HBASE-19116 Currently the tail of hfiles with CellComparator* classname makes it so hbase1 can't open hbase2 written hfiles; fix

Serializing, if appropriate, write the hbase-1.x version of the
Comparator to the hfile trailer so hbase-1.x files can read hbase-2.x
hfiles (they are the same format).

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/FixedFileTrailer.java

@@ -18,6 +18,7 @@
  */
 package org.apache.hadoop.hbase.io.hfile;
 
+
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.DataInput;
@@ -185,13 +186,8 @@ public class FixedFileTrailer {
     baos.writeTo(outputStream);
   }
 
-  /**
-   * Write trailer data as protobuf
-   * @param outputStream
-   * @throws IOException
-   */
-  void serializeAsPB(DataOutputStream output) throws IOException {
-    ByteArrayOutputStream baos = new ByteArrayOutputStream();
+  @org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesting
+  HFileProtos.FileTrailerProto toProtobuf() {
     HFileProtos.FileTrailerProto.Builder builder = HFileProtos.FileTrailerProto.newBuilder()
         .setFileInfoOffset(fileInfoOffset)
         .setLoadOnOpenDataOffset(loadOnOpenDataOffset)
@@ -203,16 +199,24 @@ public class FixedFileTrailer {
         .setNumDataIndexLevels(numDataIndexLevels)
         .setFirstDataBlockOffset(firstDataBlockOffset)
         .setLastDataBlockOffset(lastDataBlockOffset)
-      // TODO this is a classname encoded into an  HFile's trailer. We are going to need to have
-      // some compat code here.
-      .setComparatorClassName(comparatorClassName)
+        .setComparatorClassName(getHBase1CompatibleName(comparatorClassName))
         .setCompressionCodec(compressionCodec.ordinal());
     if (encryptionKey != null) {
       builder.setEncryptionKey(UnsafeByteOperations.unsafeWrap(encryptionKey));
     }
+    return builder.build();
+  }
+
+  /**
+   * Write trailer data as protobuf.
+   * NOTE: we run a translation on the comparator name and will serialize the old hbase-1.x where
+   * it makes sense. See {@link #getHBase1CompatibleName(String)}.
+   */
+  void serializeAsPB(DataOutputStream output) throws IOException {
+    ByteArrayOutputStream baos = new ByteArrayOutputStream();
     // We need this extra copy unfortunately to determine the final size of the
     // delimited output, see use of baos.size() below.
-    builder.build().writeDelimitedTo(baos);
+    toProtobuf().writeDelimitedTo(baos);
     baos.writeTo(output);
     // Pad to make up the difference between variable PB encoding length and the
     // length when encoded as writable under earlier V2 formats. Failure to pad
@@ -298,8 +302,6 @@ public class FixedFileTrailer {
       lastDataBlockOffset = trailerProto.getLastDataBlockOffset();
     }
     if (trailerProto.hasComparatorClassName()) {
-      // TODO this is a classname encoded into an  HFile's trailer. We are going to need to have 
-      // some compat code here.
       setComparatorClass(getComparatorClass(trailerProto.getComparatorClassName()));
     }
     if (trailerProto.hasCompressionCodec()) {
@@ -548,14 +550,46 @@ public class FixedFileTrailer {
         CellComparator comp = klass.getDeclaredConstructor().newInstance();
         // if the name wasn't one of the legacy names, maybe its a legit new
         // kind of comparator.
-        comparatorClassName = klass.getName();
+        this.comparatorClassName = klass.getName();
       }
-
     } catch (Exception e) {
       throw new RuntimeException("Comparator class " + klass.getName() + " is not instantiable", e);
     }
   }
 
+  /**
+   * If a 'standard' Comparator, write the old name for the Comparator when we serialize rather
+   * than the new name; writing the new name will make it so newly-written hfiles are not parseable
+   * by hbase-1.x, a facility we'd like to preserve across rolling upgrade and hbase-1.x clusters
+   * reading hbase-2.x produce.
+   *
+   * The Comparators in hbase-2.x work the same as they did in hbase-1.x; they compare
+   * KeyValues. In hbase-2.x they were renamed making use of the more generic 'Cell'
+   * nomenclature to indicate that we intend to move away from KeyValues post hbase-2. A naming
+   * change is not reason enough to make it so hbase-1.x cannot read hbase-2.x files given the
+   * structure goes unchanged (hfile v3). So, lets write the old names for Comparators into the
+   * hfile tails in hbase-2. Here is where we do the translation.
+   * {@link #getComparatorClass(String)} does translation going the other way.
+   *
+   * <p>The translation is done on the serialized Protobuf only.</p>
+   *
+   * @param comparator String class name of the Comparator used in this hfile.
+   * @return What to store in the trailer as our comparator name.
+   * @since hbase-2.0.0.
+   * @deprecated Since hbase-2.0.0. Will be removed in hbase-3.0.0.
+   * @see #getComparatorClass(String)
+   */
+  @Deprecated
+  private String getHBase1CompatibleName(final String comparator) {
+    if (comparator.equals(CellComparatorImpl.class.getName())) {
+      return KeyValue.COMPARATOR.getClass().getName();
+    }
+    if (comparator.equals(MetaCellComparator.class.getName())) {
+      return KeyValue.META_COMPARATOR.getClass().getName();
+    }
+    return comparator;
+  }
+
   @SuppressWarnings("unchecked")
   private static Class<? extends CellComparator> getComparatorClass(String comparatorClassName)
       throws IOException {

hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/TestFixedFileTrailer.java

@@ -33,8 +33,11 @@ import org.apache.hadoop.fs.FSDataInputStream;
 import org.apache.hadoop.fs.FSDataOutputStream;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
-import org.apache.hadoop.hbase.*;
+import org.apache.hadoop.hbase.CellComparatorImpl;
 import org.apache.hadoop.hbase.HBaseClassTestRule;
+import org.apache.hadoop.hbase.HBaseTestingUtility;
+import org.apache.hadoop.hbase.KeyValue;
+import org.apache.hadoop.hbase.shaded.protobuf.generated.HFileProtos;
 import org.apache.hadoop.hbase.testclassification.IOTests;
 import org.apache.hadoop.hbase.testclassification.SmallTests;
 import org.apache.hadoop.hbase.util.Bytes;
@@ -92,6 +95,19 @@ public class TestFixedFileTrailer {
     fs = FileSystem.get(util.getConfiguration());
   }
 
+  @Test
+  public void testComparatorIsHBase1Compatible() {
+    FixedFileTrailer t = new FixedFileTrailer(version, HFileReaderImpl.PBUF_TRAILER_MINOR_VERSION);
+    t.setComparatorClass(CellComparatorImpl.COMPARATOR.getClass());
+    assertEquals(CellComparatorImpl.COMPARATOR.getClass().getName(), t.getComparatorClassName());
+    HFileProtos.FileTrailerProto pb = t.toProtobuf();
+    assertEquals(KeyValue.COMPARATOR.getClass().getName(), pb.getComparatorClassName());
+    t.setComparatorClass(CellComparatorImpl.MetaCellComparator.META_COMPARATOR.getClass());
+    pb = t.toProtobuf();
+    assertEquals(KeyValue.META_COMPARATOR.getClass().getName(),
+        pb.getComparatorClassName());
+  }
+
   @Test
   public void testTrailer() throws IOException {
     FixedFileTrailer t = new FixedFileTrailer(version,