go.crypto/ssh: prevent channel writes after Close

Fixes golang/go#3810.

This change introduces an atomic boolean to guard the close
of the clientChan. Previously the client code was quite
lax with the ordering of the close messages and could allow
window adjustment or EOF messages to leak after Close had
been signaled.

Consolidating the changes to the serverChan will be handled
in a following CL.

R=agl, kardianos, gustav.paul
CC=golang-dev
https://golang.org/cl/6405064

ssh/session_test.go

@@ -380,6 +380,54 @@ func TestServerStdoutRespectsMaxPacketSize(t *testing.T) {
 	}
 }
 
+// TODO(dfc) currently writes succeed after Close()
+func testClientCannotSendAfterEOF(t *testing.T) {
+	conn := dial(shellHandler, t)
+	defer conn.Close()
+	session, err := conn.NewSession()
+	if err != nil {
+		t.Fatalf("Unable to request new session: %v", err)
+	}
+	defer session.Close()
+	in, err := session.StdinPipe()
+	if err != nil {
+		t.Fatalf("Unable to connect channel stdin: %v", err)
+	}
+	if err := session.Shell(); err != nil {
+		t.Fatalf("Unable to execute command: %v", err)
+	}
+	if err := in.Close(); err != nil {
+		t.Fatalf("Unable to close stdin: %v", err)
+	}
+	if _, err := in.Write([]byte("foo")); err == nil {
+		t.Fatalf("Session write should fail")
+	}
+}
+
+func TestClientCannotSendAfterClose(t *testing.T) {
+	conn := dial(shellHandler, t)
+	defer conn.Close()
+	session, err := conn.NewSession()
+	if err != nil {
+		t.Fatalf("Unable to request new session: %v", err)
+	}
+	defer session.Close()
+	in, err := session.StdinPipe()
+	if err != nil {
+		t.Fatalf("Unable to connect channel stdin: %v", err)
+	}
+	if err := session.Shell(); err != nil {
+		t.Fatalf("Unable to execute command: %v", err)
+	}
+	// close underlying channel
+	if err := session.channel.Close(); err != nil {
+		t.Fatalf("Unable to close session: %v", err)
+	}
+	if _, err := in.Write([]byte("foo")); err == nil {
+		t.Fatalf("Session write should fail")
+	}
+}
+
 type exitStatusMsg struct {
 	PeersId   uint32
 	Request   string