Create a LevelDBLogReader class for java

This is a first step towards porting our database backup comparison tool
(which was written in Crack and subsequently in Python) to Java so that we can
run it in a reasonable amount of time.  Ideally, this functionality should be
available in Java, however the only instance of it that I can find is
currently deprecated without a replacement.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165433766

java/google/registry/tools/LevelDbLogReader.java

@@ -0,0 +1,173 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// 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
+//
+//     http://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 google.registry.tools;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableList;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.file.FileSystems;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+/**
+ * Reads records from a set of LevelDB files and builds a gigantic ImmutableList from them.
+ *
+ * <p>See <a
+ * href="https://github.com/google/leveldb/blob/master/doc/log_format.md">log_format.md</a> for the
+ * leveldb log format specification.</a>
+ *
+ * <p>There are several other implementations of this, none of which appeared suitable for our use
+ * case: <a href="https://github.com/google/leveldb">The original C++ implementation</a>. <a
+ * href="https://cloud.google.com/appengine/docs/standard/java/javadoc/com/google/appengine/api/files/RecordWriteChannel">
+ * com.google.appengine.api.files.RecordWriteChannel</a> - Exactly what we need but deprecated. The
+ * referenced replacement: <a
+ * href="https://github.com/GoogleCloudPlatform/appengine-gcs-client.git">The App Engine GCS
+ * Client</a> - Does not appear to have any support for working with LevelDB.
+ */
+public final class LevelDbLogReader {
+
+  @VisibleForTesting static final int BLOCK_SIZE = 32 * 1024;
+  @VisibleForTesting static final int HEADER_SIZE = 7;
+
+  private final ByteArrayOutputStream recordContents = new ByteArrayOutputStream();
+  private final ImmutableList.Builder<byte[]> recordListBuilder = new ImmutableList.Builder<>();
+
+  /** Read a complete block, which must be exactly 32 KB. */
+  private void processBlock(byte[] block) {
+    // Read records from the block until there is no longer enough space for a record (i.e. until
+    // we're at HEADER_SIZE - 1 bytes from the end of the block).
+    int i = 0;
+    while (i < BLOCK_SIZE - (HEADER_SIZE - 1)) {
+      RecordHeader recordHeader = readRecordHeader(block, i);
+      if (recordHeader.type == ChunkType.END) {
+        // A type of zero indicates that we've reached the padding zeroes at the end of the block.
+        break;
+      }
+
+      // Copy the contents of the record into recordContents.
+      recordContents.write(block, i + HEADER_SIZE, recordHeader.size);
+
+      // If this is the last (or only) chunk in the record, store the full contents into the List.
+      if (recordHeader.type == ChunkType.FULL || recordHeader.type == ChunkType.LAST) {
+        recordListBuilder.add(recordContents.toByteArray());
+        recordContents.reset();
+      }
+
+      i += recordHeader.size + HEADER_SIZE;
+    }
+  }
+
+  /**
+   * Gets a byte from "block" as an unsigned value.
+   *
+   * <p>Java bytes are signed, which doesn't work very well for our bit-shifting operations.
+   */
+  private int getUnsignedByte(byte[] block, int pos) {
+    return block[pos] & 0xFF;
+  }
+
+  /** Reads the 7 byte record header. */
+  private RecordHeader readRecordHeader(byte[] block, int pos) {
+    // Read checksum (4 bytes, LE).
+    int checksum =
+        getUnsignedByte(block, pos)
+            | (getUnsignedByte(block, pos + 1) << 8)
+            | (getUnsignedByte(block, pos + 2) << 16)
+            | (getUnsignedByte(block, pos + 3) << 24);
+    // Read size (2 bytes, LE).
+    int size = getUnsignedByte(block, pos + 4) | (getUnsignedByte(block, pos + 5) << 8);
+    // Read type (1 byte).
+    int type = getUnsignedByte(block, pos + 6);
+
+    return new RecordHeader(checksum, size, ChunkType.fromCode(type));
+  }
+
+  /** Reads all records in the Reader into the record set. */
+  public void readFrom(InputStream source) throws IOException {
+    byte[] block = new byte[BLOCK_SIZE];
+
+    // read until we have no more.
+    while (true) {
+      int amountRead = source.read(block, 0, BLOCK_SIZE);
+      if (amountRead <= 0) {
+        break;
+      }
+      assert amountRead == BLOCK_SIZE;
+
+      processBlock(block);
+    }
+  }
+
+  /** Reads all records from the file specified by "path" into the record set. */
+  public void readFrom(Path path) throws IOException {
+    readFrom(Files.newInputStream(path));
+  }
+
+  /** Reads all records from the specified file into the record set. */
+  public void readFrom(String filename) throws IOException {
+    readFrom(FileSystems.getDefault().getPath(filename));
+  }
+
+  /**
+   * Gets the list of records constructed so far.
+   *
+   * <p>Note that this does not invalidate the internal state of the object: we return a copy and
+   * this can be called multiple times.
+   */
+  ImmutableList<byte[]> getRecords() {
+    return recordListBuilder.build();
+  }
+
+  /** Aggregates the fields in a record header. */
+  private static final class RecordHeader {
+    final int checksum;
+    final int size;
+    final ChunkType type;
+
+    public RecordHeader(int checksum, int size, ChunkType type) {
+      this.checksum = checksum;
+      this.size = size;
+      this.type = type;
+    }
+  }
+
+  @VisibleForTesting
+  enum ChunkType {
+    // Warning: these values must map to their array indices.  If this relationship is broken,
+    // you'll need to change fromCode() to not simply index into values().
+    END(0),
+    FULL(1),
+    FIRST(2),
+    MIDDLE(3),
+    LAST(4);
+
+    private final int code;
+
+    ChunkType(int code) {
+      this.code = code;
+    }
+
+    int getCode() {
+      return code;
+    }
+
+    /** Construct a record type from the numeric record type code. */
+    static ChunkType fromCode(int code) {
+      return values()[code];
+    }
+  }
+}