[GitHub] [incubator-druid] clintropolis commented on a change in pull request #8106: fix npe with sql metadata manager polling and empty database

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clintropolis commented on a change in pull request #8106: fix npe with sql metadata manager polling and empty database
URL: https://github.com/apache/incubator-druid/pull/8106#discussion_r305277531
 
 

 ##########
 File path: server/src/main/java/org/apache/druid/metadata/SQLMetadataSegmentManager.java
 ##########
 @@ -967,10 +961,18 @@ public DataSegment map(int index, ResultSet r, StatementContext ctx) throws SQLE
     // segment mark calls in rapid succession. So the snapshot update is not done outside of database poll at this time.
     // Updates outside of database polls were primarily for the user experience, so users would immediately see the
     // effect of a segment mark call reflected in MetadataResource API calls.
-    dataSourcesSnapshot = DataSourcesSnapshot.fromUsedSegments(
-        Iterables.filter(segments, Objects::nonNull), // Filter corrupted entries (see above in this method).
-        dataSourceProperties
-    );
+
+    ImmutableMap<String, String> dataSourceProperties = createDefaultDataSourceProperties();
+    if (segments == null || segments.isEmpty()) {
+      log.info("No segments found in the database!");
+      dataSourcesSnapshot = DataSourcesSnapshot.fromUsedSegments(Collections.emptyList(), dataSourceProperties);
 
 Review comment:
   Yea, I was not able to observe `segments` being `null` in my experiments, which mostly consisted of killing and restarting the mysql i was testing against. It doesn't look like it _should_ be able to be null, the javadoc for the `Query.list` method 
   ```
        * Executes the select
        * <p/>
        * Will eagerly load all results
        *
        * @throws org.skife.jdbi.v2.exceptions.UnableToCreateStatementException
        *                            if there is an error creating the statement
        * @throws org.skife.jdbi.v2.exceptions.UnableToExecuteStatementException
        *                            if there is an error executing the statement
        * @throws org.skife.jdbi.v2.exceptions.ResultSetException if there is an error dealing with the result set
   ```
   makes it look like it's either going to make a list or throw an exception.
   
   If we are confident it shouldn't happen then it should just be removed i think, but if still unsure it might make more sense to handle `segments == null` separate and log and probably not update the snapshot even if it's still null, because it's an unexpected condition that doesn't necessarily mean the same thing as a truly empty segments table. Thoughts?

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