Handle empty input in AddStringKeyStoreCommand (#39490)

This change ensures that we do not make assumptions about the length
of the input that we can read from the stdin. It still consumes only
one line, as the previous implementation

Co-authored-by: Ioannis Kakavas <ikakavas@protonmail.com>

server/src/main/java/org/elasticsearch/common/settings/AddStringKeyStoreCommand.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.common.settings;
 
 import java.io.BufferedReader;
+import java.io.CharArrayWriter;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.nio.charset.StandardCharsets;
@@ -83,8 +84,17 @@ protected void execute(Terminal terminal, OptionSet options, Environment env) th
 
         final char[] value;
         if (options.has(stdinOption)) {
-            BufferedReader stdinReader = new BufferedReader(new InputStreamReader(getStdin(), StandardCharsets.UTF_8));
-            value = stdinReader.readLine().toCharArray();
+            try (BufferedReader stdinReader = new BufferedReader(new InputStreamReader(getStdin(), StandardCharsets.UTF_8));
+                 CharArrayWriter writer = new CharArrayWriter()) {
+                int charInt;
+                while ((charInt = stdinReader.read()) != -1) {
+                    if ((char) charInt == '\r' || (char) charInt == '\n') {
+                        break;
+                    }
+                    writer.write((char) charInt);
+                }
+                value = writer.toCharArray();
+            }
         } else {
             value = terminal.readSecret("Enter value for " + setting + ": ");
         }

server/src/test/java/org/elasticsearch/common/settings/AddStringKeyStoreCommandTests.java

@@ -134,6 +134,27 @@ public void testStdinLong() throws Exception {
         assertSecureString("foo", "secret value 2");
     }
 
+    public void testStdinNoInput() throws Exception {
+        KeyStoreWrapper.create().save(env.configFile(), new char[0]);
+        setInput("");
+        execute("-x", "foo");
+        assertSecureString("foo", "");
+    }
+
+    public void testStdinInputWithLineBreaks() throws Exception {
+        KeyStoreWrapper.create().save(env.configFile(), new char[0]);
+        setInput("Typedthisandhitenter\n");
+        execute("-x", "foo");
+        assertSecureString("foo", "Typedthisandhitenter");
+    }
+
+    public void testStdinInputWithCarriageReturn() throws Exception {
+        KeyStoreWrapper.create().save(env.configFile(), new char[0]);
+        setInput("Typedthisandhitenter\r");
+        execute("-x", "foo");
+        assertSecureString("foo", "Typedthisandhitenter");
+    }
+
     public void testMissingSettingName() throws Exception {
         createKeystore("");
         terminal.addTextInput("");