Fall back to relying on PATH when the dotnet executable is not found at the expected location

src/WebJobs.Script/Workers/Http/Configuration/HttpWorkerOptionsSetup.cs

@@ -36,7 +36,7 @@ public void Configure(HttpWorkerOptions options)
                 {
                     throw new HostConfigurationException($"Missing WorkerDescription for HttpWorker");
                 }
-                httpWorkerDescription.ApplyDefaultsAndValidate(_scriptJobHostOptions.RootScriptPath);
+                httpWorkerDescription.ApplyDefaultsAndValidate(_scriptJobHostOptions.RootScriptPath, _logger);
                 options.Arguments = new WorkerProcessArguments()
                 {
                     ExecutablePath = options.Description.DefaultExecutablePath,

src/WebJobs.Script/Workers/Http/HttpWorkerDescription.cs

@@ -5,12 +5,13 @@
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.IO;
+using Microsoft.Extensions.Logging;
 
 namespace Microsoft.Azure.WebJobs.Script.Workers.Http
 {
     public class HttpWorkerDescription : WorkerDescription
     {
-        public override void ApplyDefaultsAndValidate(string workerDirectory)
+        public override void ApplyDefaultsAndValidate(string workerDirectory, ILogger logger)
         {
             if (workerDirectory == null)
             {

src/WebJobs.Script/Workers/ProcessManagement/WorkerDescription.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License. See License.txt in the project root for license information.
 
 using System.Collections.Generic;
+using Microsoft.Extensions.Logging;
 
 namespace Microsoft.Azure.WebJobs.Script.Workers
 {
@@ -27,6 +28,6 @@ public abstract class WorkerDescription
         /// </summary>
         public List<string> Arguments { get; set; }
 
-        public abstract void ApplyDefaultsAndValidate(string workerDirectory);
+        public abstract void ApplyDefaultsAndValidate(string workerDirectory, ILogger logger);
     }
 }

src/WebJobs.Script/Workers/Rpc/Configuration/RpcWorkerConfigFactory.cs

@@ -140,7 +140,7 @@ internal void AddProvider(string workerDir)
                     {
                         workerDescription.DefaultWorkerPath = GetHydratedWorkerPath(workerDescription);
                     }
-                    workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory());
+                    workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), _logger);
                     _workerDescripionDictionary[workerDescription.Language] = workerDescription;
                 }
                 catch (Exception ex)

src/WebJobs.Script/Workers/Rpc/RpcWorkerDescription.cs

@@ -7,6 +7,7 @@
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
+using Microsoft.Extensions.Logging;
 using Newtonsoft.Json;
 
 namespace Microsoft.Azure.WebJobs.Script.Workers.Rpc
@@ -65,7 +66,10 @@ public List<string> Extensions
             }
         }
 
-        public override void ApplyDefaultsAndValidate(string workerDirectory)
+        // Can be replaced for testing purposes
+        internal Func<string, bool> FileExists { private get; set; } = File.Exists;
+
+        public override void ApplyDefaultsAndValidate(string workerDirectory, ILogger logger)
         {
             if (workerDirectory == null)
             {
@@ -94,7 +98,7 @@ public override void ApplyDefaultsAndValidate(string workerDirectory)
                 throw new FileNotFoundException($"Did not find {nameof(DefaultWorkerPath)} for language: {Language}");
             }
 
-            ResolveDotNetDefaultExecutablePath();
+            ResolveDotNetDefaultExecutablePath(logger);
         }
 
         public void ValidateWorkerPath(string workerPath, OSPlatform os, Architecture architecture, string version)
@@ -139,14 +143,29 @@ private void ValidateRuntimeVersion(string version)
             }
         }
 
-        private void ResolveDotNetDefaultExecutablePath()
+        private void ResolveDotNetDefaultExecutablePath(ILogger logger)
         {
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
                 && (DefaultExecutablePath.Equals(RpcWorkerConstants.DotNetExecutableName, StringComparison.OrdinalIgnoreCase)
                     || DefaultExecutablePath.Equals(RpcWorkerConstants.DotNetExecutableNameWithExtension, StringComparison.OrdinalIgnoreCase)))
             {
                 var programFilesFolder = Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles);
-                DefaultExecutablePath = Path.Combine(programFilesFolder, RpcWorkerConstants.DotNetFolderName, DefaultExecutablePath);
+
+                var fullPath = Path.Combine(
+                                    programFilesFolder,
+                                    RpcWorkerConstants.DotNetFolderName,
+                                    RpcWorkerConstants.DotNetExecutableNameWithExtension);
+
+                if (FileExists(fullPath))
+                {
+                    DefaultExecutablePath = fullPath;
+                }
+                else
+                {
+                    logger.Log(
+                        LogLevel.Warning,
+                        $"File '{fullPath}' is not found, '{DefaultExecutablePath}' invocation will rely on the PATH environment variable.");
+                }
             }
         }
     }

test/WebJobs.Script.Tests/Workers/Rpc/RpcWorkerConfigTests.cs

@@ -244,17 +244,21 @@ public void ReadWorkerProviderFromConfig_OverrideDefaultExePath()
         [MemberData(nameof(InvalidWorkerDescriptions))]
         public void InvalidWorkerDescription_Throws(WorkerDescription workerDescription)
         {
-            Assert.Throws<ValidationException>(() => workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory()));
+            var testLogger = new TestLogger(testLanguage);
+
+            Assert.Throws<ValidationException>(() => workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), testLogger));
         }
 
