Remove "nodes/0" folder prefix from data path (#42489)

With the removal of node.max_local_storage_nodes, there is no need anymore to keep the data in
subfolders indexed by a node ordinal. This commit makes it so that ES 8.0 will store data directly in
$DATA_DIR instead of $DATA_DIR/nodes/$nodeOrdinal.

Upon startup, Elasticsearch will check to see if there is data in the old location, and automatically
move it to the new location. This automatic migration only works if $nodeOrdinal is 0, i.e., multiple
node instances have not previously run on the same data path, which required for
node.max_local_storage_nodes to explicitly be configured.

docs/reference/commands/shard-tool.asciidoc

@@ -51,14 +51,14 @@ $ bin/elasticsearch-shard remove-corrupted-data --index twitter --shard-id 0
   Please make a complete backup of your index before using this tool.
 
 
-Opening Lucene index at /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
+Opening Lucene index at /var/lib/elasticsearchdata/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
 
- >> Lucene index is corrupted at /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
+ >> Lucene index is corrupted at /var/lib/elasticsearchdata/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
 
-Opening translog at /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
+Opening translog at /var/lib/elasticsearchdata/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
 
 
- >> Translog is clean at /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
+ >> Translog is clean at /var/lib/elasticsearchdata/indices/P45vf_YQRhqjfwLMUvSqDw/0/translog/
 
 
   Corrupted Lucene index segments found - 32 documents will be lost.
@@ -93,7 +93,7 @@ POST /_cluster/reroute
 
 You must accept the possibility of data loss by changing parameter `accept_data_loss` to `true`.
 
-Deleted corrupt marker corrupted_FzTSBSuxT7i3Tls_TgwEag from /var/lib/elasticsearchdata/nodes/0/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
+Deleted corrupt marker corrupted_FzTSBSuxT7i3Tls_TgwEag from /var/lib/elasticsearchdata/indices/P45vf_YQRhqjfwLMUvSqDw/0/index/
 
 --------------------------------------------------
 

docs/reference/migration/migrate_8_0/node.asciidoc

@@ -14,3 +14,25 @@
 The `node.max_local_storage_nodes` setting was deprecated in 7.x and
 has been removed in 8.0. Nodes should be run on separate data paths
 to ensure that each node is consistently assigned to the same data path.
+
+[float]
+==== Change of data folder layout
+
+Each node's data is now stored directly in the data directory set by the
+`path.data` setting, rather than in `${path.data}/nodes/0`, because the removal
+of the `node.max_local_storage_nodes` setting means that nodes may no longer
+share a data path. At startup, Elasticsearch will automatically migrate the data
+path to the new layout. This automatic migration will not proceed if the data
+path contains data for more than one node. You should move to a configuration in
+which each node has its own data path before upgrading.
+
+If you try to upgrade a configuration in which there is data for more than one
+node in a data path then the automatic migration will fail and Elasticsearch
+will refuse to start. To resolve this you will need to perform the migration
+manually. The data for the extra nodes are stored in folders named
+`${path.data}/nodes/1`, `${path.data}/nodes/2` and so on, and you should move
+each of these folders to an appropriate location and then configure the
+corresponding node to use this location for its data path. If your nodes each
+have more than one data path in their `path.data` settings then you should move
+all the corresponding subfolders in parallel. Each node uses the same subfolder
+(e.g. `nodes/2`) across all its data paths.

qa/evil-tests/src/test/java/org/elasticsearch/env/NodeEnvironmentEvilTests.java

@@ -51,18 +51,19 @@ public void testMissingWritePermission() throws IOException {
             Settings build = Settings.builder()
                     .put(Environment.PATH_HOME_SETTING.getKey(), createTempDir().toAbsolutePath().toString())
                     .putList(Environment.PATH_DATA_SETTING.getKey(), tempPaths).build();
-            IOException exception = expectThrows(IOException.class, () -> {
+            IllegalStateException exception = expectThrows(IllegalStateException.class, () -> {
                 new NodeEnvironment(build, TestEnvironment.newEnvironment(build));
             });
-            assertTrue(exception.getMessage(), exception.getMessage().startsWith(path.toString()));
+            assertTrue(exception.getCause().getCause().getMessage(),
+                exception.getCause().getCause().getMessage().startsWith(path.toString()));
         }
     }
 
     public void testMissingWritePermissionOnIndex() throws IOException {
         assumeTrue("posix filesystem", isPosix);
         final String[] tempPaths = tmpPaths();
         Path path = PathUtils.get(randomFrom(tempPaths));
-        Path fooIndex = path.resolve("nodes").resolve("0").resolve(NodeEnvironment.INDICES_FOLDER)
+        Path fooIndex = path.resolve(NodeEnvironment.INDICES_FOLDER)
             .resolve("foo");
         Files.createDirectories(fooIndex);
         try (PosixPermissionsResetter attr = new PosixPermissionsResetter(fooIndex)) {
@@ -82,7 +83,7 @@ public void testMissingWritePermissionOnShard() throws IOException {
         assumeTrue("posix filesystem", isPosix);
         final String[] tempPaths = tmpPaths();
         Path path = PathUtils.get(randomFrom(tempPaths));
-        Path fooIndex = path.resolve("nodes").resolve("0").resolve(NodeEnvironment.INDICES_FOLDER)
+        Path fooIndex = path.resolve(NodeEnvironment.INDICES_FOLDER)
             .resolve("foo");
         Path fooShard = fooIndex.resolve("0");
         Path fooShardIndex = fooShard.resolve("index");