Light Client: Fix blockchain height method (#889)

Author: bullet-tooth

exonum-light-client/src/main/java/com/exonum/client/ExonumHttpClient.java

@@ -125,7 +125,7 @@ public Optional<TransactionResponse> getTransaction(HashCode id) {
   public long getBlockchainHeight() {
     BlocksResponse response = doGetBlocks(0, INCLUDE_EMPTY, null, NO_COMMIT_TIME);
 
-    return response.getBlocksRangeEnd();
+    return response.getBlocksRangeEnd() - 1; // Because '$.range.end' is exclusive
   }
 
   @Override

exonum-light-client/src/main/java/com/exonum/client/response/BlocksResponse.java

@@ -33,7 +33,17 @@ public class BlocksResponse {
   long blocksRangeStart;
 
   /**
-   * The largest height of the returned blocks that match the search criteria.
+   * The largest height of the returned blocks that match the search criteria, <em>plus one</em>.
+   *
+   * <p><b>WARNING: do not rely on the value, because it's unpredictable in some queries.</b>
+   * It'll be fixed in future releases. Read below if would still like to use it.
+   *
+   * <p>The value is always equal to {@code blocks[0].height + 1} for responses with blocks.
+   * For responses without blocks the value is set mostly randomly depending on request parameters
+   * and blockchain state:
+   * The value is equal to {@code heightMax + 1} if heightMax request parameter is passed.
+   * If the heightMax request parameter is greater then last committed block height at the moment
+   * then blocksRangeEnd value will be equal to {@code last_committed_block.height + 1}.
    * If some blocks in the range do not match the search criteria then:
    * {@code blocksRangeEnd - blocksRangeStart != blocks.size}.
    */

exonum-light-client/src/test/java/com/exonum/client/ExonumHttpClientBlocksIntegrationTest.java

@@ -308,10 +308,11 @@ void getLastNotEmptyBlockNoBlock() throws InterruptedException {
   void getBlockchainHeight() throws InterruptedException {
     // Mock response
     long height = 1600;
+    long heightExclusive = height + 1;
     String json = "{\n"
         + "  'range': {\n"
         + "    'start': 0,\n"
-        + "    'end': " + height + "\n"
+        + "    'end': " + heightExclusive + "\n"
         + "  },\n"
         + "  'blocks': [],\n"
         + "  'times': null\n"