different tags could be written to the same file

There was a missing synchronization in the code that maps
Strings to Integers.

pdb-api/src/test/java/org/lucares/pdb/api/StringCompressorTest.java

@@ -3,6 +3,15 @@ package org.lucares.pdb.api;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
 
 import org.lucares.utils.file.FileUtils;
 import org.testng.Assert;
@@ -49,4 +58,49 @@ public class StringCompressorTest {
 		}
 
 	}
+
+	private static final class StringInserter implements Callable<List<String>> {
+
+		private final StringCompressor stringCompressor;
+		private final int numEntries;
+
+		public StringInserter(final StringCompressor stringCompressor, final int numEntries) {
+			this.stringCompressor = stringCompressor;
+			this.numEntries = numEntries;
+		}
+
+		@Override
+		public List<String> call() throws Exception {
+
+			final List<String> result = new ArrayList<>();
+			for (int i = 0; i < numEntries; i++) {
+				final String s = UUID.randomUUID().toString();
+				stringCompressor.put(s);
+				result.add(s);
+			}
+			return result;
+		}
+	};
+
+	@Test(invocationCount = 1)
+	public void testPutConcurrently() throws InterruptedException, ExecutionException {
+		final UniqueStringIntegerPairs usip = new UniqueStringIntegerPairs();
+		final StringCompressor stringCompressor = new StringCompressor(usip);
+
+		final ExecutorService pool = Executors.newCachedThreadPool();
+
+		final int numEntries = 1000;
+		final Future<List<String>> future1 = pool.submit(new StringInserter(stringCompressor, numEntries));
+		final Future<List<String>> future2 = pool.submit(new StringInserter(stringCompressor, numEntries));
+		final Future<List<String>> future3 = pool.submit(new StringInserter(stringCompressor, numEntries));
+
+		future1.get();
+		future2.get();
+		future3.get();
+
+		pool.shutdown();
+		pool.awaitTermination(1, TimeUnit.MILLISECONDS);
+
+		Assert.assertEquals((int) usip.getHighestInteger(), 3 * numEntries - 1);
+	}
 }