Better Error Handling in Fusion CommandLine

src/HotChocolate/Fusion/src/CommandLine/Commands/ComposeCommand.cs

@@ -135,9 +135,15 @@ private static async Task ExecuteAsync(
         }
 
         var composer = new FusionGraphComposer(prefix, prefixSelf, () => new ConsoleLog(console));
-        var fusionGraph = await composer.ComposeAsync(configs.Values, flags, cancellationToken);
-        var fusionGraphDoc = Utf8GraphQLParser.Parse(SchemaFormatter.FormatAsString(fusionGraph));
+        var fusionGraph = await composer.TryComposeAsync(configs.Values, flags, cancellationToken);
+
+        if (fusionGraph is null)
+        {
+            console.WriteLine("Fusion graph composition failed.");
+            return;
+        }
 
+        var fusionGraphDoc = Utf8GraphQLParser.Parse(SchemaFormatter.FormatAsString(fusionGraph));
         var typeNames = FusionTypeNames.From(fusionGraphDoc);
         var rewriter = new Metadata.FusionGraphConfigurationToSchemaRewriter();
         var schemaDoc = (DocumentNode)rewriter.Rewrite(fusionGraphDoc, new(typeNames))!;

src/HotChocolate/Fusion/src/Composition/FusionGraphComposer.cs

@@ -122,4 +122,43 @@ public async ValueTask<Schema> ComposeAsync(
         // Return the resulting merged schema.
         return context.FusionGraph;
     }
+
+    /// <summary>
+    /// Composes the subgraph schemas into a single,
+    /// merged schema representing the fusion gateway configuration.
+    /// </summary>
+    /// <param name="configurations">
+    /// The subgraph configurations to compose.
+    /// </param>
+    /// <param name="features">
+    /// The composition feature flags.
+    /// </param>
+    /// <param name="cancellationToken">
+    /// A cancellation token that can be used to cancel the operation.
+    /// </param>
+    /// <returns>The fusion gateway configuration.</returns>
+    public async ValueTask<Schema?> TryComposeAsync(
+        IEnumerable<SubgraphConfiguration> configurations,
+        FusionFeatureFlags features = FusionFeatureFlags.None,
+        CancellationToken cancellationToken = default)
+    {
+        var log = new DefaultCompositionLog(_logFactory?.Invoke());
+
+        // Create a new composition context with the given subgraph configurations,
+        // fusion type prefix, and fusion type self option.
+        var context = new CompositionContext(
+            configurations.ToArray(),
+            log,
+            _fusionTypePrefix,
+            _fusionTypeSelf)
+        {
+            Features = features,
+            Abort = cancellationToken
+        };
+
+        // Run the merge pipeline on the composition context.
+        await _pipeline(context);
+
+        return log.HasErrors ? null : context.FusionGraph;
+    }
 }