Fix EOFs when reading from empty canon->noncanon PTY

when switching from canonical to non-canonical mode, only mark inQueue as readable if there is input to be read

pkg/sentry/fsimpl/devpts/line_discipline.go

@@ -154,7 +154,7 @@ func (l *lineDiscipline) setTermios(task *kernel.Task, args arch.SyscallArgument
 	if oldCanonEnabled && !l.termios.LEnabled(linux.ICANON) {
 		l.inQueue.mu.Lock()
 		l.inQueue.pushWaitBufLocked(l)
-		l.inQueue.readable = true
+		l.inQueue.readable = len(l.inQueue.readBuf) > 0
 		l.inQueue.mu.Unlock()
 		l.termiosMu.Unlock()
 		l.replicaWaiter.Notify(waiter.ReadableEvents)

test/syscalls/linux/pty.cc

@@ -1152,6 +1152,23 @@ TEST_F(PtyTest, SwitchNoncanonToCanonNoNewlineBig) {
   ExpectFinished(replica_);
 }
 
+// If the canonical input buffer is empty, and we switch to non-canonical
+// mode, the input buffer should not be readable.
+TEST_F(PtyTest, SwitchCanonToNoncanonEmptyInput) {
+  constexpr char kInput[] = "";
+  ASSERT_THAT(WriteFd(master_.get(), kInput, sizeof(kInput)),
+              SyscallSucceedsWithValue(sizeof(kInput)));
+
+  DisableCanonical();
+
+  // Nothing available yet.
+  char buf[2] = {};
+  ASSERT_THAT(PollAndReadFd(replica_.get(), buf, 2, kTimeoutShort),
+              PosixErrorIs(ETIMEDOUT, ::testing::StrEq("Poll timed out")));
+
+  ExpectFinished(replica_);
+}
+
 // Tests that we can write over the 4095 noncanonical limit, then read out
 // everything.
 TEST_F(PtyTest, NoncanonBigWrite) {