crypto/x509: keep smaller root cert representation in memory until needed

(from patchset 1, c12c890c64dd6372b3893af1e6f5ab11802c9e81, of
https://go-review.googlesource.com/c/go/+/230025/1, with merges fixes
due to parent commit's differents from its ps1..ps3)

Instead of parsing the PEM files and then storing the *Certificate
values forever, still parse them to see if they're valid and pick out
some fields, but then only store the decoded pem.Block.Bytes until
that cert is first needed.

Saves about 500K of memory on my (Debian stable) machine after doing a
tls.Dial or calling x509.SystemCertPool.

A more aggressive version of this is still possible: we can not keep
the pem.Block.Bytes in memory either, and re-read them from disk when
necessary. But dealing with files disappearing and even large
multi-cert PEM files changing (with offsets sliding around) made this
conservative version attractive. It doesn't change the
slurp-roots-on-startup semantics. It just does so with less memory
retained.

Change-Id: I3aea333f4749ae3b0026042ec3ff7ac015c72204

tempfork/x509/cert_pool.go

@@ -5,15 +5,26 @@
 package x509
 
 import (
+	"crypto/sha256"
 	"encoding/pem"
 	"errors"
 	"runtime"
+	"sync"
 )
 
+type sum224 [sha256.Size224]byte
+
 // CertPool is a set of certificates.
 type CertPool struct {
-	bySubjectKeyId map[string][]int // cert.SubjectKeyId => getCert index
-	byName         map[string][]int // cert.RawSubject => getCert index
+	bySubjectKeyId map[string][]int // cert.SubjectKeyId => getCert index(es)
+	byName         map[string][]int // cert.RawSubject => getCert index(es)
+
+	// haveSum maps from sum224(cert.Raw) to true. It's used only
+	// for AddCert duplicate detection, to avoid CertPool.contains
+	// calls in the AddCert path (because the contains method can
+	// call getCert and otherwise negate savings from lazy getCert
+	// funcs).
+	haveSum map[sum224]bool
 
 	// getCert contains funcs that return the certificates.
 	getCert []func() (*Certificate, error)
@@ -28,6 +39,7 @@ func NewCertPool() *CertPool {
 	return &CertPool{
 		bySubjectKeyId: make(map[string][]int),
 		byName:         make(map[string][]int),
+		haveSum:        make(map[sum224]bool),
 	}
 }
 
@@ -49,6 +61,7 @@ func (s *CertPool) copy() *CertPool {
 	p := &CertPool{
 		bySubjectKeyId: make(map[string][]int, len(s.bySubjectKeyId)),
 		byName:         make(map[string][]int, len(s.byName)),
+		haveSum:        make(map[sum224]bool, len(s.haveSum)),
 		getCert:        make([]func() (*Certificate, error), len(s.getCert)),
 		rawSubjects:    make([][]byte, len(s.rawSubjects)),
 	}
@@ -62,6 +75,9 @@ func (s *CertPool) copy() *CertPool {
 		copy(indexes, v)
 		p.byName[k] = indexes
 	}
+	for k := range s.haveSum {
+		p.haveSum[k] = true
+	}
 	copy(p.getCert, s.getCert)
 	copy(p.rawSubjects, s.rawSubjects)
 	return p
@@ -127,7 +143,7 @@ func (s *CertPool) AddCert(cert *Certificate) {
 	if cert == nil {
 		panic("adding nil Certificate to CertPool")
 	}
-	err := s.AddCertFunc(string(cert.RawSubject), string(cert.SubjectKeyId), func() (*Certificate, error) {
+	err := s.AddCertFunc(sha256.Sum224(cert.Raw), string(cert.RawSubject), string(cert.SubjectKeyId), func() (*Certificate, error) {
 		return cert, nil
 	})
 	if err != nil {
@@ -141,23 +157,16 @@ func (s *CertPool) AddCert(cert *Certificate) {
 // The rawSubject is Certificate.RawSubject and must be non-empty.
 // The subjectKeyID is Certificate.SubjectKeyId and may be empty.
 // The getCert func may be called 0 or more times.
-func (s *CertPool) AddCertFunc(rawSubject, subjectKeyID string, getCert func() (*Certificate, error)) error {
+func (s *CertPool) AddCertFunc(rawSum224 sum224, rawSubject, subjectKeyID string, getCert func() (*Certificate, error)) error {
 	if getCert == nil {
 		panic("getCert can't be nil")
 	}
 
 	// Check that the certificate isn't being added twice.
-	if len(s.byName[rawSubject]) > 0 {
-		c, err := getCert()
-		if err != nil {
-			return err
-		}
-		if dup, err := s.contains(c); dup {
-			return nil
-		} else if err != nil {
-			return err
-		}
+	if s.haveSum[rawSum224] {
+		return nil
 	}
+	s.haveSum[rawSum224] = true
 
 	n := len(s.getCert)
 	s.getCert = append(s.getCert, getCert)
@@ -187,16 +196,26 @@ func (s *CertPool) AppendCertsFromPEM(pemCerts []byte) (ok bool) {
 			continue
 		}
 
-		cert, err := ParseCertificate(block.Bytes)
+		certBytes := block.Bytes
+		cert, err := ParseCertificate(certBytes)
 		if err != nil {
 			continue
 		}
-
-		s.AddCert(cert)
+		var lazyCert struct {
+			sync.Once
+			v *Certificate
+		}
+		s.AddCertFunc(sha256.Sum224(cert.Raw), string(cert.RawSubject), string(cert.SubjectKeyId), func() (*Certificate, error) {
+			lazyCert.Do(func() {
+				// This can't fail, as the same bytes already parsed above.
+				lazyCert.v, _ = ParseCertificate(certBytes)
+				certBytes = nil
+			})
+			return lazyCert.v, nil
+		})
 		ok = true
 	}
-
-	return
+	return ok
 }
 
 // Subjects returns a list of the DER-encoded subjects of