Use a hint to indicate reading file footers only (#128816)

Author: thecoop

server/src/main/java/org/elasticsearch/index/store/FsDirectoryFactory.java

@@ -163,8 +163,6 @@ public IndexInput openInput(String name, IOContext context) throws IOException {
                 // we need to do these checks on the outer directory since the inner doesn't know about pending deletes
                 ensureOpen();
                 ensureCanRead(name);
-                // we switch the context here since mmap checks for the READONCE context by identity
-                context = context == Store.READONCE_CHECKSUM ? IOContext.READONCE : context;
                 // we only use the mmap to open inputs. Everything else is managed by the NIOFSDirectory otherwise
                 // we might run into trouble with files that are pendingDelete in one directory but still
                 // listed in listAll() from the other. We on the other hand don't want to list files from both dirs
@@ -191,7 +189,7 @@ private static String getExtension(String name) {
         }
 
         static boolean useDelegate(String name, IOContext ioContext) {
-            if (ioContext == Store.READONCE_CHECKSUM) {
+            if (ioContext.hints().contains(Store.FileFooterOnly.INSTANCE)) {
                 // If we're just reading the footer for the checksum then mmap() isn't really necessary, and it's desperately inefficient
                 // if pre-loading is enabled on this file.
                 return false;

server/src/main/java/org/elasticsearch/index/store/Store.java

@@ -146,19 +146,22 @@ public class Store extends AbstractIndexShardComponent implements Closeable, Ref
         Property.IndexScope
     );
 
+    /**
+     * A {@link org.apache.lucene.store.IOContext.FileOpenHint} that we will only read the Lucene file footer
+     */
+    public enum FileFooterOnly implements IOContext.FileOpenHint {
+        INSTANCE
+    }
+
     /**
      * Specific {@link IOContext} indicating that we will read only the Lucene file footer (containing the file checksum)
      * See {@link MetadataSnapshot#checksumFromLuceneFile}.
      */
-    public static final IOContext READONCE_CHECKSUM = createReadOnceContext();
-
-    // while equivalent, these different read once contexts are checked by identity in directory implementations
-    private static IOContext createReadOnceContext() {
-        var context = IOContext.READONCE.withHints(DataAccessHint.SEQUENTIAL, ReadOnceHint.INSTANCE);
-        assert context != IOContext.READONCE;
-        assert context.equals(IOContext.READONCE);
-        return context;
-    }
+    public static final IOContext READONCE_CHECKSUM = IOContext.READONCE.withHints(
+        DataAccessHint.SEQUENTIAL,
+        ReadOnceHint.INSTANCE,
+        FileFooterOnly.INSTANCE
+    );
 
     private final AtomicBoolean isClosed = new AtomicBoolean(false);
     private final StoreDirectory directory;
@@ -935,8 +938,6 @@ private static void checksumFromLuceneFile(
             boolean readFileAsHash,
             BytesRef writerUuid
         ) throws IOException {
-            // We select the read once context carefully here since these constants, while equivalent are
-            // checked by identity in the different directory implementations.
             var context = file.startsWith(IndexFileNames.SEGMENTS) ? IOContext.READONCE : READONCE_CHECKSUM;
             try (IndexInput in = directory.openInput(file, context)) {
                 final long length = in.length();

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/store/SearchableSnapshotDirectory.java

@@ -377,7 +377,7 @@ public IndexInput openInput(final String name, final IOContext context) throws I
             final BytesRef content = fileInfo.metadata().hash();
             return new ByteArrayIndexInput("ByteArrayIndexInput(" + name + ')', content.bytes, content.offset, content.length);
         }
-        if (context == Store.READONCE_CHECKSUM) {
+        if (context.hints().contains(Store.FileFooterOnly.INSTANCE)) {
             return ChecksumBlobContainerIndexInput.create(fileInfo.physicalName(), fileInfo.length(), fileInfo.checksum(), context);
         }
 

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/store/input/ChecksumBlobContainerIndexInput.java

@@ -115,7 +115,7 @@ private int checksumPositionOrThrow(long pos) {
     }
 
     private static void ensureReadOnceChecksumContext(IOContext context) {
-        if (context != Store.READONCE_CHECKSUM) {
+        if (context.hints().contains(Store.FileFooterOnly.INSTANCE) == false) {
             assert false : "expected READONCE_CHECKSUM but got " + context;
             throw new IllegalArgumentException("ChecksumBlobContainerIndexInput should only be used with READONCE_CHECKSUM context");
         }