set service name from DD_SERVICE_NAME env var (#226)

* add support for `DD_SERVICE_NAME` environment variable

* add comments

* add unit test

* add more unit test cases

src/Datadog.Trace/Tracer.cs

@@ -18,6 +18,7 @@ public class Tracer : IDatadogTracer
         private const string DefaultTraceAgentHost = "localhost";
         private const string DefaultTraceAgentPort = "8126";
         private const string EnvVariableName = "DD_ENV";
+        private const string ServiceNameVariableName = "DD_SERVICE_NAME";
 
         private static readonly string[] TraceAgentHostEnvironmentVariableNames =
         {
@@ -195,18 +196,33 @@ internal static Uri CreateAgentUri()
             return new Uri($"http://{host}:{port}");
         }
 
+        /// <summary>
+        /// Determines the default service name for the executing application by looking at
+        /// environment variables, hosted app name (.NET Framework on IIS only), assembly name, and process name.
+        /// </summary>
+        /// <returns>The default service name.</returns>
         private static string CreateDefaultServiceName()
         {
             try
             {
+                // allow users to override this with an environment variable
+                var serviceName = Environment.GetEnvironmentVariable(ServiceNameVariableName);
+
+                if (!string.IsNullOrWhiteSpace(serviceName))
+                {
+                    return serviceName;
+                }
+
 #if !NETSTANDARD2_0
+                // System.Web.dll is only available on .NET Framework
                 if (System.Web.Hosting.HostingEnvironment.IsHosted)
                 {
                     // if we are hosted as an ASP.NET application, return "SiteName/ApplicationVirtualPath".
                     // note that ApplicationVirtualPath includes a leading slash.
                     return (System.Web.Hosting.HostingEnvironment.SiteName + System.Web.Hosting.HostingEnvironment.ApplicationVirtualPath).TrimEnd('/');
                 }
 #endif
+
                 return Assembly.GetEntryAssembly()?.GetName().Name ??
                        Process.GetCurrentProcess().ProcessName;
             }

test/Datadog.Trace.Tests/TracerTests.cs

@@ -269,7 +269,6 @@ public void SetHostAndPortEnvironmentVariables(string host, string port, string
             Environment.SetEnvironmentVariable("DD_TRACE_AGENT_PORT", port);
 
             Uri uri = Tracer.CreateAgentUri();
-
             Assert.Equal(new Uri(expectedUri), uri);
 
             // reset the environment variables to their original values (if any) when done
@@ -293,5 +292,39 @@ public void SetEnvEnvironmentVariable(string env)
             // reset the environment variable to its original value (if any) when done
             Environment.SetEnvironmentVariable(name, originalEnv);
         }
+
+        [Theory]
+        // if no service name is specified, fallback to a best guess (e.g. assembly name, process name)
+        [InlineData(null, null, null, null)]
+        // if only one is set, use that one
+        [InlineData("envService", null, null, "envService")]
+        [InlineData(null, "tracerService", null, "tracerService")]
+        [InlineData(null, null, "spanService", "spanService")]
+        // if more than one is set, follow precedence: span > tracer > env > default
+        [InlineData(null, "tracerService", "spanService", "spanService")]
+        [InlineData("envService", null, "spanService", "spanService")]
+        [InlineData("envService", "tracerService", null, "tracerService")]
+        [InlineData("envService", "tracerService", "spanService", "spanService")]
+        public void SetServiceName(string envServiceName, string tracerServiceName, string spanServiceName, string expectedServiceName)
+        {
+            var name = "DD_SERVICE_NAME";
+            string originalEnv = Environment.GetEnvironmentVariable(name);
+            Environment.SetEnvironmentVariable(name, envServiceName);
+
+            var tracer = new Tracer(_writerMock.Object, defaultServiceName: tracerServiceName);
+            Span span = tracer.StartSpan("operationName", serviceName: spanServiceName);
+
+            if (expectedServiceName == null)
+            {
+                Assert.Contains(span.ServiceName, TestRunners.ValidNames);
+            }
+            else
+            {
+                Assert.Equal(expectedServiceName, span.ServiceName);
+            }
+
+            // reset the environment variable to its original values (if any) when done
+            Environment.SetEnvironmentVariable(name, originalEnv);
+        }
     }
 }