[Profiling] Limit TopN functions to available data

With this commit we consider a case in the TopN functions API where the
specified limit is larger than the available number of TopN functions.
Currently this throws an error (`IndexOutOfBoundException`). With this
check in place we just return the list as is.

x-pack/plugin/profiling/src/main/java/org/elasticsearch/xpack/profiling/action/TransportGetTopNFunctionsAction.java

@@ -120,7 +120,7 @@ static GetTopNFunctionsResponse buildTopNFunctions(GetStackTracesResponse respon
         return builder.build();
     }
 
-    private static class TopNFunctionsBuilder {
+    static class TopNFunctionsBuilder {
         private final Integer limit;
         private final HashMap<String, TopNFunction> topNFunctions;
 
@@ -141,7 +141,7 @@ public GetTopNFunctionsResponse build() {
                 sumTotalCount += topNFunction.getTotalCount();
             }
             // limit at the end so global stats are independent of the limit
-            if (limit != null && limit > 0) {
+            if (limit != null && limit > 0 && limit < functions.size()) {
                 functions = functions.subList(0, limit);
             }
             return new GetTopNFunctionsResponse(sumSelfCount, sumTotalCount, functions);

x-pack/plugin/profiling/src/test/java/org/elasticsearch/xpack/profiling/action/TopNFunctionsBuilderTests.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.profiling.action;
+
+import org.elasticsearch.test.ESTestCase;
+
+public class TopNFunctionsBuilderTests extends ESTestCase {
+    public void testBuildFunctions() {
+        TransportGetTopNFunctionsAction.TopNFunctionsBuilder builder = new TransportGetTopNFunctionsAction.TopNFunctionsBuilder(null);
+        TopNFunction foo = foo();
+        TopNFunction bar = bar();
+        builder.addTopNFunction(foo);
+        builder.addTopNFunction(bar);
+
+        GetTopNFunctionsResponse response = builder.build();
+
+        assertEquals(7L, response.getSelfCount());
+        assertEquals(14L, response.getTotalCount());
+        assertEquals(2, response.getTopN().size());
+        assertEquals(foo, response.getTopN().get(0));
+        assertEquals(bar, response.getTopN().get(1));
+    }
+
+    public void testBuildFunctionsWithLimitSmallerThanAvailableFunctionCount() {
+        TransportGetTopNFunctionsAction.TopNFunctionsBuilder builder = new TransportGetTopNFunctionsAction.TopNFunctionsBuilder(1);
+        TopNFunction foo = foo();
+        TopNFunction bar = bar();
+        builder.addTopNFunction(foo);
+        builder.addTopNFunction(bar);
+
+        GetTopNFunctionsResponse response = builder.build();
+
+        // total counts are independent of the limit
+        assertEquals(7L, response.getSelfCount());
+        assertEquals(14L, response.getTotalCount());
+        assertEquals(1, response.getTopN().size());
+        assertEquals(foo, response.getTopN().get(0));
+    }
+
+    public void testBuildFunctionsWithLimitHigherThanAvailableFunctionCount() {
+        TransportGetTopNFunctionsAction.TopNFunctionsBuilder builder = new TransportGetTopNFunctionsAction.TopNFunctionsBuilder(5);
+        TopNFunction foo = foo();
+        TopNFunction bar = bar();
+        builder.addTopNFunction(foo);
+        builder.addTopNFunction(bar);
+
+        GetTopNFunctionsResponse response = builder.build();
+
+        assertEquals(7L, response.getSelfCount());
+        assertEquals(14L, response.getTotalCount());
+        // still limited to the available two functions
+        assertEquals(2, response.getTopN().size());
+        assertEquals(foo, response.getTopN().get(0));
+        assertEquals(bar, response.getTopN().get(1));
+    }
+
+    private TopNFunction foo() {
+        TopNFunction foo = function("foo");
+        foo.addSelfCount(5L);
+        foo.addTotalCount(10L);
+        foo.addSelfAnnualCO2Tons(1.0d);
+        foo.addTotalAnnualCO2Tons(2.0d);
+        foo.addSelfAnnualCostsUSD(32.2d);
+        foo.addTotalAnnualCostsUSD(64.4d);
+        return foo;
+    }
+
+    private TopNFunction bar() {
+        TopNFunction bar = function("bar");
+        bar.addSelfCount(2L);
+        bar.addTotalCount(4L);
+        bar.addSelfAnnualCO2Tons(0.5d);
+        bar.addTotalAnnualCO2Tons(1.0d);
+        bar.addSelfAnnualCostsUSD(16.0d);
+        bar.addTotalAnnualCostsUSD(32.0d);
+        return bar;
+    }
+
+    private TopNFunction function(String name) {
+        return new TopNFunction(name, 3, false, 0, name, "main.c", 1, "demo");
+    }
+}