More logging and better error handling for download endpoint

src/SIL.Machine.AspNetCore/Configuration/IMachineBuilderExtensions.cs

@@ -336,6 +336,7 @@ public static IMachineBuilder AddServalTranslationEngineService(
         {
             options.Interceptors.Add<CancellationInterceptor>();
             options.Interceptors.Add<UnimplementedInterceptor>();
+            options.Interceptors.Add<InvalidOperationInterceptor>();
         });
         builder.AddServalPlatformService(connectionString);
         engineTypes ??=

src/SIL.Machine.AspNetCore/Services/InvalidOperationInterceptor.cs

@@ -0,0 +1,20 @@
+namespace SIL.Machine.AspNetCore.Services;
+
+public class InvalidOperationInterceptor : Interceptor
+{
+    public override async Task<TResponse> UnaryServerHandler<TRequest, TResponse>(
+        TRequest request,
+        ServerCallContext context,
+        UnaryServerMethod<TRequest, TResponse> continuation
+    )
+    {
+        try
+        {
+            return await continuation(request, context);
+        }
+        catch (InvalidOperationException)
+        {
+            throw new RpcException(new Status(StatusCode.InvalidArgument, "This operation is not valid at this time."));
+        }
+    }
+}

src/SIL.Machine.AspNetCore/Services/NmtEngineService.cs

@@ -15,7 +15,8 @@ public class NmtEngineService(
     IBuildJobService buildJobService,
     ILanguageTagService languageTagService,
     ClearMLMonitorService clearMLMonitorService,
-    ISharedFileService sharedFileService
+    ISharedFileService sharedFileService,
+    ILogger<NmtEngineService> logger
 ) : ITranslationEngineService
 {
     private readonly IDistributedReaderWriterLockFactory _lockFactory = lockFactory;
@@ -26,6 +27,7 @@ ISharedFileService sharedFileService
     private readonly ClearMLMonitorService _clearMLMonitorService = clearMLMonitorService;
     private readonly ILanguageTagService _languageTagService = languageTagService;
     private readonly ISharedFileService _sharedFileService = sharedFileService;
+    private readonly ILogger<NmtEngineService> _logger = logger;
 
     public const string ModelDirectory = "models/";
 
@@ -126,6 +128,7 @@ public async Task<ModelDownloadUrl> GetModelDownloadUrlAsync(
         CancellationToken cancellationToken = default
     )
     {
+        _logger.LogInformation("Getting model download URL for engine {engineId}.", engineId);
         TranslationEngine engine = await GetEngineAsync(engineId, cancellationToken);
         if (engine.IsModelPersisted != true)
             throw new NotSupportedException(
@@ -137,17 +140,47 @@ public async Task<ModelDownloadUrl> GetModelDownloadUrlAsync(
         string filepath = GetModelPath(engineId, engine.BuildRevision);
         bool fileExists = await _sharedFileService.ExistsAsync(filepath, cancellationToken);
         if (!fileExists)
-            throw new FileNotFoundException(
-                $"The model should exist to be downloaded but is not there for BuildRevision {engine.BuildRevision}."
+        {
+            _logger.LogError(
+                "The model should exist to be downloaded but is not there for BuildRevision {engineBuildRevision}.",
+                engine.BuildRevision
+            );
+            throw new RpcException(
+                new Status(
+                    StatusCode.DataLoss,
+                    $"The model should exist to be downloaded but is not there for BuildRevision {engine.BuildRevision}."
+                )
             );
+        }
         var expiresAt = DateTime.UtcNow.AddMinutes(MinutesToExpire);
-        var modelInfo = new ModelDownloadUrl
+        _logger.LogInformation(
+            "Model download URL will expire at {expiresAt} for engine {engineId} and revision {engineBuildRevision}.",
+            expiresAt,
+            engineId,
+            engine.BuildRevision
+        );
+        try
         {
-            Url = await _sharedFileService.GetDownloadUrlAsync(filepath, expiresAt),
-            ModelRevision = engine.BuildRevision,
-            ExipiresAt = expiresAt
-        };
-        return modelInfo;
+            var modelInfo = new ModelDownloadUrl
+            {
+                Url = await _sharedFileService.GetDownloadUrlAsync(filepath, expiresAt),
+                ModelRevision = engine.BuildRevision,
+                ExipiresAt = expiresAt
+            };
+            _logger.LogInformation(
+                "Model download URL: {modelInfoUrl} for engine {engineId} and revision {engineBuildRevision}.",
+                modelInfo.Url,
+                engineId,
+                engine.BuildRevision
+            );
+            return modelInfo;
+        }
+        catch (Exception e)
+        {
+            throw new RpcException(
+                new Status(StatusCode.Internal, "Error occurred while getting the model download URL.", e)
+            );
+        }
     }
 
     public Task<IReadOnlyList<TranslationResult>> TranslateAsync(

tests/SIL.Machine.AspNetCore.Tests/Services/NmtEngineServiceTests.cs

@@ -209,7 +209,8 @@ private NmtEngineService CreateService()
                 BuildJobService,
                 new LanguageTagService(),
                 ClearMLMonitorService,
-                SharedFileService
+                SharedFileService,
+                Substitute.For<ILogger<NmtEngineService>>()
             );
         }