         [Theory]
         [MemberData(nameof(ValidWorkerDescriptions))]
         public void ValidateWorkerDescription_Succeeds(WorkerDescription workerDescription)
         {
+            var testLogger = new TestLogger(testLanguage);
+
             try
             {
                 RpcWorkerConfigTestUtilities.CreateTestWorkerFileInCurrentDir();
-                workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory());
+                workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), testLogger);
             }
             finally
             {
@@ -267,26 +271,88 @@ public void ValidateWorkerDescription_Succeeds(WorkerDescription workerDescripti
         [InlineData("DotNet")]
         [InlineData("dotnet.exe")]
         [InlineData("DOTNET.EXE")]
-        public void ValidateWorkerDescription_ResolvesDotNetDefaultWorkerExecutablePath(string defaultExecutablePath)
+        public void ValidateWorkerDescription_ResolvesDotNetDefaultWorkerExecutablePath_WhenExpectedFileExists(
+            string defaultExecutablePath)
         {
+            var testLogger = new TestLogger(testLanguage);
+
             var expectedExecutablePath =
                 RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-                    ? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles), "dotnet", defaultExecutablePath)
+                    ? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles), "dotnet", "dotnet.exe")
                     : defaultExecutablePath;
 
-            var workerDescription = new RpcWorkerDescription { Language = testLanguage, Extensions = new List<string>(), DefaultExecutablePath = defaultExecutablePath };
-            workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory());
+            var workerDescription = new RpcWorkerDescription
+            {
+                Language = testLanguage,
+                Extensions = new List<string>(),
+                DefaultExecutablePath = defaultExecutablePath,
+                FileExists = path =>
+                                {
+                                    Assert.Equal(expectedExecutablePath, path);
+                                    return true;
+                                }
+            };
+
+            workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), testLogger);
             Assert.Equal(expectedExecutablePath, workerDescription.DefaultExecutablePath);
         }
 
+        [Theory]
+        [InlineData("dotnet")]
+        [InlineData("DotNet")]
+        [InlineData("dotnet.exe")]
+        [InlineData("DOTNET.EXE")]
+        public void ValidateWorkerDescription_DoesNotModifyDefaultWorkerExecutablePathAndWarns_WhenExpectedFileDoesNotExist(
+            string defaultExecutablePath)
+        {
+            var testLogger = new TestLogger(testLanguage);
+
+            var expectedExecutablePath =
+                RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+                    ? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles), "dotnet", "dotnet.exe")
+                    : defaultExecutablePath;
+
+            var workerDescription = new RpcWorkerDescription
+            {
+                Language = testLanguage,
+                Extensions = new List<string>(),
+                DefaultExecutablePath = defaultExecutablePath,
+                FileExists = path =>
+                                {
+                                    Assert.Equal(expectedExecutablePath, path);
+                                    return false;
+                                }
+            };
+
+            workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), testLogger);
+            Assert.Equal(defaultExecutablePath, workerDescription.DefaultExecutablePath);
+            Assert.True(testLogger.GetLogMessages().Any(message => message.Level == LogLevel.Warning
+                                                                   && message.FormattedMessage.Contains(defaultExecutablePath)
+                                                                   && message.FormattedMessage.Contains(expectedExecutablePath)));
+        }
+
         [Theory]
         [InlineData(@"D:\CustomExecutableFolder\dotnet")]
         [InlineData(@"/CustomExecutableFolder/dotnet")]
         [InlineData("AnythingElse")]
-        public void ValidateWorkerDescription_DoesNotModifyDefaultWorkerExecutablePath_WhenDoesNotStrictlyMatchDotNet(string defaultExecutablePath)
+        public void ValidateWorkerDescription_DoesNotModifyDefaultWorkerExecutablePath_WhenDoesNotStrictlyMatchDotNet(
+            string defaultExecutablePath)
         {
-            var workerDescription = new RpcWorkerDescription { Language = testLanguage, Extensions = new List<string>(), DefaultExecutablePath = defaultExecutablePath };
-            workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory());
+            var testLogger = new TestLogger(testLanguage);
+
+            var workerDescription = new RpcWorkerDescription
+            {
+                Language = testLanguage,
+                Extensions = new List<string>(),
+                DefaultExecutablePath = defaultExecutablePath,
+                FileExists = path =>
+                                {
+                                    Assert.True(false, "FileExists should not be called");
+                                    return false;
+                                }
+            };
+
+            workerDescription.ApplyDefaultsAndValidate(Directory.GetCurrentDirectory(), testLogger);
             Assert.Equal(defaultExecutablePath, workerDescription.DefaultExecutablePath);
         }