Enable querying entirely cold datasources (apache#16676)

Add ability to query entirely cold datasources.

server/src/main/java/org/apache/druid/client/coordinator/CoordinatorClient.java

@@ -69,4 +69,9 @@ public interface CoordinatorClient
    * Returns a new instance backed by a ServiceClient which follows the provided retryPolicy
    */
   CoordinatorClient withRetryPolicy(ServiceRetryPolicy retryPolicy);
+
+  /**
+   * Retrieves list of datasources with used segments.
+   */
+  ListenableFuture<Set<String>> fetchDataSourcesWithUsedSegments();
 }

server/src/main/java/org/apache/druid/client/coordinator/CoordinatorClientImpl.java

@@ -188,4 +188,17 @@ public CoordinatorClientImpl withRetryPolicy(ServiceRetryPolicy retryPolicy)
   {
     return new CoordinatorClientImpl(client.withRetryPolicy(retryPolicy), jsonMapper);
   }
+
+  @Override
+  public ListenableFuture<Set<String>> fetchDataSourcesWithUsedSegments()
+  {
+    final String path = "/druid/coordinator/v1/metadata/datasources";
+    return FutureUtils.transform(
+        client.asyncRequest(
+            new RequestBuilder(HttpMethod.GET, path),
+            new BytesFullResponseHandler()
+        ),
+        holder -> JacksonUtils.readValue(jsonMapper, holder.getContent(), new TypeReference<Set<String>>() {})
+    );
+  }
 }

server/src/main/java/org/apache/druid/segment/metadata/AbstractSegmentMetadataCache.java

@@ -200,7 +200,7 @@ public abstract class AbstractSegmentMetadataCache<T extends DataSourceInformati
    * Map of datasource and generic object extending DataSourceInformation.
    * This structure can be accessed by {@link #cacheExec} and {@link #callbackExec} threads.
    */
-  protected final ConcurrentMap<String, T> tables = new ConcurrentHashMap<>();
+  protected final ConcurrentHashMap<String, T> tables = new ConcurrentHashMap<>();
 
   /**
    * This lock coordinates the access from multiple threads to those variables guarded by this lock.
@@ -269,9 +269,10 @@ protected void cacheExecLoop()
               final boolean wasRecentFailure = DateTimes.utc(lastFailure)
                                                         .plus(config.getMetadataRefreshPeriod())
                                                         .isAfterNow();
+
               if (isServerViewInitialized &&
                   !wasRecentFailure &&
-                  (!segmentsNeedingRefresh.isEmpty() || !dataSourcesNeedingRebuild.isEmpty()) &&
+                  shouldRefresh() &&
                   (refreshImmediately || nextRefresh < System.currentTimeMillis())) {
                 // We need to do a refresh. Break out of the waiting loop.
                 break;
@@ -334,6 +335,7 @@ protected void cacheExecLoop()
     }
   }
 
+
   /**
    * Lifecycle start method.
    */
@@ -361,6 +363,15 @@ public void refreshWaitCondition() throws InterruptedException
     // noop
   }
 
+  /**
+   * Refresh is executed only when there are segments or datasources needing refresh.
+   */
+  @SuppressWarnings("GuardedBy")
+  protected boolean shouldRefresh()
+  {
+    return (!segmentsNeedingRefresh.isEmpty() || !dataSourcesNeedingRebuild.isEmpty());
+  }
+
   public void awaitInitialization() throws InterruptedException
   {
     initialized.await();
@@ -373,6 +384,7 @@ public void awaitInitialization() throws InterruptedException
    *
    * @return schema information for the given datasource
    */
+  @Nullable
   public T getDatasource(String name)
   {
     return tables.get(name);