fix(index): safely seed nested worktree indexes

cmd/index.go

@@ -251,11 +251,28 @@ func runIndexer(cmd *cobra.Command, cfg *config.ConfigService, emb *embedder.Fai
 	}
 	defer lock.Release()
 
-	// Cancel context on SIGTERM or SIGINT so the indexer stops cleanly and
-	// the deferred lock.Release() runs before exit.
+	// Install signal handling before donor seeding: copying a large sibling
+	// index can take seconds, and cancellation must be able to close/remove the
+	// seed temp file before the process exits.
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGTERM, syscall.SIGINT)
 	defer stop()
 
+	// Reuse a sibling worktree's index for a brand-new database instead of
+	// re-embedding from scratch. The index lock serializes CLI indexers, while
+	// SeedFromDonor's seed lock also serializes this copy with MCP callers.
+	// A forced rebuild cannot reuse the copied embeddings, so skip the donor
+	// copy in that mode.
+	force, _ := cmd.Flags().GetBool("force")
+	if !force {
+		seedFromDonorIfNew(ctx, dbPath, projectPath, emb.ModelName(), logger, seedOptions{
+			status: p.Info,
+		})
+	}
+	if ctx.Err() != nil {
+		logger.Info("indexing cancelled by signal", "project", projectPath)
+		return
+	}
+
 	idx, setupErr := setupIndexer(cfg, emb, dbPath, logger)
 	if setupErr != nil {
 		err = setupErr
@@ -264,7 +281,7 @@ func runIndexer(cmd *cobra.Command, cfg *config.ConfigService, emb *embedder.Fai
 	defer func() { _ = idx.Close() }()
 
 	start := time.Now()
-	stats, err = performIndexing(ctx, cmd, idx, projectPath, p)
+	stats, err = performIndexing(ctx, force, idx, projectPath, p)
 	elapsed = time.Since(start).Round(time.Millisecond)
 	if err != nil && ctx.Err() != nil {
 		// A signal arrived; treat as clean exit.
@@ -274,9 +291,7 @@ func runIndexer(cmd *cobra.Command, cfg *config.ConfigService, emb *embedder.Fai
 	return
 }
 
-func performIndexing(ctx context.Context, cmd *cobra.Command, idx *index.Indexer, projectPath string, p *tui.Progress) (index.Stats, error) {
-	force, _ := cmd.Flags().GetBool("force")
-
+func performIndexing(ctx context.Context, force bool, idx *index.Indexer, projectPath string, p *tui.Progress) (index.Stats, error) {
 	progress := p.AsProgressFunc()
 
 	if force {

cmd/seed.go

@@ -0,0 +1,92 @@
+// Copyright 2026 Aeneas Rekkas
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+
+	"github.com/ory/lumen/internal/config"
+	"github.com/ory/lumen/internal/index"
+)
+
+type seedOptions struct {
+	findDonor func(string, string) string
+	seed      func(context.Context, string, string, string) (bool, error)
+	status    func(string)
+}
+
+// seedFromDonorIfNew seeds dbPath from a sibling worktree's index when dbPath
+// does not yet exist, so a fresh git worktree reuses an already-indexed
+// worktree's embeddings instead of re-embedding every file from scratch.
+//
+// Callers may additionally hold the index lock for dbPath, but SeedFromDonor
+// has its own advisory lock so CLI and MCP callers cannot duplicate the copy.
+// Seeding is best-effort — any failure is logged and indexing continues with a
+// from-scratch build. The returned warning is suitable for surfacing to MCP
+// clients. When dbPath already exists it is a single stat on the hot path.
+func seedFromDonorIfNew(ctx context.Context, dbPath, projectPath, model string, logger *slog.Logger, opts seedOptions) string {
+	if _, err := os.Stat(dbPath); !os.IsNotExist(err) {
+		// Exists already, or stat failed for some other reason — nothing to do.
+		return ""
+	}
+
+	findDonor := opts.findDonor
+	if findDonor == nil {
+		findDonor = config.FindDonorIndex
+	}
+	donorPath := findDonor(projectPath, model)
+	if donorPath == "" {
+		return ""
+	}
+
+	logger.Info("seeding index from donor worktree",
+		"project_path", projectPath,
+		"donor_path", donorPath,
+	)
+	if opts.status != nil {
+		opts.status("Seeding index from sibling worktree...")
+	}
+
+	seed := opts.seed
+	if seed == nil {
+		seed = index.SeedFromDonorContext
+	}
+	seeded, err := seed(ctx, donorPath, dbPath, projectPath)
+	if err != nil {
+		logger.Warn("seed from donor worktree failed",
+			"project_path", projectPath,
+			"donor_path", donorPath,
+			"error", err,
+		)
+		warning := fmt.Sprintf("index seeded from scratch (sibling copy failed: %v)", err)
+		if opts.status != nil {
+			opts.status(fmt.Sprintf("Sibling index copy failed: %v; indexing from scratch.", err))
+		}
+		return warning
+	}
+	if seeded && opts.status != nil {
+		opts.status("Seeded index from sibling worktree.")
+	} else if opts.status != nil {
+		if _, statErr := os.Stat(dbPath); statErr == nil {
+			opts.status("Index was seeded by another process.")
+		} else {
+			opts.status("Sibling index could not be reused; indexing from scratch.")
+		}
+	}
+	return ""
+}

