Fix dab validate error messaging when logger is not available (#3311)

## Why make this change?

Closes #3268

## What is this change?

When `dab validate` or `dab start` encounters a config parsing error
(e.g. missing entities/autoentities), the CLI previously dumped the full
exception message and stack trace to stderr. This made the output noisy
and unhelpful. After this change, only a clean, descriptive validation
message is shown.

The key design changes:

- `TryParseConfig` is now a pure function that returns an `out string?
parseError` message on failure instead of writing to `Console.Error` or
`ILogger` internally. Error reporting is the caller's responsibility.
- `FileSystemRuntimeConfigLoader.TryLoadConfig` writes the `parseError`
to `Console.Error` (because config is parsed before the DI container and
logger are available, so the log buffer would never be flushed on a
parse failure) and sets an instance-scoped `IsParseErrorEmitted` flag so
CLI callers (`ConfigGenerator`) can avoid logging duplicate messages.
- `ConfigGenerator.IsConfigValid` now has an explicit early-return path
when config parsing fails (via `runtimeConfigProvider.TryGetConfig`),
using `IsParseErrorEmitted` to suppress duplicate output.
- `ValidateOptions.Handler` uses `LogError("Config is invalid.")`.
- Comments referencing which method emits the error to `Console.Error`
corrected to `TryLoadConfig` (not `TryParseConfig`).

## How was this tested?

* `ValidateConfigTests.cs` — Added
`TestValidateConfigWithNoEntitiesProducesCleanError` (new test verifying
clean error message, no stack traces).
* `EnvironmentTests.cs` — Updated
`FailureToStartEngineWhenEnvVarNamedWrong` to match the new single-line
clean stderr format.
* `EndToEndTests.cs` — Simplified assertions in
`TestExitOfRuntimeEngineWithInvalidConfig` (this test is Ignored but
updated for consistency).
* `RuntimeConfigLoaderTests.cs` — Updated
`FailLoadMultiDataSourceConfigDuplicateEntities` to assert
`loader.IsParseErrorEmitted` is true.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Souvik Ghosh <souvikofficial04@gmail.com>

Author: 3 people

src/Cli.Tests/EndToEndTests.cs

@@ -1173,10 +1173,6 @@ public async Task TestExitOfRuntimeEngineWithInvalidConfig(
             Assert.IsNotNull(output);
             StringAssert.Contains(output, $"Deserialization of the configuration file failed.", StringComparison.Ordinal);
 
-            output = await process.StandardOutput.ReadLineAsync();
-            Assert.IsNotNull(output);
-            StringAssert.Contains(output, $"Error: Failed to parse the config file: {TEST_RUNTIME_CONFIG_FILE}.", StringComparison.Ordinal);
-
             output = await process.StandardOutput.ReadLineAsync();
             Assert.IsNotNull(output);
             StringAssert.Contains(output, $"Failed to start the engine.", StringComparison.Ordinal);

src/Cli.Tests/EnvironmentTests.cs

@@ -163,8 +163,8 @@ public async Task FailureToStartEngineWhenEnvVarNamedWrong()
         );
 
         string? output = await process.StandardError.ReadLineAsync();
-        Assert.AreEqual("Deserialization of the configuration file failed during a post-processing step.", output);
-        output = await process.StandardError.ReadToEndAsync();
+        Assert.IsNotNull(output);
+        // Clean error message on stderr with no stack trace.
         StringAssert.Contains(output, "A valid Connection String should be provided.", StringComparison.Ordinal);
         process.Kill();
     }

src/Cli.Tests/ValidateConfigTests.cs

@@ -199,6 +199,34 @@ public void TestValidateConfigFailsWithNoEntities()
         }
     }
 
