x509roots/fallback/bundle: add bundle package to export root certs

Fixes golang/go#69898

Change-Id: Idbb1bbe48016a622414c84a56fe26f48bfe712c8
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/687155
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>

x509roots/fallback/bundle/roots.go

@@ -0,0 +1,73 @@
+// Copyright 2025 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 bundle contains the bundle of root certificates parsed from the NSS
+// trust store, using x509roots/nss.
+package bundle
+
+import (
+	"crypto/x509"
+	_ "embed"
+	"fmt"
+	"iter"
+	"time"
+)
+
+//go:embed bundle.der
+var rawCerts []byte
+
+// Root represents a root certificate parsed from the NSS trust store.
+type Root struct {
+	// Certificate is the DER-encoded certificate (read-only; do not modify!).
+	Certificate []byte
+
+	// Constraint is nil if the root is unconstrained. If Constraint is non-nil,
+	// the certificate has additional constraints that cannot be encoded in
+	// X.509, and when building a certificate chain anchored with this root the
+	// chain should be passed to this function to check its validity. If using a
+	// [crypto/x509.CertPool] the root should be added using
+	// [crypto/x509.CertPool.AddCertWithConstraint].
+	Constraint func([]*x509.Certificate) error
+}
+
+// Roots returns the bundle of root certificates from the NSS trust store. The
+// [Root.Certificate] slice must be treated as read-only and should not be
+// modified.
+func Roots() iter.Seq[Root] {
+	return func(yield func(Root) bool) {
+		for _, unparsed := range unparsedCertificates {
+			root := Root{
+				Certificate: rawCerts[unparsed.certStartOff : unparsed.certStartOff+unparsed.certLength],
+			}
+			// parse possible constraints, this should check all fields of unparsedCertificate.
+			if unparsed.distrustAfter != "" {
+				distrustAfter, err := time.Parse(time.RFC3339, unparsed.distrustAfter)
+				if err != nil {
+					panic(fmt.Sprintf("failed to parse distrustAfter %q: %s", unparsed.distrustAfter, err))
+				}
+				root.Constraint = func(chain []*x509.Certificate) error {
+					for _, c := range chain {
+						if c.NotBefore.After(distrustAfter) {
+							return fmt.Errorf("certificate issued after distrust-after date %q", distrustAfter)
+						}
+					}
+					return nil
+				}
+			}
+			if !yield(root) {
+				return
+			}
+		}
+	}
+}
+
+type unparsedCertificate struct {
+	cn           string
+	sha256Hash   string
+	certStartOff int
+	certLength   int
+
+	// possible constraints
+	distrustAfter string
+}