crypto/tls: add Config.GetConfigForClient

GetConfigForClient allows the tls.Config to be updated on a per-client
basis.

Fixes #16066.
Fixes #15707.
Fixes #15699.

Change-Id: I2c675a443d557f969441226729f98502b38901ea
Reviewed-on: https://go-review.googlesource.com/30790
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

common.go

@@ -303,7 +303,27 @@ type Config struct {
 	// If GetCertificate is nil or returns nil, then the certificate is
 	// retrieved from NameToCertificate. If NameToCertificate is nil, the
 	// first element of Certificates will be used.
-	GetCertificate func(clientHello *ClientHelloInfo) (*Certificate, error)
+	GetCertificate func(*ClientHelloInfo) (*Certificate, error)
+
+	// GetConfigForClient, if not nil, is called after a ClientHello is
+	// received from a client. It may return a non-nil Config in order to
+	// change the Config that will be used to handle this connection. If
+	// the returned Config is nil, the original Config will be used. The
+	// Config returned by this callback may not be subsequently modified.
+	//
+	// If GetConfigForClient is nil, the Config passed to Server() will be
+	// used for all connections.
+	//
+	// Uniquely for the fields in the returned Config, session ticket keys
+	// will be duplicated from the original Config if not set.
+	// Specifically, if SetSessionTicketKeys was called on the original
+	// config but not on the returned config then the ticket keys from the
+	// original config will be copied into the new config before use.
+	// Otherwise, if SessionTicketKey was set in the original config but
+	// not in the returned config then it will be copied into the returned
+	// config before use. If neither of those cases applies then the key
+	// material from the returned config will be used for session tickets.
+	GetConfigForClient func(*ClientHelloInfo) (*Config, error)
 
 	// RootCAs defines the set of root certificate authorities
 	// that clients use when verifying server certificates.
@@ -398,13 +418,17 @@ type Config struct {
 
 	serverInitOnce sync.Once // guards calling (*Config).serverInit
 
-	// mutex protects sessionTicketKeys
+	// mutex protects sessionTicketKeys and originalConfig.
 	mutex sync.RWMutex
 	// sessionTicketKeys contains zero or more ticket keys. If the length
 	// is zero, SessionTicketsDisabled must be true. The first key is used
 	// for new tickets and any subsequent keys can be used to decrypt old
 	// tickets.
 	sessionTicketKeys []ticketKey
+	// originalConfig is set to the Config that was passed to Server if
+	// this Config is returned by a GetConfigForClient callback. It's used
+	// by serverInit in order to copy session ticket keys if needed.
+	originalConfig *Config
 }
 
 // ticketKeyNameLen is the number of bytes of identifier that is prepended to
@@ -434,12 +458,18 @@ func ticketKeyFromBytes(b [32]byte) (key ticketKey) {
 // Clone returns a shallow clone of c.
 // Only the exported fields are copied.
 func (c *Config) Clone() *Config {
+	var sessionTicketKeys []ticketKey
+	c.mutex.RLock()
+	sessionTicketKeys = c.sessionTicketKeys
+	c.mutex.RUnlock()
+
 	return &Config{
 		Rand:                        c.Rand,
 		Time:                        c.Time,
 		Certificates:                c.Certificates,
 		NameToCertificate:           c.NameToCertificate,
 		GetCertificate:              c.GetCertificate,
+		GetConfigForClient:          c.GetConfigForClient,
 		RootCAs:                     c.RootCAs,
 		NextProtos:                  c.NextProtos,
 		ServerName:                  c.ServerName,
@@ -457,6 +487,8 @@ func (c *Config) Clone() *Config {
 		DynamicRecordSizingDisabled: c.DynamicRecordSizingDisabled,
 		Renegotiation:               c.Renegotiation,
 		KeyLogWriter:                c.KeyLogWriter,
+		sessionTicketKeys:           sessionTicketKeys,
+		// originalConfig is deliberately not duplicated.
 	}
 }
 
@@ -465,6 +497,11 @@ func (c *Config) serverInit() {
 		return
 	}
 
+	var originalConfig *Config
+	c.mutex.Lock()
+	originalConfig, c.originalConfig = c.originalConfig, nil
+	c.mutex.Unlock()
+
 	alreadySet := false
 	for _, b := range c.SessionTicketKey {
 		if b != 0 {
@@ -474,13 +511,21 @@ func (c *Config) serverInit() {
 	}
 
 	if !alreadySet {
-		if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
+		if originalConfig != nil {
+			copy(c.SessionTicketKey[:], originalConfig.SessionTicketKey[:])
+		} else if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
 			c.SessionTicketsDisabled = true
 			return
 		}
 	}
 
-	c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}
+	if originalConfig != nil {
+		originalConfig.mutex.RLock()
+		c.sessionTicketKeys = originalConfig.sessionTicketKeys
+		originalConfig.mutex.RUnlock()
+	} else {
+		c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}
+	}
 }
 
 func (c *Config) ticketKeys() []ticketKey {