+    /// <summary>
+    /// Validates that when the config has no entities or autoentities, TryParseConfig
+    /// sets a clean error message (not a raw exception with stack trace) and
+    /// IsConfigValid returns false without throwing.
+    /// Regression test for https://github.com/Azure/data-api-builder/issues/3268
+    /// </summary>
+    [TestMethod]
+    public void TestValidateConfigWithNoEntitiesProducesCleanError()
+    {
+        string configWithoutEntities = $"{{{SAMPLE_SCHEMA_DATA_SOURCE},{RUNTIME_SECTION}}}";
+
+        // Verify TryParseConfig produces a clean error without stack traces.
+        bool parsed = RuntimeConfigLoader.TryParseConfig(configWithoutEntities, out _, out string? parseError);
+
+        Assert.IsFalse(parsed, "Config with no entities should fail to parse.");
+        Assert.IsNotNull(parseError, "parseError should be set when config parsing fails.");
+        StringAssert.Contains(parseError,
+            "Configuration file should contain either at least the entities or autoentities property",
+            "Parse error should contain the clean validation message.");
+        Assert.IsFalse(parseError.Contains("StackTrace"),
+            "Stack trace should not be present in parse error.");
+
+        // Verify IsConfigValid also returns false cleanly (no exception thrown).
+        ((MockFileSystem)_fileSystem!).AddFile(TEST_RUNTIME_CONFIG_FILE, configWithoutEntities);
+        ValidateOptions validateOptions = new(TEST_RUNTIME_CONFIG_FILE);
+        Assert.IsFalse(ConfigGenerator.IsConfigValid(validateOptions, _runtimeConfigLoader!, _fileSystem!));
+    }
+
     /// <summary>
     /// This Test is used to verify that the validate command is able to catch when data source field is missing.
     /// </summary>

src/Cli/Commands/ValidateOptions.cs

@@ -38,7 +38,7 @@ public int Handler(ILogger logger, FileSystemRuntimeConfigLoader loader, IFileSy
             }
             else
             {
-                logger.LogError("Config is invalid. Check above logs for details.");
+                logger.LogError("Config is invalid.");
             }
 
             return isValidConfig ? CliReturnCode.SUCCESS : CliReturnCode.GENERAL_ERROR;

src/Cli/ConfigGenerator.cs

@@ -2564,7 +2564,14 @@ public static bool TryStartEngineWithOptions(StartOptions options, FileSystemRun
             // Replaces all the environment variables while deserializing when starting DAB.
             if (!loader.TryLoadKnownConfig(out RuntimeConfig? deserializedRuntimeConfig, replaceEnvVar: true))
             {
-                _logger.LogError("Failed to parse the config file: {runtimeConfigFile}.", runtimeConfigFile);
+                // When IsParseErrorEmitted is true, TryLoadConfig already emitted the
+                // detailed error to Console.Error. Only log a generic message to avoid
+                // duplicate output (stderr + stdout).
+                if (!loader.IsParseErrorEmitted)
+                {
+                    _logger.LogError("Failed to parse the config file: {runtimeConfigFile}.", runtimeConfigFile);
+                }
+
                 return false;
             }
             else
@@ -2641,6 +2648,19 @@ public static bool IsConfigValid(ValidateOptions options, FileSystemRuntimeConfi
 
             RuntimeConfigProvider runtimeConfigProvider = new(loader);
 
+            if (!runtimeConfigProvider.TryGetConfig(out RuntimeConfig? _))
+            {
+                // When IsParseErrorEmitted is true, TryLoadConfig already emitted the
+                // detailed error to Console.Error. Only log a generic message to avoid
+                // duplicate output (stderr + stdout).
+                if (!loader.IsParseErrorEmitted)
+                {
+                    _logger.LogError("Failed to parse the config file.");
+                }
+
+                return false;
+            }
+
             ILogger<RuntimeConfigValidator> runtimeConfigValidatorLogger = LoggerFactoryForCli.CreateLogger<RuntimeConfigValidator>();
             RuntimeConfigValidator runtimeConfigValidator = new(runtimeConfigProvider, fileSystem, runtimeConfigValidatorLogger, true);
 

src/Config/FileSystemRuntimeConfigLoader.cs

@@ -84,6 +84,12 @@ public class FileSystemRuntimeConfigLoader : RuntimeConfigLoader
     /// </summary>
     public string ConfigFilePath { get; internal set; }
 
