crypto/tls: don't overwrite Config.sessionTicketKeys if already set

[mark-kubacki]

With Go 1.6.2 and earlier and in tls.Config, serverInit() overwrites effects of a prior SetSessionTicketKeys().

Initializing SessionTicketKey to an arbitrary non-null value does not sort this in case of TLS ticket key rotation: SetSessionTicketKeys() could have already been called several times without being allowed to update that lone field because »after [tls.Config has] been passed to a TLS function it must not be modified«.

I expected serverInit() to:

diff --git a/src/crypto/tls/common.go b/src/crypto/tls/common.go
index c68ebfe..94be97e 100644
--- a/src/crypto/tls/common.go
+++ b/src/crypto/tls/common.go
@@ -388,6 +388,9 @@ func (c *Config) serverInit() {
        if c.SessionTicketsDisabled {
                return
        }
+       if len(c.ticketKeys()) > 0 {
+               return
+       }

        alreadySet := false
        for _, b := range c.SessionTicketKey {

[mark-kubacki]

go/src/crypto/tls/common.go

Lines 424 to 435 in 57e459e

	func (c *Config) SetSessionTicketKeys(keys [][32]byte) {
	if len(keys) == 0 {
	panic("tls: keys must have at least one key")
	}
	newKeys := make([]ticketKey, len(keys))
	for i, bytes := range keys {
	newKeys[i] = ticketKeyFromBytes(bytes)
	}
	c.mutex.Lock()
	c.sessionTicketKeys = newKeys

go/src/crypto/tls/common.go

Lines 387 to 407 in 57e459e

	func (c *Config) serverInit() {
	if c.SessionTicketsDisabled {
	return
	}
	alreadySet := false
	for _, b := range c.SessionTicketKey {
	if b != 0 {
	alreadySet = true
	break
	}
	}
	if !alreadySet {
	if _, err := io.ReadFull(c.rand(), c.SessionTicketKey[:]); err != nil {
	c.SessionTicketsDisabled = true
	return
	}
	}
	c.sessionTicketKeys = []ticketKey{ticketKeyFromBytes(c.SessionTicketKey)}

[mark-kubacki]

Given that this is a minor bugfix, any chance to see this in Go 1.7 or 1.8?

[gopherbot]

CL https://golang.org/cl/27317 mentions this issue.