cmd/seed_test.go

@@ -0,0 +1,126 @@
+// Copyright 2026 Aeneas Rekkas
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"context"
+	"errors"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func seedTestLogger() *slog.Logger {
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}
+
+func TestSeedFromDonorIfNew(t *testing.T) {
+	tests := []struct {
+		name     string
+		setupDB  bool // pre-create the destination DB
+		donor    string
+		seedErr  error // error returned by the seed stub
+		wantFind bool  // donor discovery should run
+		wantSeed bool  // seed should run
+	}{
+		{
+			name:     "skips when DB already exists",
+			setupDB:  true,
+			donor:    "/donor.db",
+			wantFind: false,
+			wantSeed: false,
+		},
+		{
+			name:     "seeds when DB missing and donor found",
+			donor:    "/donor.db",
+			wantFind: true,
+			wantSeed: true,
+		},
+		{
+			name:     "no seed when no donor found",
+			donor:    "",
+			wantFind: true,
+			wantSeed: false,
+		},
+		{
+			name:     "seed error is swallowed",
+			donor:    "/donor.db",
+			seedErr:  errors.New("copy failed"),
+			wantFind: true,
+			wantSeed: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dbPath := filepath.Join(t.TempDir(), "index.db")
+			if tt.setupDB {
+				if err := os.WriteFile(dbPath, []byte("existing"), 0o644); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			var findCalled, seedCalled bool
+			var gotDonor, gotDst string
+			var statuses []string
+			warning := seedFromDonorIfNew(
+				context.Background(),
+				dbPath,
+				"/project",
+				"model",
+				seedTestLogger(),
+				seedOptions{
+					findDonor: func(_, _ string) string {
+						findCalled = true
+						return tt.donor
+					},
+					seed: func(_ context.Context, donor, dst, projectPath string) (bool, error) {
+						seedCalled = true
+						gotDonor, gotDst = donor, dst
+						if projectPath != "/project" {
+							t.Errorf("seed project path = %q, want /project", projectPath)
+						}
+						return tt.seedErr == nil, tt.seedErr
+					},
+					status: func(message string) {
+						statuses = append(statuses, message)
+					},
+				},
+			)
+
+			if findCalled != tt.wantFind {
+				t.Errorf("donor discovery called = %v, want %v", findCalled, tt.wantFind)
+			}
+			if seedCalled != tt.wantSeed {
+				t.Errorf("seed called = %v, want %v", seedCalled, tt.wantSeed)
+			}
+			if tt.wantSeed && (gotDonor != tt.donor || gotDst != dbPath) {
+				t.Errorf("seed called with (%q, %q), want (%q, %q)", gotDonor, gotDst, tt.donor, dbPath)
+			}
+			if tt.seedErr != nil {
+				if warning == "" {
+					t.Error("expected warning when seed fails")
+				}
+				if len(statuses) != 2 {
+					t.Fatalf("expected start and failure status, got %v", statuses)
+				}
+			} else if warning != "" {
+				t.Errorf("unexpected warning: %q", warning)
+			}
+		})
+	}
+}

cmd/stdio.go

@@ -200,7 +200,7 @@ type indexerCache struct {
 	embedTimeout      time.Duration                                                                                                            // override for tests; 0 means defaultEmbedTimeout
 	staleEmbedTimeout time.Duration                                                                                                            // override for tests; 0 means defaultStaleEmbedTimeout
 	findDonorFunc     func(string, string) string                                                                                              // nil uses config.FindDonorIndex
-	seedFunc          func(string, string) (bool, error)                                                                                       // nil uses index.SeedFromDonor
+	seedFunc          func(context.Context, string, string, string) (bool, error)                                                              // nil uses index.SeedFromDonorContext
 	ensureFreshFunc   func(ctx context.Context, idx *index.Indexer, projectDir string, progress index.ProgressFunc) (bool, index.Stats, error) // nil uses idx.EnsureFresh
 	log               *slog.Logger
 	wg                sync.WaitGroup     // tracks background reindex goroutines
@@ -484,7 +484,6 @@ func (ic *indexerCache) getOrCreate(projectPath string, preferredRoot string, mo
 	}
 
 	// Seed from sibling worktree if this is a new index.
-	var seedWarning string
 	isNewDB := false
 	if _, statErr := os.Stat(dbPath); os.IsNotExist(statErr) {
 		isNewDB = true
@@ -494,29 +493,15 @@ func (ic *indexerCache) getOrCreate(projectPath string, preferredRoot string, mo
 			"model", modelName,
 			"index_version", config.IndexVersion,
 		)
-		findDonor := ic.findDonorFunc
-		if findDonor == nil {
-			findDonor = config.FindDonorIndex
-		}
-		if donorPath := findDonor(effectiveRoot, modelName); donorPath != "" {
-			ic.logger().Info("seeding index from donor worktree",
-				"effective_root", effectiveRoot,
-				"donor_path", donorPath,
-			)
-			seedFn := ic.seedFunc
-			if seedFn == nil {
-				seedFn = index.SeedFromDonor
-			}
-			if _, seedErr := seedFn(donorPath, dbPath); seedErr != nil {
-				ic.logger().Warn("seed from donor worktree failed",
-					"effective_root", effectiveRoot,
-					"donor_path", donorPath,
-					"error", seedErr,
-				)
-				seedWarning = fmt.Sprintf("index seeded from scratch (sibling copy failed: %v)", seedErr)
-			}
-		}
 	}
+	seedCtx := ic.closeCtx
+	if seedCtx == nil {
+		seedCtx = context.Background()
+	}
+	seedWarning := seedFromDonorIfNew(seedCtx, dbPath, effectiveRoot, modelName, ic.logger(), seedOptions{
+		findDonor: ic.findDonorFunc,
+		seed:      ic.seedFunc,
+	})
 
 	idx, err := index.NewIndexer(dbPath, ic.embedder, ic.cfg.MaxChunkTokens())
 	if err != nil {