+    /// <summary>
+    /// Indicates whether the most recent TryLoadConfig call encountered a parse error
+    /// that was already emitted to Console.Error.
+    /// </summary>
+    public bool IsParseErrorEmitted { get; private set; }
+
     public FileSystemRuntimeConfigLoader(
         IFileSystem fileSystem,
         HotReloadEventHandler<HotReloadEventArgs>? handler = null,
@@ -205,6 +211,7 @@ public bool TryLoadConfig(
         bool? isDevMode = null,
         DeserializationVariableReplacementSettings? replacementSettings = null)
     {
+        IsParseErrorEmitted = false;
         if (_fileSystem.File.Exists(path))
         {
             SendLogToBufferOrLogger(LogLevel.Information, $"Loading config file from {_fileSystem.Path.GetFullPath(path)}.");
@@ -241,11 +248,12 @@ public bool TryLoadConfig(
             // Use default replacement settings if none provided
             replacementSettings ??= new DeserializationVariableReplacementSettings();
 
+            string? parseError = null;
             if (!string.IsNullOrEmpty(json) && TryParseConfig(
                 json,
                 out RuntimeConfig,
+                out parseError,
                 replacementSettings,
-                logger: null,
                 connectionString: _connectionString))
             {
                 if (TrySetupConfigFileWatcher())
@@ -281,6 +289,12 @@ public bool TryLoadConfig(
                 RuntimeConfig = LastValidRuntimeConfig;
             }
 
+            if (parseError is not null)
+            {
+                Console.Error.WriteLine(parseError);
+                IsParseErrorEmitted = true;
+            }
+
             config = null;
             return false;
         }

src/Config/RuntimeConfigLoader.cs

@@ -13,7 +13,6 @@
 using Azure.DataApiBuilder.Product;
 using Azure.DataApiBuilder.Service.Exceptions;
 using Microsoft.Data.SqlClient;
-using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Primitives;
 using Npgsql;
 using static Azure.DataApiBuilder.Config.DabConfigEvents;
@@ -179,16 +178,34 @@ protected void SignalConfigChanged(string message = "")
     /// </summary>
     /// <param name="json">JSON that represents the config file.</param>
     /// <param name="config">The parsed config, or null if it parsed unsuccessfully.</param>
+    /// <param name="parseError">A clean error message when parsing fails, or null on success.</param>
     /// <param name="replacementSettings">Settings for variable replacement during deserialization. If null, no variable replacement will be performed.</param>
-    /// <param name="logger">logger to log messages</param>
     /// <param name="connectionString">connectionString to add to config if specified</param>
     /// <returns>True if the config was parsed, otherwise false.</returns>
     public static bool TryParseConfig(string json,
         [NotNullWhen(true)] out RuntimeConfig? config,
         DeserializationVariableReplacementSettings? replacementSettings = null,
-        ILogger? logger = null,
         string? connectionString = null)
     {
+        return TryParseConfig(json, out config, out _, replacementSettings, connectionString);
+    }
+
+    /// <summary>
+    /// Parses a JSON string into a <c>RuntimeConfig</c> object for single database scenario.
+    /// </summary>
+    /// <param name="json">JSON that represents the config file.</param>
+    /// <param name="config">The parsed config, or null if it parsed unsuccessfully.</param>
+    /// <param name="parseError">A clean error message when parsing fails, or null on success.</param>
+    /// <param name="replacementSettings">Settings for variable replacement during deserialization. If null, no variable replacement will be performed.</param>
+    /// <param name="connectionString">connectionString to add to config if specified</param>
+    /// <returns>True if the config was parsed, otherwise false.</returns>
+    public static bool TryParseConfig(string json,
+        [NotNullWhen(true)] out RuntimeConfig? config,
+        out string? parseError,
+        DeserializationVariableReplacementSettings? replacementSettings = null,
+        string? connectionString = null)
+    {
+        parseError = null;
         // First pass: extract AzureKeyVault options if AKV replacement is requested
         if (replacementSettings?.DoReplaceAkvVar is true)
         {
@@ -263,18 +280,9 @@ public static bool TryParseConfig(string json,
             ex is JsonException ||
             ex is DataApiBuilderException)
         {
-            string errorMessage = ex is JsonException ? "Deserialization of the configuration file failed." :
-                "Deserialization of the configuration file failed during a post-processing step.";
-
-            // logger can be null when called from CLI
-            if (logger is null)
-            {
-                Console.Error.WriteLine(errorMessage + $"\n" + $"Message:\n {ex.Message}\n" + $"Stack Trace:\n {ex.StackTrace}");
-            }
-            else
-            {
-                logger.LogError(exception: ex, message: errorMessage);
-            }
+            parseError = ex is DataApiBuilderException
+                ? ex.Message
+                : $"Deserialization of the configuration file failed. {ex.Message}";
 
