Allow absent inputs to use import contexts

docs/config.md

@@ -78,9 +78,14 @@ non-root destination. Unknown modes, invalid destinations, and invalid or
 conflicting sequence registrations fail before source acquisition. Overlay and
 CLI coordinates remain rooted at the application root. Ordinary `Load` recognizes
 source-root collection declarations whether or not imports are enabled; an
-`imports` declaration still requires imports to be enabled. `NoSource` rejects
-imports, active overlays, and filesystem context as invalid combinations but
-accepts overrides and remains an absent source rather than an empty document.
+`imports` declaration still requires imports to be enabled. `NoSource` may enable
+imports, but it has no main declarations to traverse. It accepts an explicit
+filesystem context: a host context requires an absolute existing `Root` and may
+omit `BaseDir` to use that root as its internal import base; a native context
+requires its namespace, `BaseDir`, and `Root`. Supplied context is validated even
+when imports are disabled. Active ordinary overlays remain invalid for
+`NoSource`. It accepts overrides and remains absent rather than contributing an
+empty document.
 
 Every source, options, schema, version, defaults, decode, resolution, or validation
 failure returns a nil result and a `*LoadError`. Successful results contain caller-owned `Config`, detached

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Configuration Discovery Implementation Plan
 
-Status: in progress. Stages 1, 2, and 3 are complete.
+Status: in progress. Stages 1, 2, 3, and 4 are complete.
 
 Implement the target behavior in [the discovery roadmap](discovery.md). That
 document owns the selected conventions and OD-1 through OD-5; this plan owns the
@@ -271,7 +271,7 @@ running defaults/resolution. A selected source's later read failure is not a
 fallback signal. Run focused discovery/source tests and the everyday check;
 mark and commit.
 
-## Stage 4 — Unify Absent Input Acquisition And Context Validation
+## Stage 4 — Unify Absent Input Acquisition And Context Validation (Complete)
 
 This stage changes `NoSource()` deliberately, before attaching discovery to loads.
 

internal/engine/filesystem.go

@@ -9,6 +9,9 @@ import (
 )
 
 func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	if source.kind == sourceNone && context.FS == nil {
+		return validateAbsentHostFilesystemContext(context)
+	}
 	if nativeContext(source, context) {
 		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
 		if diagnostic != nil {
@@ -33,10 +36,54 @@ func validateFilesystemContext(source Source, context *FilesystemContext, main p
 	return nil
 }
 
+func validateAbsentHostFilesystemContext(context *FilesystemContext) *Diagnostic {
+	rootPath, err := canonicalExistingPath(context.Root)
+	if err != nil {
+		value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+		return &value
+	}
+	basePath := rootPath
+	if context.BaseDir != "" {
+		basePath, err = canonicalExistingPath(context.BaseDir)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+			return &value
+		}
+	}
+	if !containedPath(rootPath, basePath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "import base must stay inside the configuration root", nil)
+		return &value
+	}
+	for _, candidate := range []string{rootPath, basePath} {
+		info, err := os.Stat(candidate)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return &value
+		}
+	}
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
+		value.Primary = nil
+		return &value
+	}
+	if err := root.Close(); err != nil {
+		value := importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)
+		return &value
+	}
+	return nil
+}
+
 func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Diagnostic {
 	switch source.kind {
 	case sourceNone:
-		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
+		if context.FS != nil {
+			if context.FS.namespace == nil || nilFilesystem(context.FS.namespace.filesystem) || context.BaseDir == "" || context.Root == "" || !fs.ValidPath(context.BaseDir) || !fs.ValidPath(context.Root) {
+				return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "native filesystem context requires a namespace, base, and root directory")}
+			}
+		} else if context.Root == "" || !filepath.IsAbs(context.Root) || context.BaseDir != "" && !filepath.IsAbs(context.BaseDir) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "absent-input filesystem root and base directories must be absolute host paths")}
+		}
 	case sourceFile:
 		if context.FS != nil {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "host file sources cannot use a native filesystem context")}

internal/engine/glob_test.go

@@ -268,6 +268,48 @@ func TestFilesystemContextIsValidatedWithoutImportsOrActiveOverlay(t *testing.T)
 	}
 }
 
