}
+
+	systemA.AddCert(tc)
+	if systemA.Equal(systemB) {
+		t.Error("empty system pool equals non-empty system pool")
+	}
+
+	systemB.AddCert(tc)
+	if !systemA.Equal(systemB) {
+		t.Error("two non-empty system pools not equal")
+	}
+
+	systemA.AddCert(otherTC)
+	if systemA.Equal(systemB) {
+		t.Error("non-equal system pools equal")
+	}
+}