             config = null;
             return false;

src/Core/Configurations/RuntimeConfigProvider.cs

@@ -188,6 +188,7 @@ public async Task<bool> Initialize(
         if (RuntimeConfigLoader.TryParseConfig(
                 configuration,
                 out RuntimeConfig? runtimeConfig,
+                out _,
                 replacementSettings: null))
         {
             _configLoader.RuntimeConfig = runtimeConfig;
@@ -269,7 +270,7 @@ public async Task<bool> Initialize(
 
         IsLateConfigured = true;
 
-        if (RuntimeConfigLoader.TryParseConfig(jsonConfig, out RuntimeConfig? runtimeConfig, replacementSettings))
+        if (RuntimeConfigLoader.TryParseConfig(jsonConfig, out RuntimeConfig? runtimeConfig, out _, replacementSettings))
         {
             _configLoader.RuntimeConfig = runtimeConfig.DataSource.DatabaseType switch
             {

src/Service.Tests/Configuration/ConfigurationTests.cs

@@ -2500,8 +2500,7 @@ public async Task TestSPRestDefaultsForManuallyConstructedConfigs(
                 configJson,
                 out RuntimeConfig deserializedConfig,
                 replacementSettings: new(),
-                logger: null,
-                GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL));
+                connectionString: GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL));
             string configFileName = "custom-config.json";
             File.WriteAllText(configFileName, deserializedConfig.ToJson());
             string[] args = new[]
@@ -2588,8 +2587,7 @@ public async Task SanityTestForRestAndGQLRequestsWithoutMultipleMutationFeatureF
                 configJson,
                 out RuntimeConfig deserializedConfig,
                 replacementSettings: new(),
-                logger: null,
-                GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL)));
+                connectionString: GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL)));
             string configFileName = "custom-config.json";
             File.WriteAllText(configFileName, deserializedConfig.ToJson());
             string[] args = new[]
@@ -3619,8 +3617,7 @@ public async Task ValidateStrictModeAsDefaultForRestRequestBody(bool includeExtr
                 configJson,
                 out RuntimeConfig deserializedConfig,
                 replacementSettings: new(),
-                logger: null,
-                GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL));
+                connectionString: GetConnectionStringFromEnvironmentConfig(environment: TestCategory.MSSQL));
             const string CUSTOM_CONFIG = "custom-config.json";
             File.WriteAllText(CUSTOM_CONFIG, deserializedConfig.ToJson());
             string[] args = new[]

src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs

@@ -96,10 +96,11 @@ public async Task FailLoadMultiDataSourceConfigDuplicateEntities(string configPa
         Console.SetError(sw);
 
         loader.TryLoadConfig("dab-config.json", out RuntimeConfig _);
-        string error = sw.ToString();
 
-        Assert.IsTrue(error.StartsWith("Deserialization of the configuration file failed during a post-processing step."));
-        Assert.IsTrue(error.Contains("An item with the same key has already been added."));
+        Assert.IsTrue(loader.IsParseErrorEmitted,
+            "IsParseErrorEmitted should be true when config parsing fails.");
+        Assert.IsFalse(string.IsNullOrWhiteSpace(sw.ToString()),
+            "An error message should have been emitted to Console.Error.");
     }
 
     /// <summary>