CLI: Fix prompting for yes/no to handle console returning null (#23320)

Console.readText may return null in certain cases. This commit fixes a
bug in Terminal.promptYesNo which assumed a non-null return value.  It
also adds a test for this, and modifies mock terminal to be able to
handle null input values.

core/src/main/java/org/elasticsearch/cli/Terminal.java

@@ -100,10 +100,11 @@ public abstract class Terminal {
     public final boolean promptYesNo(String prompt, boolean defaultYes) {
         String answerPrompt = defaultYes ? " [Y/n]" : " [y/N]";
         while (true) {
-            String answer = readText(prompt + answerPrompt).toLowerCase(Locale.ROOT);
-            if (answer.isEmpty()) {
+            String answer = readText(prompt + answerPrompt);
+            if (answer == null || answer.isEmpty()) {
                 return defaultYes;
             }
+            answer = answer.toLowerCase(Locale.ROOT);
             boolean answerYes = answer.equals("y");
             if (answerYes == false && answer.equals("n") == false) {
                 println("Did not understand answer '" + answer + "'");

core/src/test/java/org/elasticsearch/cli/TerminalTests.java

@@ -52,6 +52,8 @@ public class TerminalTests extends ESTestCase {
         assertTrue(terminal.promptYesNo("Answer?", true));
         terminal.addTextInput("");
         assertFalse(terminal.promptYesNo("Answer?", false));
+        terminal.addTextInput(null);
+        assertFalse(terminal.promptYesNo("Answer?", false));
     }
 
     public void testPromptYesNoReprompt() throws Exception {

test/framework/src/main/java/org/elasticsearch/cli/MockTerminal.java

@@ -25,7 +25,9 @@ import java.io.PrintWriter;
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayDeque;
+import java.util.ArrayList;
 import java.util.Deque;
+import java.util.List;
 
 /**
  * A terminal for tests which captures all output, and
@@ -35,8 +37,18 @@ public class MockTerminal extends Terminal {
 
     private final ByteArrayOutputStream buffer = new ByteArrayOutputStream();
     private final PrintWriter writer = new PrintWriter(new OutputStreamWriter(buffer, StandardCharsets.UTF_8));
-    private final Deque<String> textInput = new ArrayDeque<>();
-    private final Deque<String> secretInput = new ArrayDeque<>();
+
+    // A deque would be a perfect data structure for the FIFO queue of input values needed here. However,
+    // to support the valid return value of readText being null (defined by Console), we need to be able
+    // to store nulls. However, java the java Deque api does not allow nulls because it uses null as
+    // a special return value from certain methods like peek(). So instead of deque, we use an array list here,
+    // and keep track of the last position which was read. It means that we will hold onto all input
+    // setup for the mock terminal during its lifetime, but this is normally a very small amount of data
+    // so in reality it will not matter.
+    private final List<String> textInput = new ArrayList<>();
+    private int textIndex = 0;
+    private final List<String> secretInput = new ArrayList<>();
+    private int secretIndex = 0;
 
     public MockTerminal() {
         super("\n"); // always *nix newlines for tests
@@ -44,18 +56,18 @@ public class MockTerminal extends Terminal {
 
     @Override
     public String readText(String prompt) {
-        if (textInput.isEmpty()) {
+        if (textIndex >= textInput.size()) {
             throw new IllegalStateException("No text input configured for prompt [" + prompt + "]");
         }
-        return textInput.removeFirst();
+        return textInput.get(textIndex++);
     }
 
     @Override
     public char[] readSecret(String prompt) {
-        if (secretInput.isEmpty()) {
+        if (secretIndex >= secretInput.size()) {
             throw new IllegalStateException("No secret input configured for prompt [" + prompt + "]");
         }
-        return secretInput.removeFirst().toCharArray();
+        return secretInput.get(secretIndex++).toCharArray();
     }
 
     @Override
@@ -65,12 +77,12 @@ public class MockTerminal extends Terminal {
 
     /** Adds an an input that will be return from {@link #readText(String)}. Values are read in FIFO order. */
     public void addTextInput(String input) {
-        textInput.addLast(input);
+        textInput.add(input);
     }
 
     /** Adds an an input that will be return from {@link #readText(String)}. Values are read in FIFO order. */
     public void addSecretInput(String input) {
-        secretInput.addLast(input);
+        secretInput.add(input);
     }
 
     /** Returns all output written to this terminal. */