rpc: treat protocol handshake errors as permanent

treat handshake errors as permanent on the client

The issue was observed by 100% CPU usage due to lack ofrate-limiting in
dataconn.ReqPing retries=> safeguard that

rpc/rpc_client.go

@@ -142,6 +142,11 @@ func (c *Client) WaitForConnectivity(ctx context.Context) error {
 			// dataClient uses transport.Connecter, which doesn't expose FailFast(false)
 			// => we need to mask dial timeouts
 			if err, ok := dataErr.(interface{ Temporary() bool }); ok && err.Temporary() {
+				// Rate-limit pings here in case Temporary() is a mis-classification
+				// or returns immediately (this is a tight loop in that case)
+				// TODO keep this in lockstep with controlClient
+				// 		=> don't use FailFast for control, but check that both control and data worked
+				time.Sleep(envconst.Duration("ZREPL_RPC_DATACONN_PING_SLEEP", 1*time.Second))
 				continue
 			}
 			// it's not a dial timeout, 

rpc/transportmux/transportmux.go

@@ -7,10 +7,10 @@ package transportmux
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"net"
 	"time"
-	"fmt"
 
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/transport"
@@ -111,7 +111,7 @@ func Demux(ctx context.Context, rawListener transport.AuthenticatedListener, lab
 				if ctx.Err() != nil {
 					return
 				}
-				getLog(ctx).WithError(err).Error("accept error")
+				getLog(ctx).WithError(err).WithField("errType", fmt.Sprintf("%T", err)).Error("accept error")
 				continue
 			}
 			closeConn := func() {

rpc/versionhandshake/versionhandshake.go

@@ -26,14 +26,22 @@ type HandshakeError struct {
 	msg string
 	// If not nil, the underlying IO error that caused the handshake to fail.
 	IOError error
+	isAcceptError bool
 }
 
 var _ net.Error = &HandshakeError{}
 
 func (e HandshakeError) Error() string { return e.msg }
 
-// Always true to enable usage in a net.Listener.
+// Like with net.OpErr (Go issue 6163), a client failing to handshake
-func (e HandshakeError) Temporary() bool { return true }
+// should be a temporary Accept error toward the Listener .
+func (e HandshakeError) Temporary() bool {
+	if  e.isAcceptError {
+		return true
+	}
+	te, ok := e.IOError.(interface{ Temporary() bool });
+	return ok && te.Temporary()
+}
 
 // If the underlying IOError was net.Error.Timeout(), Timeout() returns that value.
 // Otherwise false.
@@ -142,14 +150,14 @@ func (m *HandshakeMessage) DecodeReader(r io.Reader, maxLen int) error {
 	return nil
 }
 
-func DoHandshakeCurrentVersion(conn net.Conn, deadline time.Time) error {
+func DoHandshakeCurrentVersion(conn net.Conn, deadline time.Time) *HandshakeError {
 	// current protocol version is hardcoded here
 	return DoHandshakeVersion(conn, deadline, 1)
 }
 
 const HandshakeMessageMaxLen = 16 * 4096
 
-func DoHandshakeVersion(conn net.Conn, deadline time.Time, version int) error {
+func DoHandshakeVersion(conn net.Conn, deadline time.Time, version int) *HandshakeError {
 	ours := HandshakeMessage{
 		ProtocolVersion: version,
 		Extensions: nil,

rpc/versionhandshake/versionhandshake_transport_wrappers.go

@@ -55,6 +55,7 @@ func (l HandshakeListener) Accept(ctx context.Context) (*transport.AuthConn, err
 		dl = time.Now().Add(l.timeout) // shadowing
 	}
 	if err := DoHandshakeCurrentVersion(conn, dl); err != nil {
+		err.isAcceptError = true
 		conn.Close()
 		return nil, err
 	}