Fix non-deterministic generated code involving interfaces and fragments.

docs/CHANGELOG.md

@@ -36,8 +36,9 @@ Note that genqlient is now tested from Go 1.20 through Go 1.22.
 ### Bug fixes:
 - The presence of negative pointer directives, i.e., `# @genqlient(pointer: false)` are now respected even in the when `optional: pointer` is set in the configuration file.
 - Made name collisions between query/mutation arguments and local function variables less likely.
-- Fix generation issue related to golang type implementation of complex graphql union fragments
+- Fix generation issue related to golang type implementation of complex graphql union fragments.
 - Bind correctly to types in the same package as the generated code.
+- Fix non-deterministic generated code when querying graphql interfaces via named fragments.
 
 ## v0.6.0
 

generate/convert.go

@@ -841,6 +841,8 @@ func (g *generator) convertNamedFragment(fragment *ast.FragmentDefinition) (goTy
 		return goType, nil
 	case ast.Interface, ast.Union:
 		implementationTypes := g.schema.GetPossibleTypes(typ)
+		// Make sure we generate stable output by sorting the types by name when we get them
+		sort.Slice(implementationTypes, func(i, j int) bool { return implementationTypes[i].Name < implementationTypes[j].Name })
 		goType := &goInterfaceType{
 			GoName:          fragment.Name,
 			SharedFields:    fields,