Add exception type for READONLY errors #1943

Redis instances in read-only mode will reply with a `READONLY` error
message when a write is attempted. This change allows users to
specifically handle these errors.

Original pull request: #1944.

src/main/java/io/lettuce/core/RedisReadOnlyException.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.lettuce.core;
+
+/**
+ * Exception that gets thrown when Redis replies with a {@code READONLY} error response.
+ *
+ * @author anothertobi
+ */
+@SuppressWarnings("serial")
+public class RedisReadOnlyException extends RedisCommandExecutionException {
+
+    /**
+     * Create a {@code RedisReadOnlyException} with the specified detail message.
+     *
+     * @param msg the detail message.
+     */
+    public RedisReadOnlyException(String msg) {
+        super(msg);
+    }
+
+    /**
+     * Create a {@code RedisReadOnlyException} with the specified detail message and nested exception.
+     *
+     * @param msg the detail message.
+     * @param cause the nested exception.
+     */
+    public RedisReadOnlyException(String msg, Throwable cause) {
+        super(msg, cause);
+    }
+
+}

src/main/java/io/lettuce/core/internal/ExceptionFactory.java

@@ -134,6 +134,10 @@ public static RedisCommandExecutionException createExecutionException(String mes
                 return cause != null ? new RedisLoadingException(message, cause) : new RedisLoadingException(message);
             }
 
+            if (message.startsWith("READONLY")) {
+                return cause != null ? new RedisReadOnlyException(message, cause) : new RedisReadOnlyException(message);
+            }
+
             return cause != null ? new RedisCommandExecutionException(message, cause)
                     : new RedisCommandExecutionException(message);
         }

src/test/java/io/lettuce/core/ExceptionFactoryUnitTests.java

@@ -70,6 +70,16 @@ void shouldCreateLoadingException() {
                 .hasRootCauseInstanceOf(IllegalStateException.class);
     }
 
+    @Test
+    void shouldCreateReadOnlyException() {
+
+        assertThat(ExceptionFactory.createExecutionException("READONLY foo bar")).isInstanceOf(RedisReadOnlyException.class)
+                .hasMessage("READONLY foo bar").hasNoCause();
+        assertThat(ExceptionFactory.createExecutionException("READONLY foo bar", new IllegalStateException()))
+                .isInstanceOf(RedisReadOnlyException.class).hasMessage("READONLY foo bar")
+                .hasRootCauseInstanceOf(IllegalStateException.class);
+    }
+
     @Test
     void shouldFormatExactUnits() {