+func TestNoSourceImportsAndFilesystemContext(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	for _, imports := range []bool{false, true} {
+		name := "imports disabled"
+		if imports {
+			name = "imports enabled"
+		}
+		t.Run(name, func(t *testing.T) {
+			result, err := testLoad[importTestConfig](NoSource(), Options[importTestConfig]{
+				Imports: imports, Filesystem: &FilesystemContext{Root: directory},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if sources := result.Sources(); len(sources) != 0 {
+				t.Fatalf("NoSource sources = %#v", sources)
+			}
+			if _, ok := result.Metadata().Lookup(Keys("items", "root")); ok {
+				t.Fatal("NoSource contributed explicit input")
+			}
+		})
+	}
+
+	outside := t.TempDir()
+	for _, test := range []struct {
+		name    string
+		context FilesystemContext
+		code    Code
+	}{
+		{name: "missing root", context: FilesystemContext{BaseDir: directory}, code: CodeOptionsInvalid},
+		{name: "relative root", context: FilesystemContext{Root: "."}, code: CodeOptionsInvalid},
+		{name: "missing root directory", context: FilesystemContext{Root: filepath.Join(directory, "missing")}, code: CodeImportInvalid},
+		{name: "base outside root", context: FilesystemContext{BaseDir: outside, Root: directory}, code: CodeImportOutsideRoot},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			requireLoadCode(t, NoSource(), Options[importTestConfig]{Filesystem: &test.context}, test.code)
+		})
+	}
+}
+
 func writeGlobRoot(t *testing.T, directory, pattern string, required bool) string {
 	t.Helper()
 	requiredLine := ""

internal/engine/imports_test.go

@@ -398,7 +398,7 @@ label: helper
 	childPath := writeImportFile(t, directory, "records/children/child.yml", "value: nested\n")
 
 	loaded, err := LoadLayered(
-		Input{Source: NoSource()}, testSecretBinding(),
+		Input{Source: NoSource(), Imports: true}, testSecretBinding(),
 		LayeredOptions[config]{Underlays: []Underlay{{Inputs: []Input{{
 			Source: File(itemPath), Imports: true, Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem,
 		}}}}},

internal/engine/load.go

@@ -21,8 +21,9 @@ type Definition[T any] struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay. Overrides contain only values explicitly supplied by an application CLI.
 type Options[T any] struct {
 	Definition              Definition[T]
 	Imports                 bool
@@ -39,7 +40,9 @@ type Options[T any] struct {
 	importDefaultsValidated bool
 }
 
-// FilesystemContext supplies filesystem roots for imports and overlays.
+// FilesystemContext supplies filesystem roots for imports and overlays. For an
+// absent host input, Root is required and BaseDir may be omitted to use Root as
+// the import base. Native contexts still require both BaseDir and Root.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string

internal/engine/load_input.go

@@ -200,7 +200,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 			versionState.mainProcessing = &processed
 		}
 	}
-	if options.Imports {
+	if options.Imports && source.kind != sourceNone {
 		if options.deferAssembly {
 			input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessingDeferred(source, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy, true)
 		} else {
@@ -218,7 +218,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		if versionProcessing != nil {
 			input.document = versionProcessing.payload
 		}
-		if options.Filesystem != nil && !activeOverlay(options.Overlay) {
+		if options.Filesystem != nil && (source.kind == sourceNone || !activeOverlay(options.Overlay)) {
 			if diagnostic := validateFilesystemContext(source, options.Filesystem, parsed); diagnostic != nil {
 				return input, []Diagnostic{*diagnostic}
 			}

internal/engine/options.go

@@ -31,9 +31,6 @@ func selectOptions[T any](source Source, supplied []Options[T]) (Options[T], []D
 	if options.Filesystem != nil {
 		diagnostics = append(diagnostics, filesystemOptionDiagnostics(source, options.Filesystem)...)
 	}
-	if source.kind == sourceNone && options.Imports {
-		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot enable imports"))
-	}
 	if source.kind == sourceNone && options.Overlay != "" && options.Overlay != "default" {
 		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot select an active overlay"))
 	}

internal/engine/override_lifecycle_test.go

@@ -129,35 +129,42 @@ func TestPostDecodeOverrideLifecycleMatchesOrdinaryAndLayeredLoads(t *testing.T)
 func TestNoSourceOverridesReachResolutionAfterDefaults(t *testing.T) {
 	t.Parallel()
 	type config struct {
-		Count   int `yaml:"count"`
-		Derived int `yaml:"derived"`
+		Count   int  `yaml:"count"`
+		Enabled bool `yaml:"enabled"`
+		Derived int  `yaml:"derived"`
 	}
 	calls := 0
 	result, err := testLoad(NoSource(), Options[config]{
 		Definition: Definition[config]{
 			Defaults: func() config {
 				calls++
-				return config{Count: 2}
+				return config{Count: 2, Enabled: true}
 			},
 			Resolve: func(value *config, resolution *Resolution) error {
-				info, ok := resolution.InputInfo(Keys("count"))
-				if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
-					t.Errorf("resolution count info = %#v, %v", info, ok)
+				for _, key := range []string{"count", "enabled"} {
+					info, ok := resolution.InputInfo(Keys(key))
+					if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+						t.Errorf("resolution %s info = %#v, %v", key, info, ok)
+					}
 				}
 				Default(resolution, Keys("count"), &value.Count, 10, "preserve CLI value")
+				Default(resolution, Keys("enabled"), &value.Enabled, true, "preserve CLI value")
 				Default(resolution, Keys("derived"), &value.Derived, value.Count*3, "derive from CLI count")
 				return nil
 			},
 		},
-		Overrides: []Override{{Path: Keys("count"), Value: 4, Flag: "--count"}},
+		Overrides: []Override{
+			{Path: Keys("count"), Value: 0, Flag: "--count"},
+			{Path: Keys("enabled"), Value: false, Flag: "--enabled"},
+		},
 	})
 	if err != nil {
 		t.Fatal(err)
 	}
-	if calls != 1 || result.Config.Count != 4 || result.Config.Derived != 12 {
+	if calls != 1 || result.Config.Count != 0 || result.Config.Enabled || result.Config.Derived != 0 {
 		t.Fatalf("defaults-only CLI result = %#v, defaults calls %d", result.Config, calls)
 	}
-	if sources := result.Sources(); len(sources) != 1 || sources[0].Layer != LayerCLI {
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Layer != LayerCLI || sources[1].Layer != LayerCLI {
 		t.Fatalf("defaults-only CLI sources = %#v", sources)
 	}
 }

