Cover search exhaustion and fallback selection

docs/roadmap/remediation.md

@@ -74,6 +74,8 @@ sequence-item, and budget checks to verify the extraction.
 
 ## Finding 3 — Exhaustion and lazy fallback lack focused regression coverage
 
+**Status** — Complete. Public tests cover search exhaustion, root-only main context, and missing fallbacks when they win or are shadowed.
+
 **Priority P3.** The context-selection test uses an existing fallback when
 checking that main derivation wins. It would still pass if the loader
 prematurely opened that fallback. The no-base tests use `NoSource()`, and

facade_main_relative_integration_test.go

@@ -122,3 +122,76 @@ func TestMainRelativeSourceVersionEvidence(t *testing.T) {
 		}
 	}
 }
+
+func TestMainRelativeOptionalSearchExhaustion(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "overlays"), 0700); err != nil {
+		t.Fatal(err)
+	}
+	for name, content := range map[string]string{
+		"fragment.yml":      "settings: {companion: loaded}\n",
+		"overlays/live.yml": "count: 9\n",
+	} {
+		if err := os.WriteFile(filepath.Join(root, name), []byte(content), 0600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	search := configkit.SearchConfig("", configkit.SearchPaths(filepath.Join(root, "absent.yml")))
+	missing := filepath.Join(root, "missing-fallback")
+	fallback := &configkit.FilesystemContext{Root: missing, BaseDir: missing}
+
+	for _, tc := range []struct {
+		name            string
+		mainContext     *configkit.FilesystemContext
+		bindingFallback *configkit.FilesystemContext
+		overlayFallback *configkit.FilesystemContext
+		overlay         string
+		wantCount       int
+		wantSources     int
+	}{
+		{name: "skip without context", wantCount: 3, wantSources: 1},
+		{
+			name: "root-only main context wins", mainContext: &configkit.FilesystemContext{Root: root},
+			bindingFallback: fallback, overlayFallback: fallback, overlay: "live", wantCount: 9, wantSources: 3,
+		},
+		{name: "selected missing companion fallback", bindingFallback: fallback},
+		{name: "selected missing overlay fallback", overlay: "live", overlayFallback: fallback},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			underlay := configkit.Input{
+				Source:  configkit.Bytes("companion", []byte("count: 3\nconfig: {imports: [{path: fragment.yml}]}\n")),
+				Imports: true, MainRelative: &configkit.MainRelativeContext{Fallback: tc.bindingFallback},
+			}
+			result, err := configkit.LoadLayered(
+				configkit.Input{Source: search, Filesystem: tc.mainContext},
+				configkit.LayeredOptions[layeredFacadeConfig]{
+					Underlays: []configkit.Underlay{{Inputs: []configkit.Input{underlay}}},
+					Overlay:   tc.overlay, OverlayFallback: tc.overlayFallback,
+				},
+			)
+			if tc.wantSources == 0 {
+				if err == nil {
+					t.Fatal("missing selected fallback succeeded")
+				}
+				diagnostics := err.(*configkit.LoadError).Diagnostics()
+				if len(diagnostics) != 1 || diagnostics[0].Code != configkit.CodeImportInvalid {
+					t.Fatalf("diagnostics = %#v", diagnostics)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			sources := result.Sources()
+			if result.Config.Count != tc.wantCount || len(sources) != tc.wantSources {
+				t.Fatalf("config = %#v, sources = %#v", result.Config, sources)
+			}
+			if sources[0].Layer != configkit.LayerUnderlay || len(sources[0].EmptyImportSelections) != 0 {
+				t.Fatalf("declaring source = %#v", sources[0])
+			}
+			if tc.mainContext != nil && (result.Config.Settings["companion"] != "loaded" || sources[2].Layer != configkit.LayerOverlay) {
+				t.Fatalf("config = %#v, sources = %#v", result.Config, sources)
+			}
+		})
+	}
+}