4 files changed, 85 insertions, 7 deletions
diff --git a/src/crypto/tls/tls.go b/src/crypto/tls/tls.go
index 8509b7dc0d..b30f0b8fe4 100644
--- a/src/crypto/tls/tls.go
+++ b/src/crypto/tls/tls.go
@@ -22,6 +22,7 @@ import (
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"internal/godebug"
 	"net"
 	"os"
 	"strings"
@@ -222,11 +223,14 @@ func (d *Dialer) DialContext(ctx context.Context, network, addr string) (net.Con
 	return c, nil
 }
 
-// LoadX509KeyPair reads and parses a public/private key pair from a pair
-// of files. The files must contain PEM encoded data. The certificate file
-// may contain intermediate certificates following the leaf certificate to
-// form a certificate chain. On successful return, Certificate.Leaf will
-// be nil because the parsed form of the certificate is not retained.
+// LoadX509KeyPair reads and parses a public/private key pair from a pair of
+// files. The files must contain PEM encoded data. The certificate file may
+// contain intermediate certificates following the leaf certificate to form a
+// certificate chain. On successful return, Certificate.Leaf will be populated.
+//
+// Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
 func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 	certPEMBlock, err := os.ReadFile(certFile)
 	if err != nil {
@@ -239,9 +243,14 @@ func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 	return X509KeyPair(certPEMBlock, keyPEMBlock)
 }
 
+var x509keypairleaf = godebug.New("x509keypairleaf")
+
 // X509KeyPair parses a public/private key pair from a pair of
-// PEM encoded data. On successful return, Certificate.Leaf will be nil because
-// the parsed form of the certificate is not retained.
+// PEM encoded data. On successful return, Certificate.Leaf will be populated.
+//
+// Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
 func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 	fail := func(err error) (Certificate, error) { return Certificate{}, err }
 
@@ -296,6 +305,12 @@ func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 		return fail(err)
 	}
 
+	if x509keypairleaf.Value() != "0" {
+		cert.Leaf = x509Cert
+	} else {
+		x509keypairleaf.IncNonDefault()
+	}
+
 	cert.PrivateKey, err = parsePrivateKey(keyDERBlock.Bytes)
 	if err != nil {
 		return fail(err)
diff --git a/src/crypto/tls/tls_test.go b/src/crypto/tls/tls_test.go
index 69b57de1e6..158b459976 100644
--- a/src/crypto/tls/tls_test.go
+++ b/src/crypto/tls/tls_test.go
@@ -8,13 +8,19 @@ import (
 	"bytes"
 	"context"
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
 	"crypto/x509"
+	"crypto/x509/pkix"
 	"encoding/json"
+	"encoding/pem"
 	"errors"
 	"fmt"
 	"internal/testenv"
 	"io"
 	"math"
+	"math/big"
 	"net"
 	"os"
 	"reflect"
@@ -1945,3 +1951,54 @@ func TestHandshakeKyber(t *testing.T) {
 		})
 	}
 }
+
+func TestX509KeyPairPopulateCertificate(t *testing.T) {
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	keyDER, err := x509.MarshalPKCS8PrivateKey(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	keyPEM := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: keyDER})
+	tmpl := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject:      pkix.Name{CommonName: "test"},
+	}
+	certDER, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, key.Public(), key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	certPEM := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: certDER})
+
+	t.Run("x509keypairleaf=0", func(t *testing.T) {
+		t.Setenv("GODEBUG", "x509keypairleaf=0")
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf != nil {
+			t.Fatal("Leaf should not be populated")
+		}
+	})
+	t.Run("x509keypairleaf=1", func(t *testing.T) {
+		t.Setenv("GODEBUG", "x509keypairleaf=1")
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf == nil {
+			t.Fatal("Leaf should be populated")
+		}
+	})
+	t.Run("GODEBUG unset", func(t *testing.T) {
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf == nil {
+			t.Fatal("Leaf should be populated")
+		}
+	})
+}
diff --git a/src/internal/godebugs/table.go b/src/internal/godebugs/table.go
index eaa95d5aa9..df99334cb0 100644
--- a/src/internal/godebugs/table.go
+++ b/src/internal/godebugs/table.go
@@ -54,6 +54,7 @@ var All = []Info{
 	{Name: "tlsunsafeekm", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "winreadlinkvolume", Package: "os", Changed: 22, Old: "0"},
 	{Name: "winsymlink", Package: "os", Changed: 22, Old: "0"},
+	{Name: "x509keypairleaf", Package: "crypto/tls", Changed: 23, Old: "0"},
 	{Name: "x509negativeserial", Package: "crypto/x509", Changed: 23, Old: "1"},
 	{Name: "x509seriallength", Package: "crypto/x509", Changed: 23, Old: "1"},
 	{Name: "x509sha1", Package: "crypto/x509"},
diff --git a/src/runtime/metrics/doc.go b/src/runtime/metrics/doc.go
index 2dd8ce261c..c89e176986 100644
--- a/src/runtime/metrics/doc.go
+++ b/src/runtime/metrics/doc.go
@@ -326,6 +326,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the os package
 		due to a non-default GODEBUG=winsymlink=... setting.
 
+	/godebug/non-default-behavior/x509keypairleaf:events
+		The number of non-default behaviors executed by the crypto/tls
+		package due to a non-default GODEBUG=x509keypairleaf=...
+		setting.
+
 	/godebug/non-default-behavior/x509negativeserial:events
 		The number of non-default behaviors executed by the crypto/x509
 		package due to a non-default GODEBUG=x509negativeserial=...