[PAN-3012] Print warning when there are comments in genesis file (#1838)

The technique is to parse with comments disabled, and if a parse exception is
detected re-parse with comments allowed, and if successful complain at warn.

To turn this option off we just unwrap the try and delete the catch and update
the test.

config/src/main/java/tech/pegasys/pantheon/config/GenesisConfigFile.java

@@ -19,12 +19,17 @@
 import java.util.Optional;
 import java.util.stream.Stream;
 
+import com.fasterxml.jackson.core.JsonParseException;
 import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.google.common.collect.Streams;
 import com.google.common.io.Resources;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 
 public class GenesisConfigFile {
 
+  static final Logger LOG = LogManager.getLogger();
+
   public static final GenesisConfigFile DEFAULT =
       new GenesisConfigFile(JsonUtil.createEmptyObjectNode());
 
@@ -53,10 +58,23 @@ public static GenesisConfigFile development() {
   }
 
   public static GenesisConfigFile fromConfig(final String jsonString) {
-    // TODO: Should we disable comments?
-    final boolean allowComments = true;
-    final ObjectNode rootNode = JsonUtil.objectNodeFromString(jsonString, allowComments);
-    return fromConfig(rootNode);
+    try {
+      final ObjectNode rootNode = JsonUtil.objectNodeFromString(jsonString, false);
+      return fromConfig(rootNode);
+    } catch (final RuntimeException re) {
+      if (re.getCause() instanceof JsonParseException) {
+        // we had a runtime exception cause by a jsom parse exception.
+        // try again with comments enabled
+        final ObjectNode rootNode = JsonUtil.objectNodeFromString(jsonString, true);
+        // if we get here comments is what broke things, warn and move on.
+        LOG.warn(
+            "The provided genesis file contains comments. "
+                + "In a future release of Pantheon this will not be supported.");
+        return fromConfig(rootNode);
+      } else {
+        throw re;
+      }
+    }
   }
 
   public static GenesisConfigFile fromConfig(final ObjectNode config) {

config/src/test/java/tech/pegasys/pantheon/config/GenesisConfigFileTest.java

@@ -203,6 +203,17 @@ public void shouldGetLargeChainId() {
                 "31415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095"));
   }
 
+  @Test
+  public void acceptComments() {
+    // this test will change in the future to reject comments.
+    final GenesisConfigFile config =
+        GenesisConfigFile.fromConfig(
+            "{\"config\": { \"chainId\": 2017 }\n/* C comment }*/\n//C++ comment }\n}");
+
+    assertThat(config.getConfigOptions().getChainId()).contains(new BigInteger("2017"));
+    // Unfortunately there is no good (non-flakey) way to assert logs.
+  }
+
   private GenesisConfigFile configWithProperty(final String key, final String value) {
     return GenesisConfigFile.fromConfig("{\"" + key + "\":\"" + value + "\"}");
   }