internal/engine/source.go

@@ -174,7 +174,10 @@ func Mapping(label string, values map[string]any) Source {
 	return source
 }
 
-// NoSource explicitly selects a defaults-only input. It creates no source record.
+// NoSource selects an absent input for defaults and resolution. It creates no
+// source record and has no main declarations to traverse when imports are enabled.
+// An explicit filesystem context may be supplied; active overlays are not
+// available for absent inputs.
 func NoSource() Source {
 	return Source{kind: sourceNone}
 }

internal/engine/source_version_test.go

@@ -539,6 +539,41 @@ func TestSourceVersionNoSourceMainRetainsAbsentCallback(t *testing.T) {
 			if !called || len(result.diagnostics) != 0 || result.evidence != nil || result.payload != nil {
 				t.Fatalf("NoSource result = %#v, callback called = %v", result, called)
 			}
+
+			type config struct{}
+			for _, reject := range []bool{false, true} {
+				called = false
+				definition := Definition[config]{Version: &VersionRule{
+					Path: Keys("version"),
+					Check: func(version Version, _ *Validation) error {
+						called = true
+						if version != (Version{}) {
+							t.Errorf("NoSource load version = %#v", version)
+						}
+						if reject {
+							return errors.New("rejected absent main")
+						}
+						return nil
+					},
+				}}
+				loaded, loadErr := testLoad(NoSource(), Options[config]{
+					Definition: definition, Imports: true, SourceVersionMode: test.mode,
+				})
+				if !called {
+					t.Fatal("NoSource load did not call the source-version checker")
+				}
+				if reject {
+					var failure *LoadError
+					if !errors.As(loadErr, &failure) {
+						t.Fatalf("rejected NoSource load = %v, want *LoadError", loadErr)
+					}
+					requireSourceVersionDiagnostic(t, failure.Diagnostics(), CodeVersionHookFailed, "/version")
+					continue
+				}
+				if loadErr != nil || loaded == nil || len(loaded.Sources()) != 0 {
+					t.Fatalf("accepted NoSource load = %#v, %v", loaded, loadErr)
+				}
+			}
 		})
 	}
 }

lifecycle.go

@@ -83,8 +83,9 @@ type ImportDefaults struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay. Overrides contain only values explicitly supplied by an application CLI.
 // Enabled source-version modes interpret Definition.Version.Path in each
 // document's local coordinates. Placement projects checks and main protection
 // through the input destination; item placement adds the source's item
@@ -109,9 +110,11 @@ type Options[T any] struct {
 }
 
 // FilesystemContext supplies roots for imports and overlays. With FS nil,
-// BaseDir and Root use absolute host paths. With FS set, they use slash-relative
-// paths in that namespace ("." denotes its root). File sources derive BaseDir
-// from the selected file; a native file's explicit Root limits its scope.
+// BaseDir and Root use absolute host paths. For NoSource, Root is required and
+// BaseDir may be omitted to use Root as the import base. With FS set, BaseDir
+// and Root use slash-relative paths in that namespace ("." denotes its root).
+// File sources derive BaseDir from the selected file; a native file's explicit
+// Root limits its scope.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string

source.go

@@ -47,7 +47,10 @@ func Mapping(label string, values map[string]any) Source {
 	return Source{value: engine.Mapping(label, values)}
 }
 
-// NoSource explicitly selects a defaults-only input. It creates no source record.
+// NoSource selects an absent input for defaults and resolution. It creates no
+// source record and has no main declarations to traverse when imports are enabled.
+// An explicit filesystem context may be supplied; active overlays are not
+// available for absent inputs.
 func NoSource() Source { return Source{value: engine.NoSource()} }
 
 func (s Source) toEngine() engine.Source { return s.value }