Fix concurrent gauge creation (#7116)

* Added failing test.
CHANGELOG_BEGIN
CHANGELOG_END

* Fixed test.

* Added missing header.

* Reformatted.

* Fixed concurrency issue for CacheMetrics as well.

* Reworded test case description a bit.

* Code tidying.

* Use da_scala_test_suite instead.

ledger/metrics/BUILD.bazel

@@ -4,6 +4,7 @@
 load(
     "//bazel_tools:scala.bzl",
     "da_scala_library",
+    "da_scala_test_suite",
 )
 
 da_scala_library(
@@ -23,3 +24,14 @@ da_scala_library(
         "@maven//:io_dropwizard_metrics_metrics_jvm",
     ],
 )
+
+da_scala_test_suite(
+    name = "metrics-tests",
+    size = "small",
+    srcs = glob(["src/test/scala/**/*.scala"]),
+    deps = [
+        ":metrics",
+        "@maven//:io_dropwizard_metrics_metrics_core",
+        "@maven//:org_scalatest_scalatest_2_12",
+    ],
+)

ledger/metrics/src/main/scala/com/daml/metrics/CacheMetrics.scala

@@ -23,8 +23,9 @@ final class CacheMetrics(
   def registerWeightGauge(weightGauge: Gauge[Long]): Unit =
     register(prefix :+ "weight", () => weightGauge)
 
-  private def register[T](name: MetricName, gaugeSupplier: MetricSupplier[Gauge[_]]): Gauge[T] = {
+  private def register[T](name: MetricName, gaugeSupplier: MetricSupplier[Gauge[_]]): Gauge[T] =
-    registry.remove(name)
+    registry.synchronized {
-    registry.gauge(name, gaugeSupplier).asInstanceOf[Gauge[T]]
+      registry.remove(name)
-  }
+      registry.gauge(name, gaugeSupplier).asInstanceOf[Gauge[T]]
+    }
 }

ledger/metrics/src/main/scala/com/daml/metrics/Metrics.scala

@@ -8,10 +8,13 @@ import com.codahale.metrics._
 
 final class Metrics(val registry: MetricRegistry) {
 
-  private def gauge[T](name: MetricName, metricSupplier: MetricSupplier[Gauge[_]]): Gauge[T] = {
+  private[metrics] def gauge[T](
-    registry.remove(name)
+      name: MetricName,
-    registry.gauge(name, metricSupplier).asInstanceOf[Gauge[T]]
+      metricSupplier: MetricSupplier[Gauge[_]]): Gauge[T] =
-  }
+    registry.synchronized {
+      registry.remove(name)
+      registry.gauge(name, metricSupplier).asInstanceOf[Gauge[T]]
+    }
 
   object test {
     private val Prefix: MetricName = MetricName("test")

ledger/metrics/src/test/scala/com/daml/metrics/CacheMetricsSpec.scala

@@ -0,0 +1,27 @@
+// Copyright (c) 2020 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package com.daml.metrics
+
+import com.codahale.metrics.MetricRegistry
+import org.scalatest.{AsyncWordSpec, Matchers}
+
+import scala.concurrent.{ExecutionContext, Future}
+
+class CacheMetricsSpec extends AsyncWordSpec with Matchers {
+  "gauge registrations" should {
+    "succeed on multiple threads in parallel for the same metric registry" in {
+      val cacheMetrics = new CacheMetrics(new MetricRegistry, MetricName.DAML)
+      implicit val executionContext: ExecutionContext = ExecutionContext.global
+      val instances =
+        (1 to 1000).map(_ =>
+          Future {
+            cacheMetrics.registerSizeGauge(() => 1L)
+            cacheMetrics.registerWeightGauge(() => 2L)
+        })
+      Future.sequence(instances).map { _ =>
+        succeed
+      }
+    }
+  }
+}

ledger/metrics/src/test/scala/com/daml/metrics/MetricsSpec.scala

@@ -0,0 +1,24 @@
+// Copyright (c) 2020 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package com.daml.metrics
+
+import com.codahale.metrics.MetricRegistry
+import org.scalatest.{AsyncWordSpec, Matchers}
+
+import scala.concurrent.{ExecutionContext, Future}
+
+class MetricsSpec extends AsyncWordSpec with Matchers {
+  "gauge registration" should {
+    "succeed on multiple threads in parallel for the same metric name" in {
+      val metrics = new Metrics(new MetricRegistry)
+      implicit val executionContext: ExecutionContext = ExecutionContext.global
+      val metricName = MetricName.DAML :+ "a" :+ "test"
+      val instances =
+        (1 to 1000).map(_ => Future(metrics.gauge[Double](metricName, () => () => 1.0)))
+      Future.sequence(instances).map { _ =>
+        succeed
+      }
+    }
+  }
+}