feat(creds): retrieve peer credentials when using unix socket

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

creds/peercreds/peercreds.go

@@ -0,0 +1,68 @@
+package peercreds
+
+import (
+	"context"
+	"errors"
+	"net"
+
+	"github.com/tailscale/peercred"
+	"google.golang.org/grpc/credentials"
+)
+
+var _ credentials.TransportCredentials = (*peerCreds)(nil)
+
+var common = credentials.CommonAuthInfo{SecurityLevel: credentials.PrivacyAndIntegrity}
+
+func New() credentials.TransportCredentials {
+	return &peerCreds{info: credentials.ProtocolInfo{
+		SecurityProtocol: "peercred",
+		ProtocolVersion:  "0.1",
+	}}
+}
+
+type peerCreds struct {
+	info credentials.ProtocolInfo
+}
+
+// AuthInfo we’ll attach to the gRPC peer
+type AuthInfo struct {
+	credentials.CommonAuthInfo
+	Creds *peercred.Creds
+}
+
+func (AuthInfo) AuthType() string { return "peercred" }
+
+func (t *peerCreds) ClientHandshake(_ context.Context, _ string, conn net.Conn) (net.Conn, credentials.AuthInfo, error) {
+	return t.handshakeConn(conn)
+}
+
+func (t *peerCreds) ServerHandshake(conn net.Conn) (net.Conn, credentials.AuthInfo, error) {
+	return t.handshakeConn(conn)
+}
+
+func (t *peerCreds) Info() credentials.ProtocolInfo {
+	return t.info
+}
+
+func (t *peerCreds) Clone() credentials.TransportCredentials {
+	return &peerCreds{info: t.info}
+}
+
+func (t *peerCreds) OverrideServerName(name string) error {
+	t.info.ServerName = name
+	return nil
+}
+
+func (t *peerCreds) handshakeConn(conn net.Conn) (net.Conn, credentials.AuthInfo, error) {
+	if conn.RemoteAddr().Network() != "unix" {
+		return nil, nil, errors.New("peercred only works with unix domain sockets")
+	}
+	creds, err := peercred.Get(conn)
+	if err != nil {
+		if errors.Is(err, peercred.ErrNotImplemented) {
+			return nil, nil, errors.New("peercred not implemented on this OS")
+		}
+		return nil, nil, err
+	}
+	return conn, AuthInfo{Creds: creds, CommonAuthInfo: common}, nil
+}