ARROW-4532: [Java] fix bug causing very large varchar value buffers

java/vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthVector.java

@@ -1100,7 +1100,7 @@ public void setSafe(int index, int isSet, int start, int end, ArrowBuf buffer) {
     assert index >= 0;
     final int dataLength = end - start;
     fillEmpties(index);
-    handleSafe(index, end);
+    handleSafe(index, dataLength);
     BitVectorHelper.setValidityBit(validityBuffer, index, isSet);
     final int startOffset = offsetBuffer.getInt(index * OFFSET_WIDTH);
     offsetBuffer.setInt((index + 1) * OFFSET_WIDTH, startOffset + dataLength);

java/vector/src/test/java/org/apache/arrow/vector/TestValueVector.java

@@ -1394,6 +1394,51 @@ public void testFillEmptiesNotOverfill() {
     }
   }
 
+  @Test
+  public void testSetSafeWithArrowBufNoExcessAllocs() {
+    final int numValues = BaseFixedWidthVector.INITIAL_VALUE_ALLOCATION * 2;
+    final byte[] valueBytes = "hello world".getBytes();
+    final int valueBytesLength = valueBytes.length;
+    final int isSet = 1;
+
+    try (
+        final VarCharVector fromVector = newVector(VarCharVector.class, EMPTY_SCHEMA_PATH,
+            MinorType.VARCHAR, allocator);
+        final VarCharVector toVector = newVector(VarCharVector.class, EMPTY_SCHEMA_PATH,
+            MinorType.VARCHAR, allocator)) {
+      /*
+       * Populate the from vector with 'numValues' with byte-arrays, each of size 'valueBytesLength'.
+       */
+      fromVector.setInitialCapacity(numValues);
+      fromVector.allocateNew();
+      for (int i = 0; i < numValues; ++i) {
+        fromVector.setSafe(i, valueBytes, 0 /*start*/, valueBytesLength);
+      }
+      fromVector.setValueCount(numValues);
+      ArrowBuf fromDataBuffer = fromVector.getDataBuffer();
+      assertTrue(numValues * valueBytesLength <= fromDataBuffer.capacity());
+
+      /*
+       * Copy the entries one-by-one from 'fromVector' to 'toVector', but use the setSafe with
+       * ArrowBuf API (instead of setSafe with byte-array).
+       */
+      toVector.setInitialCapacity(numValues);
+      toVector.allocateNew();
+      for (int i = 0; i < numValues; i++) {
+        int start = fromVector.getstartOffset(i);
+        int end = fromVector.getstartOffset(i + 1);
+        toVector.setSafe(i, isSet, start, end, fromDataBuffer);
+      }
+
+      /*
+       * Since the 'fromVector' and 'toVector' have the same initial capacity, and were populated
+       * with the same varchar elements, the allocations and hence, the final capacity should be
+       * the same.
+       */
+      assertEquals(fromDataBuffer.capacity(), toVector.getDataBuffer().capacity());
+    }
+  }
+
   @Test
   public void testCopyFromWithNulls() {
     try (final VarCharVector vector = newVector(VarCharVector.class, EMPTY_SCHEMA_PATH, MinorType.VARCHAR, allocator);