Fixing bug in Flex Consumption HostId generation in placeholder mode (#9391)

Author: mathewc

src/WebJobs.Script/Host/ScriptHostIdProvider.cs

@@ -54,13 +54,16 @@ internal static HostIdResult GetDefaultHostId(IEnvironment environment, ScriptAp
             {
                 // in Flex Consumption, we use a Guid based host ID without truncation.
                 string uniqueSlotName = environment?.GetAzureWebsiteUniqueSlotName();
-                byte[] hash;
-                using (MD5 md5 = MD5.Create())
+                if (!string.IsNullOrEmpty(uniqueSlotName))
                 {
-                    hash = md5.ComputeHash(Encoding.UTF8.GetBytes(uniqueSlotName));
-                }
+                    byte[] hash;
+                    using (MD5 md5 = MD5.Create())
+                    {
+                        hash = md5.ComputeHash(Encoding.UTF8.GetBytes(uniqueSlotName));
+                    }
 
-                result.HostId = new Guid(hash).ToString().Replace("-", string.Empty).ToLowerInvariant();
+                    result.HostId = new Guid(hash).ToString().Replace("-", string.Empty).ToLowerInvariant();
+                }
 
                 return result;
             }

test/WebJobs.Script.Tests/ScriptHostIdProviderTests.cs

@@ -117,6 +117,47 @@ public void GetDefaultHostId_FlexConsumption_ReturnsExpectedResult(string siteNa
             Assert.False(result.IsTruncated);
         }
 
+        [Fact]
+        public void GetDefaultHostId_PlaceholderMode_ReturnsExpectedResult()
+        {
+            var options = new ScriptApplicationHostOptions
+            {
+                ScriptPath = @"c:\testscripts"
+            };
+
+            // In placeholder mode, site name and other settings aren't available to compute the
+            // default HostId, so we expect null.
+            var environment = new TestEnvironment();
+            environment.SetEnvironmentVariable(EnvironmentSettingNames.AzureWebsiteInstanceId, "123123");
+
+            Assert.False(environment.IsFlexConsumptionSku());
+
+            var result = ScriptHostIdProvider.GetDefaultHostId(environment, options);
+
+            Assert.Equal(null, result.HostId);
+        }
+
+        [Fact]
+        public void GetDefaultHostId_PlaceholderMode_FlexConsumption_ReturnsExpectedResult()
+        {
+            var options = new ScriptApplicationHostOptions
+            {
+                ScriptPath = @"c:\testscripts"
+            };
+
+            // In placeholder mode, site name and other settings aren't available to compute the
+            // default HostId, so we expect null.
+            var environment = new TestEnvironment();
+            environment.SetEnvironmentVariable(EnvironmentSettingNames.ContainerName, "testContainer");
+            environment.SetEnvironmentVariable(EnvironmentSettingNames.LegionServiceHost, "legionhost");
+
+            Assert.True(environment.IsFlexConsumptionSku());
+
+            var result = ScriptHostIdProvider.GetDefaultHostId(environment, options);
+
+            Assert.Equal(null, result.HostId);
+        }
+
         [Fact]
         public void GetDefaultHostId_SelfHost_ReturnsExpectedResult()
         {