add thread safety test. AddMetric lock.

Author: hjgraca

libraries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs

@@ -52,6 +52,11 @@ public class Metrics : IMetrics, IDisposable
     /// </summary>
     private readonly bool _captureColdStartEnabled;
 
+    // <summary>
+    // Shared synchronization object
+    // </summary>
+    private readonly object _lockObj = new();
+
     /// <summary>
     ///     Creates a Metrics object that provides features to send metrics to Amazon Cloudwatch using the Embedded metric
     ///     format (EMF). See
@@ -98,17 +103,20 @@ void IMetrics.AddMetric(string key, double value, MetricUnit unit, MetricResolut
                 "'AddMetric' method requires a valid metrics value. Value must be >= 0.", nameof(value));
         }
 
-        var metrics = _context.GetMetrics();
-        
-        if (metrics.Count > 0 && 
-            (metrics.Count == PowertoolsConfigurations.MaxMetrics ||
-             metrics.FirstOrDefault(x => x.Name == key)
-                 ?.Values.Count == PowertoolsConfigurations.MaxMetrics))
+        lock (_lockObj)
         {
-            _instance.Flush(true);
+            var metrics = _context.GetMetrics();
+        
+            if (metrics.Count > 0 && 
+                (metrics.Count == PowertoolsConfigurations.MaxMetrics ||
+                 metrics.FirstOrDefault(x => x.Name == key)
+                     ?.Values.Count == PowertoolsConfigurations.MaxMetrics))
+            {
+                _instance.Flush(true);
+            }
+
+            _context.AddMetric(key, value, unit, metricResolution);
         }
-
-        _context.AddMetric(key, value, unit, metricResolution);
     }
 
     /// <summary>

libraries/tests/AWS.Lambda.Powertools.Metrics.Tests/Handlers/FunctionHandler.cs

@@ -13,8 +13,11 @@
  * permissions and limitations under the License.
  */
 
+using System;
 using System.Globalization;
+using System.Linq;
 using System.Threading.Tasks;
+using AWS.Lambda.Powertools.Common;
 
 namespace AWS.Lambda.Powertools.Metrics.Tests.Handlers;
 
@@ -41,4 +44,16 @@ public async Task<string> HandleTestSecondCall(string input)
 
         return input.ToUpper(CultureInfo.InvariantCulture);
     }
+    
+    [Metrics(Namespace = "ns", Service = "svc")]
+    public async Task<string> HandleMultipleThreads(string input)
+    {
+        await Parallel.ForEachAsync(Enumerable.Range(0, Environment.ProcessorCount * 2), async (x, _) =>
+        {
+            Metrics.AddMetric("MyMetric", 1);
+            await Task.Delay(1);
+        });
+
+        return input.ToUpper(CultureInfo.InvariantCulture);
+    }
 }

libraries/tests/AWS.Lambda.Powertools.Metrics.Tests/Handlers/FunctionHandlerTests.cs

@@ -49,4 +49,16 @@ public async Task When_Metrics_Add_Metadata_Second_Invocation_Should_Not_Throw_E
         exception = await Record.ExceptionAsync( () => handler.HandleTestSecondCall("whatever"));
         Assert.Null(exception);
     }
+    
+    [Fact]
+    public async Task When_Metrics_Add_Metadata_FromMultipleThread_Should_Not_Throw_Exception()
+    {
+        // Arrange
+        Metrics.ResetForTest();
+        var handler = new FunctionHandler();
+
+        // Act
+        var exception = await Record.ExceptionAsync(() => handler.HandleMultipleThreads("whatever"));
+        Assert.Null(exception);
+    }
 }