Use empty array instead of uninitialized array

Enumerating uninitialized array will throw InvalidOperationException

Author: balag0

src/WebJobs.Script/Config/ScriptJobHostOptions.cs

@@ -30,7 +30,7 @@ public string RootScriptPath
             get => _rootScriptPath;
             set
             {
-                _directorySnapshot = default;
+                _directorySnapshot = ImmutableArray<string>.Empty;
                 _rootScriptPath = value;
             }
         }
@@ -39,7 +39,7 @@ public ImmutableArray<string> RootScriptDirectorySnapshot
         {
             get
             {
-                if (_rootScriptPath != null && _directorySnapshot.IsDefault)
+                if (_rootScriptPath != null && _directorySnapshot.IsDefaultOrEmpty)
                 {
                     // take a startup time function directory snapshot so we can detect function additions/removals
                     // we'll also use this snapshot when reading function metadata as part of startup
@@ -51,7 +51,7 @@ public ImmutableArray<string> RootScriptDirectorySnapshot
                     }
                     catch (DirectoryNotFoundException)
                     {
-                        _directorySnapshot = default;
+                        _directorySnapshot = ImmutableArray<string>.Empty;
                     }
                 }
 

test/WebJobs.Script.Tests/FunctionMetadataManagerTests.cs

@@ -5,7 +5,9 @@
 using System.Collections.Generic;
 using System.IO;
 using System.IO.Abstractions.TestingHelpers;
+using Microsoft.Azure.WebJobs.Script.Rpc;
 using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
 using Newtonsoft.Json.Linq;
 using Xunit;
 
@@ -36,6 +38,24 @@ public void DeterminePrimaryScriptFile_MultipleFiles_SourceFileSpecified(string
             Assert.Equal(@"c:\functions\queueTrigger.py", scriptFile, StringComparer.OrdinalIgnoreCase);
         }
 
+        [Theory]
+        [InlineData(@"NonExistentPath")]
+        [InlineData(null)]
+        public void InitializesEmptyOrMissingDirectorySnapshot(string rootScriptPath)
+        {
+            var scriptConfig = new ScriptJobHostOptions()
+            {
+                RootScriptPath = rootScriptPath
+            };
+
+            IOptions<ScriptJobHostOptions> scriptOptions = new OptionsManager<ScriptJobHostOptions>(new TestOptionsFactory<ScriptJobHostOptions>(scriptConfig));
+            IOptions<LanguageWorkerOptions> languageWorkerOptions = new OptionsManager<LanguageWorkerOptions>(new TestOptionsFactory<LanguageWorkerOptions>(new LanguageWorkerOptions()));
+
+            var functionMetadataManager = new FunctionMetadataManager(scriptOptions, languageWorkerOptions, NullLoggerFactory.Instance);
+            Assert.False(functionMetadataManager.Functions.IsDefault);
+            Assert.True(functionMetadataManager.Functions.IsEmpty);
+        }
+
         [Fact]
         public void DeterminePrimaryScriptFile_RelativeSourceFileSpecified()
         {