[client] Guard the shared OAuth flow state with the server mutex

reuseOAuthFlow read flow, expiresAt, waitCancel and info without holding
s.mutex, while startSSOLogin and WaitSSOLogin write them under it. Reading the
fields one at a time could also answer with auth info from a flow that was
already replaced, or cancel a wait that no longer belongs to the flow just
judged stale. Take one snapshot under the lock and decide from it.

WaitSSOLogin read oauthAuthFlow.flow twice outside the lock; both now use a
value snapshotted in the critical section that already installs actCancel.

Its stale waitCancel was read and called in a separate section from the one
installing the new one, so two racing calls could read the same predecessor and
leave one wait uncancelled. Swap the two in a single critical section. Both
cancels run after unlocking: the displaced wait takes s.mutex as it unwinds.

client/server/server.go

@@ -738,15 +738,26 @@ func (s *Server) startSSOLogin(ctx context.Context, msg *proto.LoginRequest, con
 // reuseOAuthFlow returns the cached auth info when the previous flow targets
 // the same client and still has enough life left, and otherwise cancels the
 // stale wait and returns nil so the caller requests a fresh flow.
+//
+// The whole decision runs off one snapshot taken under s.mutex: a concurrent
+// WaitSSOLogin replaces waitCancel and expires the flow, so reading the fields
+// one at a time could cancel a wait that no longer belongs to the flow just
+// judged stale, or answer with auth info from a flow that was already replaced.
+// The cancel itself is called after unlocking — it runs arbitrary teardown, and
+// WaitSSOLogin takes s.mutex on the way out.
 func (s *Server) reuseOAuthFlow(ctx context.Context, oAuthFlow auth.OAuthFlow, state statusSetter) *proto.LoginResponse {
-	if s.oauthAuthFlow.flow == nil || s.oauthAuthFlow.flow.GetClientID(ctx) != oAuthFlow.GetClientID(ctx) {
+	s.mutex.Lock()
+	current := s.oauthAuthFlow
+	s.mutex.Unlock()
+
+	if current.flow == nil || current.flow.GetClientID(ctx) != oAuthFlow.GetClientID(ctx) {
 		return nil
 	}
 
-	if !s.oauthAuthFlow.expiresAt.After(time.Now().Add(90 * time.Second)) {
+	if !current.expiresAt.After(time.Now().Add(90 * time.Second)) {
 		log.Warnf("canceling previous waiting execution")
-		if s.oauthAuthFlow.waitCancel != nil {
-			s.oauthAuthFlow.waitCancel()
+		if current.waitCancel != nil {
+			current.waitCancel()
 		}
 		return nil
 	}
@@ -755,9 +766,9 @@ func (s *Server) reuseOAuthFlow(ctx context.Context, oAuthFlow auth.OAuthFlow, s
 	state.Set(internal.StatusNeedsLogin)
 	return &proto.LoginResponse{
 		NeedsSSOLogin:           true,
-		VerificationURI:         s.oauthAuthFlow.info.VerificationURI,
-		VerificationURIComplete: s.oauthAuthFlow.info.VerificationURIComplete,
-		UserCode:                s.oauthAuthFlow.info.UserCode,
+		VerificationURI:         current.info.VerificationURI,
+		VerificationURIComplete: current.info.VerificationURIComplete,
+		UserCode:                current.info.UserCode,
 	}
 }
 
@@ -831,9 +842,10 @@ func (s *Server) WaitSSOLogin(callerCtx context.Context, msg *proto.WaitSSOLogin
 	}
 
 	s.actCancel = cancel
+	flow := s.oauthAuthFlow.flow
 	s.mutex.Unlock()
 
-	if s.oauthAuthFlow.flow == nil {
+	if flow == nil {
 		return nil, gstatus.Errorf(codes.Internal, "oauth flow is not initialized")
 	}
 
@@ -860,18 +872,23 @@ func (s *Server) WaitSSOLogin(callerCtx context.Context, msg *proto.WaitSSOLogin
 		return nil, gstatus.Errorf(codes.InvalidArgument, "sso user code is invalid")
 	}
 
-	if s.oauthAuthFlow.waitCancel != nil {
-		s.oauthAuthFlow.waitCancel()
-	}
-
 	waitCTX, cancel := context.WithCancel(ctx)
 	defer cancel()
 
+	// Swap in this wait's cancel and take over the one it displaces in a single
+	// critical section, so two WaitSSOLogin calls racing here cannot both read
+	// the same predecessor and leave one wait uncancelled. Cancelling happens
+	// after the unlock: the displaced wait takes s.mutex as it unwinds.
 	s.mutex.Lock()
+	staleCancel := s.oauthAuthFlow.waitCancel
 	s.oauthAuthFlow.waitCancel = cancel
 	s.mutex.Unlock()
 
-	tokenInfo, err := s.oauthAuthFlow.flow.WaitToken(waitCTX, flowInfo)
+	if staleCancel != nil {
+		staleCancel()
+	}
+
+	tokenInfo, err := flow.WaitToken(waitCTX, flowInfo)
 	if err != nil {
 		s.mutex.Lock()
 		s.oauthAuthFlow.expiresAt = time.Now()