Improve error message in 8.x to 7.x downgrade

server/src/main/java/org/elasticsearch/env/NodeEnvironment.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.node.DiscoveryNodeRole;
+import org.elasticsearch.common.io.Channels;
 import org.elasticsearch.core.CheckedFunction;
 import org.elasticsearch.common.Randomness;
 import org.elasticsearch.core.SuppressForbidden;
@@ -53,12 +54,19 @@
 import java.io.Closeable;
 import java.io.IOException;
 import java.io.UncheckedIOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
+import java.nio.charset.CharacterCodingException;
+import java.nio.charset.CharsetDecoder;
+import java.nio.charset.CodingErrorAction;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.AtomicMoveNotSupportedException;
 import java.nio.file.DirectoryStream;
 import java.nio.file.FileStore;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
+import java.nio.file.StandardOpenOption;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -252,6 +260,17 @@ public NodeEnvironment(Settings settings, Environment environment) throws IOExce
         NodeLock nodeLock = null;
 
         try {
+            for (Path path : environment.dataFiles()) {
+                final Path nodesPath = path.resolve(NODES_FOLDER);
+                if (Files.exists(nodesPath) && Files.isDirectory(nodesPath) == false) {
+                    throw new IllegalStateException(
+                        "data path [" + path + "] is not compatible with Elasticsearch v" + Version.CURRENT +
+                            ", perhaps it has already been upgraded to a later version",
+                        new IllegalStateException(
+                            "[" + nodesPath + "] is a file which contains [" + readFileContents(nodesPath) + "]"));
+                }
+            }
+
             sharedDataPath = environment.sharedDataFile();
             IOException lastException = null;
             int maxLocalStorageNodes = MAX_LOCAL_STORAGE_NODES_SETTING.get(settings);
@@ -326,6 +345,26 @@ public NodeEnvironment(Settings settings, Environment environment) throws IOExce
         }
     }
 
+    private static String readFileContents(Path nodesPath) throws IOException {
+        final int maxBytes = 256;
+
+        try (FileChannel fileChannel = FileChannel.open(nodesPath, StandardOpenOption.READ)) {
+            final ByteBuffer byteBuffer = ByteBuffer.allocate(maxBytes);
+            final int len = Channels.readFromFileChannel(fileChannel, 0, byteBuffer);
+            byteBuffer.flip();
+
+            final CharsetDecoder charsetDecoder = StandardCharsets.UTF_8
+                .newDecoder()
+                .onMalformedInput(CodingErrorAction.REPORT)
+                .onUnmappableCharacter(CodingErrorAction.REPORT);
+            try {
+                return charsetDecoder.decode(byteBuffer) + (len == maxBytes ? "..." : "");
+            } catch (CharacterCodingException e) {
+                return "<unreadable>";
+            }
+        }
+    }
+
     /**
      * Resolve a specific nodes/{node.id} path for the specified path and node lock id.
      *

server/src/test/java/org/elasticsearch/env/NodeEnvironmentTests.java

@@ -11,12 +11,12 @@
 import org.apache.lucene.util.LuceneTestCase;
 import org.apache.lucene.util.SetOnce;
 import org.elasticsearch.cluster.node.DiscoveryNodeRole;
-import org.elasticsearch.core.SuppressForbidden;
-import org.elasticsearch.core.PathUtils;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.AbstractRunnable;
 import org.elasticsearch.common.util.set.Sets;
+import org.elasticsearch.core.PathUtils;
+import org.elasticsearch.core.SuppressForbidden;
 import org.elasticsearch.core.internal.io.IOUtils;
 import org.elasticsearch.gateway.MetadataStateFormat;
 import org.elasticsearch.index.Index;
@@ -26,8 +26,11 @@
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.IndexSettingsModule;
 import org.elasticsearch.test.NodeRoles;
+import org.hamcrest.Matcher;
 
 import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
@@ -39,13 +42,21 @@
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.BiConsumer;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
+import static org.elasticsearch.env.NodeEnvironment.NODES_FOLDER;
 import static org.elasticsearch.test.NodeRoles.nonDataNode;
 import static org.elasticsearch.test.NodeRoles.nonMasterNode;
+import static org.hamcrest.CoreMatchers.endsWith;
 import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.Matchers.allOf;
 import static org.hamcrest.Matchers.arrayWithSize;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.instanceOf;
+import static org.hamcrest.Matchers.lessThanOrEqualTo;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.startsWith;
 
@@ -526,6 +537,48 @@ public void testEnsureNoShardDataOrIndexMetadata() throws IOException {
         verifyFailsOnShardData(noDataNoMasterSettings, indexPath, shardDataDirName);
     }
 
+    public void testDowngradeFrom8xErrorMessage() throws IOException {
+        final Settings settings = buildEnvSettings(Settings.EMPTY);
+        final Environment environment = TestEnvironment.newEnvironment(settings);
+        // noinspection EmptyTryBlock we're just creating the directory structure
+        try (NodeEnvironment ignored = new NodeEnvironment(settings, environment)) {
+        }
+        final Path nodesPath = randomFrom(environment.dataFiles()).resolve(NODES_FOLDER);
+        assertTrue(Files.isDirectory(nodesPath));
+        IOUtils.rm(nodesPath);
+
+        final BiConsumer<byte[], Matcher<String>> testCaseConsumer = (fileContent, causeMatcher) -> {
+            try {
+                Files.write(nodesPath, fileContent);
+                final IllegalStateException e = expectThrows(IllegalStateException.class, () -> new NodeEnvironment(settings, environment));
+                assertThat(
+                    e.getMessage(),
+                    allOf(containsString("is not compatible"), containsString("perhaps it has already been upgraded to a later version")));
+                assertThat(e.getCause(), instanceOf(IllegalStateException.class));
+                assertThat(
+                    e.getCause().getMessage().length(),
+                    lessThanOrEqualTo(nodesPath.toString().length() + "[] is a file which contains [...]".length() + 256));
+                assertThat(e.getCause().getMessage(), causeMatcher);
+            } catch (IOException e) {
+                throw new UncheckedIOException(e);
+            }
+        };
+
+        testCaseConsumer.accept(
+            "file content which should be reported in the error message".getBytes(StandardCharsets.UTF_8),
+            endsWith("is a file which contains [file content which should be reported in the error message]")
+        );
+
+        testCaseConsumer.accept(
+            Stream.iterate("x", x -> x).limit(10000).collect(Collectors.joining()).getBytes(StandardCharsets.UTF_8),
+            allOf(containsString("is a file which contains [xxxxxx"), endsWith("xxx...]"))
+        );
+
+        final byte[] content = randomByteArrayOfLength(1024);
+        content[between(0, 255)] = (byte) 0xff; // never valid in a UTF-8 string
+        testCaseConsumer.accept(content, endsWith("is a file which contains [<unreadable>]"));
+    }
+
     private void verifyFailsOnShardData(Settings settings, Path indexPath, String shardDataDirName) {
         IllegalStateException ex = expectThrows(IllegalStateException.class,
             "Must fail creating NodeEnvironment on a data path that has shard data if node does not have data role",